Rough idle after maintenance

Hi, I have a problem with a '98 996 3.4. The car is 80 000 km and runs very good before I made the maintenance. First of all, I remove the spark plugs (which were BERU models) and put the new ones (which were NGK provided by a car parts supplier). Then I tried to start the car.... and it started. The idle was very bad and the injection indicator light comes on. I stopped the ignition and check the exhaust temperature after some seconds, one was cold. First mistake, I forgot to plug one spark plug. I plugged back, then I started again to try the car which seemed to run good, in spite of the light was still on.... Then I disconnected the battery and connected it. The car runed good, and the light was off. All was good. For the next step of the maintenance, I've done the oil draining and filling, oil filter, air filter and the drive belt in the same time, without particular problem. When the airbox was removed, I switch the igntion on (but not the engine) to check the oil level at the Board Computer. The MAF was obviously also unplugged and I realized after that a new fault code was generated. After this step, I started the car, and since runs badly. The idle is rough, and the engine seem not to have all its power. I read the fault code and reset all of them, then check the fault code again, nothing. I tried to put the old plugs and the idle is better, but there' still a problem. The sympyoms are the idle, which fluctuate and is rough (and seems to be about stalling), and there is one backfire when I push the throttle pedal quickly to the floor, when the car is at the idle. In spite of them, the car runs nod bad when driving. I thought about catalysor/lambda probe, MAF (but I've read that it doensn't control during idle), idle throttle potentiometer, idle valve, broken oil filling tube... But everything goes well before, and I don't know why it could fail... Someone of this forum have an idea of this problem ?

Let's start at the beginning. What was the part number of the new plugs?

New were BKR6EQUP. But I've put the old ones since.

You might remove and clean the idle control valve and make sure the throttlebody is clean. Remember, for a few drive cycles after dsconnecting the battery, your engine will not idle smoothly until it relearns your engine's parameters. Additionally, if you are going to use NGK plugs, use the BKR6EK. The one's you got are for the 3.6.

I just had a similar problem. Mine was solved because one of the coils was not put on correctly. What was the fault or CEL code? If it was a specific cylinder, then it is surely that one of the coils needs to be put on again.

I tried yesterday to remove the control valve and inspect the throttle body. The valve seems to be good, like the throttle body. When all is plugged, ignition on and engine off, the valve make a noise and is lighty shaking, so I think it works fine. I also read that we have to drive a few times in order to "stabilize" the injection in the workshop manual, but the battery was already disconnected without any problem. For the fault code, I don't have a reader here. When checked some days ago, all the codes were results of (big) mistakes I made, and after reseting, any code comed. I probably will try another code test this week, and check again the coils (but the engine is very good at middle and high speed).

All hoses are tight all around the throttle body. seated properly?

I would start by purchasing new spark plugs. Also purchase the spark plug tube o-rings (two per tube, one inner and one outer). Replace the plugs, replace the o-rings. Make sure the coil packs are not damaged and properly seated on the spark plugs. I 've had similar issues after replacing spark plugs. I used E3 brand spark plugs (don't). Stick with the proper OEM plugs. One coil pack was not seated properly. One set of 0-rings was bad which allowed oil to foul the spark plug. After getting everything straight, the car ran nice. Improved gas mileage too.

Check all coils for proper seating and cracks, the coils are commonly cracked when re-torque-ing them. I wouldn't start with o-rings and tubes at this point until you have solved the problem. No sense in adding any more variables.

How a coil can be not seated properly, when tightened ? And when you had this kind of problem, the cars doesn't run well at idle, but good at middle and high speed ?

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Yes, this is what I said too. The coils need to be put back very carefully so they seat on each plug properly. You need to put the coil on squarely on each plug. If there is a slight angle when you put them on, you can easily get a rough idle and misfires.

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Ok thank you, I think I will check again during this week. In your case, did it generate a fault code ?

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Yes it did, but not immediately. The car worked OK at mid and high speed, but from 2000-3000 RPM I had a problem. Then the idle started to get rough.

