996 idle issue

at traffic lights, the rpm somtimes goes down to 100 rev and the engine oil light pops up for a second then the idle goes back up to normal. What can cause this? How to fix?

The oil light comes on becasue the idle drops to low to pump the oil.. Fix the idle and you will fix the pressure thing. Ok Seriously on the 928 this is caused by the Idle air stabilizer. Not sure if we have one on the 996.

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What Shark Attack said. To add eliminate the AOS. One AOS failure mode can cause the RPMs to fluctuate over a few seconds period -- drop then climb and repeat -- and as the rpms drop oil pressure drops cause oil pressure related to how fast engine spinning. Sincerely, Macster.

Strong recommendation to get it into your dealer ASAP. The oil pump on the head that drives off the cam sprocket has been known to fail. If not corrected immediately it can mean a new engine.

Sorry, but that is just not the case with OP's car. Its got a running issue that causes the presure to drop - its rpm related and not a mechanical concern. The same thing can happen in summer w/o the idle issue if you run light oils. Your idle needs to be fixed and I am not the best at those. Might be a vacuum leak, an AOS, a sensor issue, dirty MAF or throttle body. You need to diagnos it and fix it, the oil issue will be gone - as long as you run good oil too

what year? check the ICV and throttle body.... they might be sticking and need cleaning.... and clean you MAF.

its a 00 C2. where is the ICV located? How to clean it?

I think that only the '99's have the ICV. Once they went with electronic throttle it was eliminated. On the '99 it is on the side of the throttle body, and you can use carb cleaner to spray around inside of pivot points of moving plate. Easier to clean if you remove it. (2 screws) Pete

if it is an 00 I would guess the AOS is going out if it has not already gone out. If it never has been replaced I would do so anyway and then clean your MAF and throttle body as the oil residue will lead to errors in the MAF readings and gumming of the throttle plate.

i did the oil cap test and AOS seems to be ok. i also cleaned both MAF and TB eventhough they looked clean to me in the first place. the car went out a major service in the dealer where they changed spark plugs and flush the fuel system. The car run like a breeze except for that drop at full stop and trans on N. Please, help.

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The check for excessive vacuum at the oil filler tube is not 100% conclusive the AOS is ok. There are at least 2 failure modes the AOS can experience and they differ in the symptoms they manifest. IOWs, had you found excessive vacuum at the oil filler tube opening that would have confirmed the AOS was bad. That you didn't find this vacuum does not eliminate the AOS from consideration. The cars (2000 and newer) equipped with E-Gas do not have an idle control valve. The E-Gas system controls the engine's idle speed. I don't think this will help, but follow the instructions in the owners manual regarding disconnecting the battery, as if you were going to replace the battery. Before you disconnect the battery note the radio's security code. Be sure all major electrical loads are removed. Do not leave the head lights on for instance. Turn the key to the off position and remove the key from the ignition switch. Disconnect the battery. Wait some time. Minutes is probably too much time, but what the heck. Reconnect the battery. Now let the E-Gas perform a recalibration of its internal circuits. This is done automatically after a battery disconnect when you turn the ignition key on and wait at least 10 seconds *before* starting the engine. You'll have to re-enter the radio's security code and reset the window stops as well after you reconnect the battery. If this sees the idle now stable and remaining so great. I don't think it will help but you might consider trying it. But if this doesn't help it may convince you the cause of the symptom lies elsewhere. I've not heard of this as a problem with Porsches but in some cars the A/C compressor engaged switch/sensor can act up and "fool" the engine controller into thinking the compressor is on when it is off or off when it is on. This causes the engine controller to attempt to adapt to the expected increase in parasitic engine load by slightly increasing the engine speed. Then the engine controller recognizes the engine speed at idle too high and it backs the speed down again to the nominal idle speed. The result in the engine's idle speed varies by some noticable but usually small amount but varies by more than the idle speed should vary under normal operating conditions and when everything's working just fine. Not sure what you can do about the above. Trouble-shooting this generally requires the use of the PST2 (Porsche System Tester 2) diagnostics computer to monitor the MAF and the engine idle speed and the presence/absence of parasitic engine load signals to the engine controller. If you haven't already done so, you can try this: Disconnect the MAF at the wiring harness. Do not remove the MAF from teh intake pipe. Ideally you want the engine dead cold. Now when you start the engine with the MAF disconnected the engine may behave a bit different cause it will have been used to having the MAF present and supplying the engine controller the amount of air the engine is using. Start the engine and let it idle and warm up and then drive the car as you do when the idle symptom appears. The idea is to as close as possible duplicate the same conditions with the MAF disconnected as when it was connected. Try your best, give the engine every opportunity to manifest the symptom. Try to avoid treating the car the engine any different than you did when the MAF was connected. If the symptom to the best of your ability does not appear, depending upon how confident you are regarding your efforts to give the symptom the chance to appear, this strongly suggests the MAF at fault. If the symptom appears then this almost certainly eliminates the MAF from the list of possible sources of the symptom. Note: With the MAF disconnected the check engine light may come on. You will probably have to clear this error associated with the CEL cause it may not clear on its own after the MAF is reconnected. Sincerely, Macster.

great write up Macster,, i will follow your instructions and let you know.