I would do the O-rings since they only cost a couple of dollars each and with all the time and effort involved with replacing spark plugs spending a couple of extra dollars to keep the oil out seems wise.

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The old plugs were clean, and there isn't any track of oil. The problem appeared as soon as the engine was runned with the new plugs. But I agree with you about the replacement during changing the spark plugs, even I don't remember this o-rings...

The o-rings he is referring to are attached to the spark plug tubes. The tubes are inside the cylinder and are what the spark plugs slide into. You can remove the tubes by first removing the spark plug and then popping the tube out. Some people have come up with interesting methods to do this, the best way I found was to gently twist the end of a small rubber-handled screw driver into the tube and then pop the tube out. The tube has o-rings on either end. You may also have a crack in one of the tubes since they are plastic and can get cracked if the spark plugs are inserted abruptly at an angle.

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if it is good at higher RPM, it really screams coil pack to me do a code test, it will tell you which cylinder is misfiring (if any) and you can then inspect that coil more extensively for cracks or wear

I have some news. Yesterday, my father drived the car to his job because we planned to make a default code read during the afternoon. When he arrived at his job, the 996 was running very smooth. We test the quick full load at idle, there's always a noise but not the misfiring like before (I think the noise is a flap or simply the air acceleration in intake). Today, the car is still working well, the problem seems to be fixed, and we don't know exactly why... Maybe the "learning" is complete by the DME (after a bad learning with bad plugs given by a BAD SUPPLIER), but we drive the car more than 50 km and for about an hour, and it would be a long learning.

Since this issue, I changed a coil which the resistance was different from the other ones, and after 30 minutes of running, the car was running as good as before. Some news : I don't drive the car enough to keep the battery power (winter) and then, it was down when trying to start. Ok, disconnected it, pluged the charger for some hours. After connecting the battery again, the car start but my misfires problem came back ! After some minutes of running, the Check Engine Light lights up. I plugged my OBD reader which shows me 4 codes : - P-0318 - P-031E - P-031D - P-031C (not sure, just remembering) The P0318 gives a "rough road sensor". I don't have a PSM, and even, no link with the misfires. I don't know what P0318* are. So If I replaced the 0 by 1 (for specific manufacturer codes), I read P1318 for the first one, and it could be a Misfire at the 6th cylinder. But no result for the other ones. What do you think about it, and these codes ? (before removing the coils, especially the 6th).

I'd just replace the sparkplug wires. Normally a person doesn't know these parts wear out too. It is even a higher likelyhood of a faulty wire after messing with it after it sat in the same position year after year getting baked. There are a few types of spark plug wires. Plain wire, exotic carbon filled tubes and maybe a couple other varieties. Any of them flake out. If you or anyone else pulls on the wire section instead of the hard socket its asking for trouble. It's pretty much the same thing as yanking the wire if you pull only the plug then bend and stress the junction. Did I mention I believe in replacing spark plug wires every few years since they carry a very important part of ignition? Plainly if you installed correct plugs for either cold or hot weather it shouldn't be running rough. Most likely problem is the spark plug wires. If they have never been replaced...............

Ok, but there's no High Tension wire because of the "one coil per plug", so what do you mean by "spark plugs wires" ?" The harness ?

Yes the individual wires for each spark plug.

I tried 2 other softwares to read the DTC codes. Seems the yesterday one shows wrong codes as the new codes are these : P0300 - Random/multiple cylinder misfire detected P0304 - Cylinder 4 misfire P0305 - Cylinder 5 misfire P0306 - Cylinder 6 misfire They don't appear at cold idle, but when some rpm are reach (but the engine is still rough at idle). So what do you think about it ? I often had misfires problem when disconnecting/connecting the battery. Lambda sensor ? ECU (hope no...) ? Variocam coil ? Wires eaten by mouses ?

hold on there... this has been a year and a 1/2? have you driven it during this time?