My money is on the MAF. Are you running a stock Air filter?

check for air intake leaks.... mostly, leaks after the MAF.... if air is coming in after the MAF, then the MAF is not reading the extra air coming in.... just throwing this out there.

i am running the stock air filter,, I have disconnected the MAF and the symptom appeared again,, I am so confused!

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Well, I like to mention that there's a risk to trying to self-diagnosis -- even with the "help" of well-intentioned people on the 'net -- a car acting up and that is the risk of having a relative minor problem turn into a major problem and cause some serious engine damage, maybe even destroy the engine. With you having no access to any data viewer that allows you to view various engine sensor inputs to the engine controller, and perform other tests, you've gone about as far as you can go. As I think I mentioned, my 02 Boxster at around 78K miles developed a mlld idle fluctuation with the idle climbing a bit then falling a bit but nothing dramatic and certainly not bad enough to cause the oil light to come on. course, I should mention there was several inches of snow on the ground and while the car was in a garage it was still cold and the engine never got real hot so the oil may not have been hot enough to be thin enough the drop in rpms enough to affect oil pressure. I messed around and did what I could to identify the source of the problem but when I encountered the high vacuum -- quite by accident -- at the oil filler tube and the engine began to smoke all the time, though not bad, I got scared and called the dealer's service department. Service manager told me "AOS" and to not drive the car, avoid running the engine. Even though I was out of work at the time I still had the car flat bedded to the dealer and had the AOS replaced. Symptoms gone and for not much money. (In fact while car in for repairs I drove my other car 2000 miles to a job interview -- I hate to fly -- and managed to get a job offer which I accepted. The Porsche rode behind a moving van on an open trailer after getting a new AOS when I moved myself 2000 miles west to take the new job.) Now even that AOS failed at I forget what mileage -- call it 180K -- and this time failed a bit differently. No idle symptoms, just after hundreds of miles drving in one day -- on a trip -- setting the check engine light. I forget the codes, but decided to press on until the MAF -- I suspected MAF -- failed and I'd limp in and buy a new one at some dealer along the way. But 2 days later upon first engine start of the day the engine smoked like crazy. I knew the problem and had the car flat bedded again to a dealer, the same dealer as it turned out that did the the 1st AOS. This second time instead of having the car flat bedded 40+ miles it was over twice the distance away. Still money well-spent. My point is that you have to decide how much longer you're going to fuss around and try to sort of solve this problem without the proper training, without the proper equipement. I'd love to be able to tell you the problem's the MAF or an air leak, and not the AOS, or even that it the AOS, or something else, but I can't with any confidence. No one that I know can unless he's a tech who's seen enough of these cars and with this symptom to know what it means. You can elect to continue to use the car and let the symptom progress to where it is more obvious where the problem lies. I do not recommend this technique though I have followed it once or twice for what I suspected to be either MAF or O2 sensor problems. I was wrong on the MAF -- it was a leaking oil filler tube cap -- and right on the sensor. I fixed the car in both cases, though the first "fix" cost me nearly $413. $400 to replace a perfectly good MAF and $13 to replace a bad oil filler tube cap. However, as I tried to convey with my story about my experience with AOSs, is tha sometimes you need to get the car to an expert and have the matter sorted out. This usually proves to be the least expensive course of action in the long run. And sometimes in the short run as well. Engine speed dropping low enough to cause the oil light to come on is not good. The engine oiling system is probably fine, the low oil pressure caused by what I suspect is a relatively big drop in rpms. In this situation the more you run the engine to try to identify the source of the problem the more risk you take that each time the oil pressure drops some bearing wear and tear is taking place, that a cam lobe or lifter face is receiving insufficient oil and wearing or galling. You could perhaps finally identify the source of the problem -- say it is the AOS and call it a $500 repair bill -- only to find the engine needs new bearings or new cams and lifters cause the low oil pressure ruined them. My advice is to get the car to a qualified shop staffed with techs who work on these cars day after day and know them inside and out. And get the car to a shop without driving it. If you insist on continuing to drive the car, running the engine, then be prepared at any time to if the engine's condition shows or gives any indication it is getting worse to shut the engine off as soon as it is safe to do so -- which I hate to say it but you need to be aware of it may already be too late -- and then get the car flat bedded to a shop. And this time don't start the engine at all cause there is no more running of the engine that's going to make the situation better, it can only make the situation worse. Sincerely, Macster.

an update,, i took the car to the dealer,, they found no codes,, cleaned the MAF,, reflash the ECU,, checked all possible causes,, chraged me $200,, later at the same night the car was out,, it happened again and the RPM dropped to 200 revs and back up again,, i am out of ideas,, please, please, help! :@

hmmmm, bad gas? try disconnecting the - battery terminal. Make sure you have the radio codes before you do... this will reset the ecu. Im out of ideas as well.