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If a car has sat a while unused and the engine acts up, runs rough, and just generally misbehaves for no *apparent* reason, if there are any electrical gremlins then rodent damage is always a possibility. A thorough inspection of the car's interior spaces and under the car on top of the under body panels for any signs of rodent nesting material or droppings, etc. is called for. Another possibility, if car has sat outside, is water has gotten into the cabin or some interior space (inside the door) and is causing the symptoms. Lastly, there was one case where one cylinder bank was generating misfires and the problem turned out to be a MAF. You can disconnect the MAF at the wiring harness and run the engine, normally, and see if the misfires reappear. If they do not then the MAF becomes the likely suspect. I do not know why a failing or bad MAF would generate misfires on just one cylinder bank but at least in this one instance it did. Sincerely, Macster.

After running the car several minutes after my first problem, with the correct plugs, it disappear and the car runs perfectly after for some month. But this rough idle occurs one more time between today and my first problem. After checking the coil resistances, one were different to the others. After replacing it, as the first time, after a little road test, all was fine. The car is parked indoor, and safe. I was joking about the mouses but obviously, I visually checked plugs and harness. I didn't think that the MAF fails because I would have not only problems at the right side cylinders. I also checked the value measured and it increases as I push the throttle. The air temperature seems also to be fine. Car running, I checked the O2 sensor values : one changes during throttle actuation, other one stays at 0. I will checked futher about it, but maybe is there my problem. I will let you know. Thanks !

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mine too, i thought that the harness "clicked" and it didn't. ran the codes and fixed it.

I made some tests today. At cold, my two 02 sensors are working. But when the engine gets hot, one drop to 0 et stay (it seems that it indicates lean mixture, but somes sources gives rich mixture...). So I tried to unplugged this sensor. Starting the engine, and... absolutely the same. Plugged it again and try to unplugged the MAF, same same same... Ok, according to the behaviour of the value of the 02 sensors and the fuel trim, I think the ECU "remember" the last x minutes of running for making it's modification. I also think my two O2 sensors are OK. So I disconnect and connect the battery again and start with MAF uplugged.... the car run smoothly, even when actuating the throttle. Of course, it's not perfect, but it's better. After I plugged it again and, as before, the engine runs very rough. After connecting, the fuel trim and lambda were more and more bad, then returned to 0. When plugged, the air flow is about 6 g/s at idle (if I remember), and grows as I accelerate. Does somebody know the correct value ? The intake temperature shows -40 (or as disconnected) when ignition on but engine off and shows around the ext. temperature, not quickly fluctuating but increasing slowy as the engine heats. I think it's just a correction which measure the air warmed by the engine compartment. So the MAF seems to works properly... I must notice that since this battery disconnection, the CEL never lights again. Worse, the car never give me a fault code for the MAF or the O2 sensor unplugged... I don't know what to trust in all these measurment. That's all... for today.

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If O2 sensor voltage reading drops to 0 exhaust gas oxygen cotent is higher than ambient air oxygen content or sensor tip is too cold (below 575F (300C). (The sensor only generates a voltage if ambient air has higher oxygen content than exhaust gases and the sensor tip is above 575F.) If O2 sensor voltage reads ok then drops drastically this suggests a problem with the 02 sensor heating element. Unplugging various sensors even O2 sensors may not produce an error code right away. The engine controller require time in closed loop mode to be able to complete all tests/checks before it can flag an error. There are tests one can do to verify the O2 sensor's heating circuit is working properly and the sensor is seeing the proper voltage to its heater circuit. Sincerely, Macster.

The sensor shows voltage at engine starting, then drop to 0. The 0 voltage increases when I unplug the MAF. I tried to hold a 2500-3000 rpm during several seconds but nothing changes.

This car makes me mad. I tried another "strategy" today : - Disconnect the battery - Ensure all is plugged and in a good condition - Connect the battery - Let the ignition on one minute then off 10 secondes (adaptation only for the DME ME 7.2, even mine is M5.2 but it takes just one minute to do). - Start the car... still bad running, even stop one time by itself ! - Let it warming, when warm, carefully hold 3000 rpm for one minute - Release throttle, the can now runs smoothly. Now, the car runs fine. After a short drive test, no problem and still smooth running. Even if it's written in the workshop manual that "the engine must run for several minutes before the engine controle module can relearn the idle speed and mixture adaptations values", I don't know if it was only an "adaptation relearn" problem solved by running, for M5.2 DME, or a strange other thing. Hope it lasts, but I don't really like a problem goes without I know why... Maybe my car is just smart and fix it by itself. But really, I'm sorry for disturbing and wasting your time, just for telling then that my car now runs fine. I would prefer "to know exactly why". Hope you understand. Hope also I can help you again for anything else.