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Did you think of leaving the car with the dealer (I assume a Porsche dealer?) to have the tech reproduce the symptom? It is very important you relate under what conditions the symptom appears so the tech is successful at reproducing the symptom. Even though there are no error codes symptoms can be very helpful to a tech in trouble shooting a problem. Absent any instrumentation to monitor the various sensor inputs -- like short term fuel trims, air flow, timing, intake and coolant temperature, etc. -- you are going to remain in the dark as to what's going on. Let's recap: Did you check for excessive crankcase vacuum by trying to remove the oil filler tube cap while engine was idling? Is the throttle body, its butterfly valve, and the area downstream of the throttle body (the intake manifold) free of any signs of contamination? Is there any amount of oil present in at the where the connector comes into the manifold behind right after the throttle body? Is the oil level stable and good or has it dropped any? Any smoking upon startup or at any other times? (A brief cloud of smoke at cold start especially if you've not run the engine much is ok. But if the cloud persists is not ok.) Does the symptom appear when the MAF disconnected? What is the over all condition of the car? Well maintained? Or something less than that. Has fuel filter been changed. Is fuel level ok? There's not a lot a novice can do beyond trying a few things. Your best bet is to return the car to the Porsche dealer, or some qualifed indy shop, and have the trouble diagnosed. This requires the tech be able to reproduce the symptom. I'm leaning towards AOS but I wouldn't want you to replace it solely based on my layman's opinion. The rpm drop is a bit more severe than I have seen (or read reported) a bad AOS causes. But it is still my 1st choice for the cause of the symptom. But it could be a fuel supply or fuel pressure problem. Or something else. To have the car exhibiting the symptom/behavior while at a dealership with the car hooked up to the Porsche diagnostics computer system and to have the various sensors and inputs available a tech should be able to eliminate many things very quickly. Besides the computer the tech has his eyes, ears and fingers to help him: For instance the tech might see the vacuum hoses that run from the AOS to the engine collapse when the rpms drop or he might feel them flex as the engine runs. This would suggest excessive vacuum and provide him a clue. If you're looking for a definite "this is what's wrong" and then taking this to the dealer and telling the dealer "this is what's worng, fix this" I don't think you're going to get that. I can't/won't say AOS. If I did and you replaced it and it wasn't the AOS I'd feel bad. Worse the problem could get worse and then the car and you'd be worse off than are now. As I think I've mentioned the problem may not be serious, now. But messing around trying various things, continuously reproducing the symptom when it obviously of no value to you since you are not able to know what it means and for us here the same applies, could be moving the problem towards a more serious level. There's a risk involved in prolonged self-diagnosis of engine symptoms. Anymore diagnosis I believe should be done by a trained qualified technican and not by you and not for the benefit of anyone here. Sincerely, Macster.

Did you check for excessive crankcase vacuum by trying to remove the oil filler tube cap while engine was idling? Yes, i can easily remove the oil cap but when removed the rpm fluctuates. Is the throttle body, its butterfly valve, and the area downstream of the throttle body (the intake manifold) free of any signs of contamination? No Is there any amount of oil present in at the where the connector comes into the manifold behind right after the throttle body? No Is the oil level stable and good or has it dropped any? Utterly stable Any smoking upon startup or at any other times? (A brief cloud of smoke at cold start especially if you've not run the engine much is ok. But if the cloud persists is not ok.) No Smoke at any time Does the symptom appear when the MAF disconnected? Yes What is the over all condition of the car? Well maintained? Or something less than that. Has fuel filter been changed. Is fuel level ok? The car is mint condition and regularly maintained. Fuel filter replaced 3 weeks ago

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The idle will change when the oil filler tube cap removed. The idle should stabilize though in a moment or two. And when you replace the cap the idle will again change and again it should stabilize and recover nicely to what it was before the cap removed and then replaced. The absence of other warning signals of pending AOS trouble lowers the odds it is AOS. I''m running out of ideas. All I can think to ask now is: Was the symptom present before the fuel filter changed, or did it appear after? Sincerely, Macster.

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the symptom was there before changing the fuel filter,, Loren is suspecting the throttle body positioner,, what do you think?

What is your idle when cold starting? What's the idle after warmed up? Is your a/c on? What's the weather? What is the fuel level? Do you run your fuel tank around 1/4 before refueling? Most important is not running the gas tank past 1/4 is a fair warning. Modern electric fuel pumps need constant fuel around them to stay cool. They burn out quick if sucking air and not bathed in liquid. Could be a weak fuel pump, water from winter storage or whatevers. If its the pump you'll know it when it finally goes which might take a year. Oh yah, does the idle run up higher than normal idle when jumping back up?

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Let's see.. Car is a 2000? IIRC that is the 1st year E-Gas (throttle by wire) available. Sure. E-Gas (pedal) could be acting up or the electro-mechanical device (throttle body positioner) on the side of the throttle body that moves the butterfly valve on command by the engine controller from the inputs it receives from the E-Gas pedel potentiometers. Both the E-Gas and the throttle body positioner are pretty sophisticated devices and should lend themselves well to proper diagnostics by some shop equipped with the proper Porsche diagnostics computer. There may be more less high tech ways to trouble shoot either of these devices but I don't know of any off hand. In short Loren's suggestion makes very good sense and you would do well to eliminate either device as the cause of the symptoms you see. Which still leaves you facing taking the car to a qualified shop that is set up to diagnose these items. Sincerely, Macster.

What is your idle when cold starting? around 900,, i believe it should jump over 1000 when cold starting,, but it doesn't,, What's the idle after warmed up? around 700 Is your a/c on? a/c is off but not showing OFF in climate display What's the weather? 30c 40-50% humidity What is the fuel level? fuel level is ok Oh yah, does the idle run up higher than normal idle when jumping back up? didn't get that!

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Couldn't think of saying does your rpm's roller coaster up and down searching out where to stabilize. I'm thinking if it is roller coastering I would forget about it being a throttle control related problem and go after air fuel systems. MAF, fuel pump, fuel, or the elctronics sensing something that says to increase throttle.

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well,, when it drops, it doesn't jump back up,, it goes back up slowly to normal,, i must say that my car doesn't have this symptom all the time,, it happens mainly after long rides,,

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i can't recall that the rpm stabilized until i put the cap back in place.

Does this car have a throttle body alignment feature? I recall with the audis, if you disconnect the battery and reconnect, you turn the key to the acces position and the throttle body aligns itself.

a thought,, everytime the rpm drops,, a sound of an electrical actuator occurs a moment before the drop,, this sound comes from the front right behind the dashboard,, it is the same sound that happens when you turn the key to ON position before a cold start,, I am starting to suspect the fuel pump now but,, a weak pump should affect the overall performance of the car,, right??

A weak pump can be adjusted for till its too far out of the elctronic management of fuel and ignition for proper running. If you had said its been warm and running the a/c I might of figured its the a/c cutting in and dragging the engine down. I imagine if refrigerant is low and or refrigerant oil it might cause a low enough rpm dragging it down. I'm not sure if you mentioned the a/c emblem is on or not when running the car. I'm a little confused why the a/c light comes on when hitting auto for using the temp for heat instead of a/c but eh. Just might be the a/c cutting on causing drag. The front right below the winshield has what looks like and a/c valves block. You have to lift the panel that also covers the airfilter area to see it. It's also the a/c servicing area for high and low pressure coolant lines. There are 134 coolant bottles that come with a pressure reader. It will read your low pressure line. Try seeing if the coolant reads low and tht might explain some drag in the system when the pump clicks on. When coolant is low the system will automaticly shut down when pressure drops too low then click back on when enough coolant has accumilated. Anyway try checking the coolant level since summer is coming.

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In my cars with autoclimate control when I press the auto button the AC is activated. The snowflake appears on the climate control LCD. The air is passed through the A/C evaporator to dry the air then this cool air is routed through the heat exchanger to warm it up. The autoclimate control varies the flaps inside the AC system to direct the proper amount of air from the AC evaporator to the heat exchanger to get the cabin temperature to the desired temperature and maintain it. I don't know about other cars but the autoclimate control in my Boxster (and Turbo) are fantastic systems. Sincerely, Macster.

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Well, it takes time. A minute is too long but it can take more than a moment or two. If after a minute the idle speed hasn't gotten better and smoother (though it may not be as smooth as it was with the cap in place) this could suggest an air leak elsewhere. With the existing air leak and then this new one introduced by removing the cap the engine controller simply can't cope. At this point it would be nice to have an OBD code reader/data viewer hooked up and examine the long and short fuel trims before removing the cap, after, and after putting the cap back on. The long term fuel trims before the cap removed if at the rich end of the adjustment almost certainly points to an air leak somewhere (or a low fuel supply or pressure problem). All doable if the car's at a properly equipped shop. Sincerely, Macster.