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Well, as I mentioned in an earlier post I came across a post by an owner who reported misfires on just one bank and after some work on his part he found it was the MAF at the root of the problem. I'd still not replace the MAF the first thing with one bank of misfires as the only symptom. If the symptom repeatable then disconnecting the MAF from the wiring harness and seeing if the symptoms reappear is one way to possibly eliminate the MAF. Lastly, my experience has been the amount of air being reported by a bad MAF can be ok. The intake air temperature reported by a bad MAF can be reasonable. What I have seen is the short term fuel trims occasionally go rail to rail. That is the readings go from -25% to +25% and all points in between. When the MAF goes bad. However the MAF doesn't always fail outright but just develops an intermittent fault/failure. At other times, the MAF works just fine and the short term fuel trims are reasonable. But when one observes the MAF's adaptation values are all over the place and not in some reasonable order -- by reasonable order I mean the adaptation values climb in increments from 0 to 25% and then go back down to -25% as opposed to the adaptation values take on the output of a randon number generator -- this is a pretty good sign the MAF's bad. An outright failure of a MAF could of course deliver more concrete symptoms of it being the MAF that is bad, but my limited experience with bad MAFs is the more intermittent failures are more common. Sincerely, Macster.

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Aghhhh!!!!! If it has been that long? Dump the fuel and be amazed with the difference. I've parked cars for a year, year and a half during a few desert vacations and don't even trust using a year or two years treatment of fuel stabilizer. Again if you leave a car sitting you should use fuel stabilizer and remember you are still running a chance of water getting in your fuel from condensation besides the fuel degrading from age. I didn't see an answer to ivangene's question so figured I'd mention how fuel goes bad through time. How old is the fuel? Fuel sitting in injectors and lines will gum up when evaporating from just sitting there. Yes the fuel doesn't need to of been exposed to air to gum injectors and lines.

Sounds like my lawnmower in the spring.

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I answered. Even the car isn't a daily driver, it is regulary driven. But probably not enough, you're right. The car doesn't stay not running for a long time. Max in winter, sometimes 2 or 3, maybe 4 month without being drived (sometimes, we let the engine running). That's why I have to charge the battery (or because I let the door a little opened ). Maybe the fuel isn't good as before, but when my problem disappears, it's still the same fuel. My problems always go after running everytime it occured. I couldn't never had a car in this state for many months ! Maybe my AFM is dirty, and the "factory settings" resetted by the battery disconnection make the engine bad running because of wrong mixture. Let it running makes the modification in the correction, and then it can runs right. Maybe it's another thing wich can be corrected after some minutes of running. Can a 996 be so intelligent ? Not really sure but it shows me all signs of it. Nice car.

Pretty frustrating. Could be a lot of things. I've had fuel pumps before they fail cause rough running on and off for months. Anyway the best thing to do for an electronicly controlled mixture and all is to set it up with proper fueling and spark. When trying what's easy and doesn't require major work on the engine the changing the fuel is a good option. A year old fuel is bad fuel. The electronic changes the ignition does to adjust mixtures and such would be adjusting to the max till it can't calibrate far enough with bad fuel. I think bad fuel would be prone to burn better in a hot engine. Even gummed up injectors probably function better when the gum is loosened by heating. When putting new fuel in there add some fuel system cleaner for the injectors. Like a lot of things its a combination of variables. There are fuel system flushes that can be used which I can't remember the name of. Lots of performance bike riders run into problems with fuel after winter storage. All in all I figure if your fuel was a year old and you replace it the engine will run better with just that change. Don't forget to add some injector cleaner before fueling.