If this were the pre electronics days I'd be thinking vapor lock being approached or......I don't YET understand the fouling of the AOS or what ever puts coolant out into the engine the wrong way but if gizmo's aren't throwing an error I kind of think it increases the chance of corrupted standard data items of air fuel or spark with out an error code detected. Once again BUT I'm just a simple guy staying away from saying or thinking I hate the modern gizmo's. I don't know if there is heat shielding that got messed up and heates your fuel. Some times symptoms like the click are just another thing thing out there with nothing to do with the problem but highly noticed when looking for symptoms. Good luck.

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Thanks Macster. I've been turning off the auto by hitting the fan adjust then turning off the a/c. Seems the heater works with out the dryer running too. Thanks for letting me know what's going on in there.

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Question- Have you taken the air cleaner off and inspected the butterfly valve on the throttle body? No way the dealership guys didn't do this but you never know. +1 for bad gas.

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yes, i did cleaned the throttle body,, 2000 Carreras have no idle control valve,, if it is bad gas,, should i take the tank off and clean it then fill it again??

Not to hijack a thread, but my 99 kind of does the same thing. When the car has been run for 30 min - 1 hour, if I'm sitting idling at a light, the RPM's sometimes fluctuate +/- 200 RPM and then it settles down to it's normal idle. It does not do this when it cold or just warmed up and mostly happens in the afternoon. I have no CEL or codes, new AOS 3K miles ago, all new O2 sensors 1K ago and cleaned the throttle a few weeks ago. It's odd, but I'm not too concerned at this point. My thinking is the throttle valve actuator is wearing out after 108K. Rick 99 996C4 87 944S

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Bad gas is too vague a diagnosis I believe to justify removing the tank and cleaning it. Or even draining it by opening up the top and siphoning the "bad" gas out. Fooling around with a gas tank is dangerous business and unless you're quite confident in your ability to handle this safely I'd leave it for the professionals. Besides there are various hoses, connectors, wiring that if you're not careful you can damage and add further to your misery. Even if you are careful, these hoses, connectors and so on are getting on in years and are brittle, dry, and will not take kindly to be handled as you make your way to the tank and back out again. Just how much work are you prepared to do to resolve this symptom? How much can you do? Without at least a code reader, data viewer to read codes, clear codes and view OBD sensor data in real time you're shooting in the dark. Even with above you have little better than candle light to pull back darkness over what's going on. Sincerely, Macster.

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In your car's case I believe it has a old style throttle. But you cleaned the throttle body. Was the butterfly operated by a cable from the pedal and lever or was there a black box on the side of the throttle body? If the former, then the throttle body has no electro-mechanical throttle valve actuator but instead relies upon an idle control valve for idle speed. Many have reported removing this ICV and cleaning it thoroughly has seen any idle variation symptoms gone. Sincerely, Macster.

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It's a 99 C4, e-gas, no idle control valve. Rick 99 996C4 87 944S

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Ok. E-Gas it is. My info is E-Gas first appeared in MY 2000 cars. Regardless, it has what it has and if it has a throttle valve control module then this could be wearing out. 108K miles is alot of miles. My car has 230K+ miles and the throttle valve control module still works ok. But different cars. Different throttle valve control modules. In both our cars' the throttle valve control module runs self-tests before every engine start to verify mechanical integrity. Then during driving the throttle valve control module is continuously monitored: Throttle valve position; throttle valve ranges; throttle valve end stages; and voltage are all continuously monitored. My point is that one would have to believe that if the device were worn enough to allow the air flow to vary enough to cause the engine speed to vary by a significant amount this would have an error code active. Can't say if this will help or not, all you can do is try it you want to. Turn off all electrical loads. Headlights, etc. Remove the key from the ignition. Following proper procedure disconnect the car's battery. After a minute or so -- seconds is long enough but a minute's better or you can wait longer if you want -- reconnect the battery. Turn the ignition key to on but not to engine start position. Do not touch the gas pedal. Wait at least 10 seconds for the throttle valve control module to recalibrate its adaptation values. If it doesn't help could be the throttle valve control module but I'd not advise you to replace it on the strength of result of the battery disconnect/throttle valve control module recalibration I mentioned above. Sincerely, Macster.

to share the same experience,, i have uploaded a video of the symptom,, http://www.youtube.com/watch?v=4Taz4CJiSnw

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When the rpm drops can you hear any other noise? I'm thinking some parasitic load kicks on. A/C compressor. Electric fans. That kind of a period -- about a minute by my watch -- between rpm drops doesn't follow a vacuum leak induced rpm drop. My WAG is a fuel supply problem but a real odd one (if I'm even correct). While engine running if you could arrange to listen to the fuel pump even going so far as to raise the front of the car and get safely under the car and with a stethescope pick a good spot to listen in on the fuel pump to see if you can hear the pump slow down or stop momentarily I think might have some value, be worth the effort. It might be of some value to listen at the gas tank filler opening with the cap removed to hear any sounds from inside the tank. If there are any fuel hoses -- rubber like -- watch them to see if they twitch or move or collapse or do anything at all when the rpms drop. If you are prepared to do the work you could set up a fuel pressure gage and watch the fuel pressure while the engine's running and see if the pressure changes. Or if you don't want to mess with a fuel pressure gage -- and these can be messy -- then with an OBD code reader/data viewer connected the car's OBDII port watch the short term fuel trims to see how they behave before, during, and after the rpm drop. I know I know kind of an all over the map series of suggestions but really this is a process of gathering info and trying to find something that provides a clue as to what's going on. Refresh my memory: Since this symptom pretty easy -- I take it -- to reproduce -- why haven't you taken the car to a Porsche dealer or a good indy shop for this symptom? Also, you were able to reproduce the symptom with the MAF disconnected at the wiring harness, yes? Sincerely, Macster.

when you unplugged the maf.... does it behave the same way? If it does, you may have a bad maf, even though you cleaned it. You really need to start eliminating hardware to properly troubleshoot.... maf, throttlebody, gas, pump, filter etc, etc... but do it one at a time, surely its one or more on the list.

When the rpm drops can you hear any other noise? I'm thinking some parasitic load kicks on. A/C compressor. Electric fans. i hear an electric actuator humming in the front right behind the dashboard Refresh my memory: Since this symptom pretty easy -- I take it -- to reproduce -- why haven't you taken the car to a Porsche dealer or a good indy shop for this symptom? i had it to the dealer and they charged me $200 for nothing Also, you were able to reproduce the symptom with the MAF disconnected at the wiring harness, yes? that's right

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how to eliminate the gas and pump since they are the suspects.

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I would start eliminating the easier and less expensive first..... I believe the gas pump is in gas tank.... but you mentioned this only happens on idle at park or neutral.... could it be that the o2 sensor are not reading properly and the ecu is adjusting accordingly (false idling)

it happens on park, neutral and on D in heavy traffic. this should throw a CEL,, shouldn't?

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Ok.... try eliminating the ac compressor, does the rpm fluctuate with the ac off? throw a cell?, not if the O2 sensor is in range.... but you mentioned that it also does it in D so.... its unlikely that all 4 sensor are bad (eg:reading constant)

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it happens with a/c off and on,, but i guess the chance of it happening is more likely the moment a/c goes on.

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where to get that?? how much is it??

Another possible issue. Throttle body alignment????? An unaligned throttle body would cuase a rough idle and poor off the line performance. Unplug battery.....step on brakes...(making sure there is no current in system) Reconnect battery turn key to position just before cranking, do not start car...you should hear throttle body cycling...let it go through full cycle. Ussually 30sec to 2min. Start car. See if this helps.

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You can pay nearly as little or much as you want. Somewhere in the middle best I think. Course it depends upon what you want to do. Since you are attempting to trouble-shoot a puzzling problem you might want to get a full feature OBDII code reader/data viewer, scan tool, diagnostics tool which the ulitimate is the Porsche System Tester 2. These are not something you can run right out and buy though. These are used by Porsche dealerships and seldom come up for sale. Price is when they do come up for sale is near I can tell north of approx. $1100. How much north I can't say. Some have reported though buying one for $1100 IIRC. Recenlty one went unsold at just over $1000. The next tool down is the Durametric. Search the web and you'll find company/product link. Below the Durametric you're in generic OBD2 scan tools. Actron makes a good one (several in fact) and there are other brands. A problem is even if you are able to look at the OBD sensor data and derived data it may not tell you anything. Even when there are error codes the failing sensor or component may not jump out at you. Sincerely, Macster.

can it be a bad alt/battery? the rpm is more likely to drop when car is hot, fans on and a/c on,,

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An increase in parasitic load from some accessory drive I guess could account for the RPM drop. Alternator accessory drive is as good as any to suspect I guess. Would have to be a pretty big load though. Something abnormally big. At idle A/C load (and electric fans, cabin fan) ups the load by just a few tenths of a percent. Maybe from 8.8hp to 9.24hp. IOWs, it requires about 1.25hp at idle to turn the alternator to provide the electricity the A/C electrical loads. Then the A/C compressor load also accounts for some of this 1.25hp. The engine controller knows the A/C is on and will compensate to keep the idle at the desired level and to accomodate the fact the compressor is cycled on and off. (With autoclimate control the compressor is variable and this is adjusted to produce more or less cooling. Much smoother operation. Seamless in fact as near as I can tell.) If you know how to hook up and use a volt/ohm meter you might be able to spot some voltage change of significance concurrent with the rpm drop and the rise. Sincerely, Macster.

Quotes source post: 7527188

I am afraid that the sound i hear with the symptom is the sound of a fan draining too much electricity due to an impairment. Also, i don't see the rpm accomodates any. It is always plays between 600-700 in all cases.

Quotes source post: 7527216

That the rpm stays stable between 600 and 700 (IIRC the rpm variation is +/-40) is the engine controller accomodating load variations as accessories are turned on and off and as for instance A/C loads increase or decrease. I again listened to the clip you posted and I can't hear any fans or any thing other than the background noise of the engine exhaust. I would think that if an accessory load was causing the engine rpms to drop by the amount I see in the clip were you outside the car at various places (one place at a time -- no being in more than one place at a time!) you couldn't help but hear the device or whatever react at the same time the engine rpms dropped and then react again as the rpms recovered. Also, I note the rpms never jump up? Which kind of is strange... unless the load goes away slower than it comes on, if it is a load. I'm leaning away from some exceptional load causing this. Apparently other loads are introduced then removed by your action and the action of various subsystems and the engine controller is able to accomodate them just fine. Without seeing fuel trims, long and short term fuel trims, ignition advance, and and engine load (just in case it is some exceptional parastitic load) you and everyone else offering help is really shooting in the dark. It could be the throttle body/E-Gas control module acting up but I'm not about to suggest replacing that. If you could borrow a throttle body from another car... Would to be one very accomodating Porsche owner though. And if you handed him back the throttle body/E-Gas control module and it didn't work right you would have to hand over a check for a new one right then and there. Then call him a cab to run him to nearest Porsche dealer to buy (order) a replacement. Ouch! Sometimes a salvage yard that sells parts salvaged from cars might let you try a salvaged part, but I don't know if there are any where you are and if they'd do this with a throttle body/E-Gas control module. You'd still be on the hook if somehow the unit was working ok before you tried it -- though how to know? -- and if not after -- again how is one going to know? -- to buy the now broken unit. Sincerely, Macster.

The temperature is up there about where the fans should kick in. Any good dark place with a white wall where you could wait for the pressure drop and observe the headlight brightness? If it is an amps available drop due to a connection I'd think it would show up in the headlights having to run off the battery (with a bad connection the batterys full amps wont be available and the alternator wont put out enough amps in the lower rpm). A big amps drop would probably drop the rpms of the fuel pump. Problem sourse in that situation would be bad cells in battery, or bad connection to a ground or positive, or just the battery post connections need a good cleaning. Fan engines going bad can suck a lot of power. Could be the radiators just need cleaning if the fans are sucking against a blockage.

Quotes source post: 7527216

Well, I ran your symptom by a couple of Porsche techs this AM. Neither had much to suggest. It is an odd symptom and not one either is familiar with. (And they between them have over 40 years experience on Porsches.) One tech suggested it might be related to a going bad or bad battery or possibly a bad connection between the battery and car (and car to battery). Along with this I can think of a marginal connection at any one of a number of connectors that are vital to the proper running of the engine. Too many to list. Without any error codes, without any ability to instrument the car and check its vital signs before/during/after the symptom appears...everyone's just shooting in the dark. I note the CEL doesn't come on, doesn't flash. No misfires then. So it is not spark, not fuel supply (that is I think these can reasonably be ruled out). Thus the only thing I can think of is it is an air supply problem: Throttle acting up. Or E-Gas pedal acting up. Can't recommend replacing the throttle body with no more than just some WAG on my part to go by. (Did you ever do the E-Gas recalibration thing I covered in an earlier post to you?) Not sure this is doable, but with everything at where it needs to be and the symptom showing up shutting off engine and disconnecting (one at a time) the E-Gas pedal from the wiring harness or the throttle body controller from the engine harness and seeing if the symptom returns is about all I can think of. And not sure the engine will even start and run with either one of these control systems disconnected. Or another worry is the engine starts then races out of control. And even if it does start and run (whether the symptom present or absent) one or more error codes likely to be logged which could require using an OBD scan tool to clear them. Sincerely, Macster.

Quotes source post: 7529038

first of all,, i want express how glad i am to be here in RL,, i thank you all for the great help and support that neither my local dealer nor local P owners seem to provide,, back to subject,, i have tried the e-gas recalibration many many times,, what are the major car-battery connection points?? one more point,, the car at cold start doesn't shout and jump to higher rpm as usual,, it starts lazy up to the regular rpm level as if it is warm already,, rings a bell?

Quotes source post: 7529123

Well, the lazy sort of start cold -- which is kind of one half the symptom (the coming back up to normal RPMs half) -- sans any backfiring or stumbling -- which means the fuel mixture is right which to me eliminates fuel pressure, fuel supply, air temperature, coolant temperature as being wrong -- sort of suggests... Nothing. That is at first I thought there might be something signficant and there might be but what means? Check that. Symptom might arise from the "bad" battery putting a heck of a load on the alternator; or going with the wiring theme a bad battery/car connection somehow accounts for this behaviour. No time to detail this. It could still be due to a faulty E-Gas pedal or throttle body controller module. And there are probably more possible causes. The major battery/car connection points are the battery terminals and the other end of those cables where they connect to the car, chassis, or wherever they connect. I've seen such poor condition battery cables that the battery cable where is connects to the chassis is so frayed/corroded that just a few strands of copper carry the electrical load and these can glow red when the starter spins the engine. This heating up and sudden cooling tends to soften the copper which makes it less likely to break so this condition can continue for a while until one or more strands finally does break and then the heat melts the others. (Years ago when I replaced battery cables -- when they were much less complex -- I'd buy heavy duty welding cable instead of regular battery cable --cause welding cable carried more current: Cable was larger in diameter and had more strands.) Starting at the battery you'll have to trace the battery cables to where they go and to what the connect to and make sure not only the battery terminal connectors are good and corrosion free (which is usually the case cause they're "easy" to get to) but the connections at the other end are likewise. Since these are less accessible they tend to get overlooked. Is the battery in good condition? Can you have an autoshop or even a battery reseller check/test the battery for you? Check the battery acid's specific gravity level for you? Sincerely, Macster.

You might try checking major connections (big plugs and sockets) for electronics in the engine compartment and the grounds in there if they are findable. One guy was having rpoblems that turned out to be a connection on his alternator. Might finger some of those wires. I just bought a durametric last night. Hopefully I wont have something like this presenting problems later on. Anyway good luck.

an update.. the humming sound that goes with the symptom is produced from the fan of the passenger side radiator,, it is to me a chicken-and-egg issue now :S does the fan drain more amp then rpm drops?? or,, rpm drops then fan is suffering from that?? the dealer promised to arrange for a used throttle body to test on my car tomorrow,, i will keep you updated,,

Quotes source post: 7532053

Hard to believe the radiator fan could consume so much power as to cause the alternator to pull the engine down that much to keep the alternator turning. Besides the engine controller/E-Gas control module would attempt to keep the rpm constant. This to me at least puts even more suspicion on the engine controller or the E-Gas control module. The latter more easy to replace, test, and much cheaper (I think) to replace should it prove to need it. You can of course replace the engine controller but as with most Porsche components you want to make darn sure the diagnosis is spot on and replacement is absolutely necessary and of course some WAG by a 'net expert, even a well-intentioned one is not such a diagnosis. My 02 Boxster's fans make noises sometimes and the idle's just fine, constant with no dips or falls. (Only barely related: In fact I found the other day the driver's side fan not running. About the same time I noticed this the coolant tank decided to split. Both fixed now. Fan problem was a low speed resistor. The coolant tank of course had to be replaced.) I don't recommend it but you could disconnect the fan electrically from the car's wiring harness to of course not only disable it but eliminate the possibility (as slim as I think it is) that the fan is somehow causing the electrical load on the alternator to cause the rpms to drop/dip like they do. Now a Porsche tech with the PST2 can command the fans to come on low or high speed and independently of each other and the engine running. If the fan causing any problems with excessive power consumption doing this and monitoriing the load on the battery should make this evident. Let's see what the throttle body/control module swap out produces. Sincerely, Macster.

there is a part "electronic throttle control" located under the dash in the driver side,, what does this do? could it be related to the symptom?

after all,, issue solved by replacing the fuel tank breather valve,, it wasn't releasing air when pressure builds up in the tank,, which was causing instant fuel cut-offs,, the whole thing cost $200 @ the dealer,,

Interesting outcome, can you send the part numbers they replaced. Mine kind of does the same thing, just not nearly as bad as yours did. Thanks Rick 99 996C4 87 944S

Quotes source post: 7583937

Hey do you know, where is that valve ? and what it looks like ? I think I isolated my problem of start in the morning. It seems the engine burn the fuel that is in the gas line, but then starves. And after waiting like 30 minutes it then works just fine. I assume that valve problem would make sense as it might be clogged and release the pressure once I wait long enough. I'd love to check it out if I knew where it is located could it be the thing located in the top left corner of this picture, you can barely see it , but there is a breather valve under there. last thing, as you can see my gas pump 'screw' is glue down... is that a normal thing to do to secure it ?

Quotes source post: 7583937

Just as a follow up to this, there is a detailed DIY for this replacement procedure over on Renntech...

Quotes source post: 8211793

Hi TomF Hi searched the link of this DIY but didn't found anything... Thanks for your help. jLaurent

Quotes source post: 7583937

Hi KJ I'm owner of a MY '99 C4 and I have to face with an idle issue for 3 or 4 months. I tried to understand myself, I let the car to a tech and no result. I thought to an air leak nearby the air intake collector when I just read your thread :-)! Could you please telling me the ref of this breather valve? I do think I have the same problem as yours. Thanks a lot for your response. jLaurent