sudden power loss

Hello Guys, My 1999 996 C4 suddenly lost power yesterday on my way home from work. I was on the motorway, around 65mph, suddenly no power - no response to accelerator. No lights that I remember, but then I was getting to a safe place. Car is transported and pushed into back into my garage. When I try to start, just a glimmer, but immediately stalls. Recently idle has been dodgy when up to temperature, but that lately improved, if that could be a clue. I've had a search of the forums, but can't find how I should start to diagnose. I have a Foxwell Porsche specific scanner. Can anyone advise, so I don't waste too much time. Thanks in advance.

Some more info, after using the scanner: DME faults P0130 O2 sensor ahead of cat bank 1 P1115 O2 sensor heater ahead of cat bank 1 P1119 O2 sensor heater ahead of cat bank 2 P0154 O2 sensor ahead of cat bank 1 Also, on the dash, the oil level check is counting down from a very large number 58:41 - see pic.

Ignore the oil pressure counter. It does dump things with the motor not running. Looking at your faults i would check 3 things. 1. Pull the air box and do a through inspection of all the wiring of the o2 harness and main harness. Mice love to eat it. 2. Pull the front o2 sensors and see if she runs. It almost looks like the cats r cloged because those codes should not prevent the car from running. 3. Check your fuel pressure. Testers r free to borrow from most local parts stores. Actually do this 2nd as its easier then pulling the o2 sensors U did say motoway....not that it matters but r u accross the pond? Your car has a throttle cable so it should be a little easier to diagnose. Other numbers we may need r short term fuel and cam deviations if u have a good scanner.

Hello De Jeeper. Yes, I am in the centre of civilisation known as England. Thanks for taking the time to respond. The car is a C4, so fly by wire. My scanner shows me pedal positions moving, and throttle potentiometers moving with pedal too. I can't hear any moving, but I presume that means the throttle is opening ok. I could check if I removed the air box. I wonder if the fuel pressure can be accessed by my scanner. I engaged some fuel pump relay and heard some gurgling from the front. Let me give you some 'actual values' - I have no idea what they mean. O2 sensing bank 1, 1.00 ditto for bank 2 O2 sensor voltage ahead of cat bank 1, 0.4V ditto bank 2, 0.45V Cat temperature 50 degrees C (it is around 7C today) O2 senor resistance ahead of cat bank 1, 0.0 Ohms O2 sensor heat ahead of cat bank 1, 4.37V ditto bank 2, 4.36V Whatever has happened has caused DME codes on faults on both banks - what could be common? DME relay? (Where is that on a cabriolet, I wonder?) Do the readings above look plausible. Thanks so far...

I know fuel pumps often fail on restarts, but maybe that�s the first thing I would check here (fuel pressure). Sometimes the fuel pump issue isn�t a locked rotor, but instead, the fuel line comes off the pump.

Thanks Mike. Would such a fuel pressure loss cause those DME codes? I'm fairly sure those codes were not present when I scanned a little while ago chasing my low idle problem.

Low fuel pressure can cause a lean condition which in turn can set an o2 code. Usually the heated o2 code is set at startup so its a little weird. I did have a issue with an intermintant o2 heater curcuit that cause an sai pump fault but thats not whats happening here. Fuel pressure itself is not monitored and must be tested with with an external gauge. There is a schrader valve in the fuel rail that is capped. Thats where u hose screws in. We can rent/loan these kits at most parts stores for free. https://shop.advanceautoparts.com/p/...ce=1&gclsrc=ds

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I ran out of gas last year and had all sorts of codes - misfires and such, and then a check engine light stayed on for a good day after. I don�t think they were cat codes though.

Thanks guys. Yesterday I checked that the throttle was opening - it did. I am wondering too, if I ran effectively out of fuel. The gauge says 1/2. I disconnected - waited - reconnected the battery and it still read half. Is it true that the level is predicted below a certain point. Re the fuel pressure. What should the pressure be? I can see the fuel filter, where is the schrader in relation to that please? I mentioned that when I enabled the final? fuel relay via scanner, I heard a load of gurgling from the front. Is that normal? I have read of problems with the in-tank fuel lines perishing. I had in mind to add a few additional litres of fuel, in case the tank is actually empty or a broken tube only picks up above a certain level. I'm picking up on your comment that O2 sensor codes should not prevent the car from starting. If I sprayed some 'engine start' would that prove anything fuel wise. Thanks again for comments so far.

If it fires in ether then its a fuel issue. Orange cap is the schreader valve. https://rennlist.com/forums/996-foru...-pressure.html

The extra couple of litres of fuel has not made a difference. I have a can of engine start, which I'll try tomorrow. Then I'll look for a way to test the fuel pressure; for that I'll probably have to buy something..

An update today. When I topped up the fuel yesterday, I tried to start it two or three times - is that enough for the fuel to get to the right places if I had run out? Today with a spray or two of 'engine start', it started, and to my surprise continued to run unaided. Could it be that the extra fuel did fix it, but I didn't try for long enough? I'll try removing a more fuel than I put in, and see if it stops again. Is there a good way to remove fuel? I have a very thin pipe and diaphragm pump which I have used successfully in the past - but it is slow. Do 996 fuel gauges go wrong? Could the fuel tank pipe perishing thing cause it not to supply fuel below a certain level? What I don't want is an intermittent problem.

Its a simple test....your fuel pump is failing or u have a bad in tank hose. If u dont want intermintant failures find the root cause and fix it. I highly doubt its the gauge unless u added about 18 gal of fuel u were not empty. These pumps work well for empting the tank. Amazon Amazon

The tank calibration is a thing for sure. There is a procedure for it, and it�s not super easy. One thing you could do is fill the tank completely, then disconnect the battery for 5-10 mins while shorting the cables cables together (no battery). The downside of doing this right now is that if you fill the tank, now the tank needs to be drained to fix any fuel pump issue. Then other thing to think about is if your car has an external fuel filter or not. My 99 C2 has one outside the tank on the car, but many cars have a filter built-in to the fuel pump inside the tank. If yours is external, you could replace the filter and see how much junk or fuel comes out. But I suspect C4S might have the filter in the tank.

Hi Mike, my fuel filter is external to the tank. I can see it near the coolant expansion tank. I'm struggling to find a video to show me how to get to the fuel pressure test point. I know De Jeefer provided a picture, but I don't know how to access it. My fuel removal via filler with thin pipe failed to pick up any fuel. Guys, do let me know if you think I'm wasting my time with my fuel level hypothesis. If not I may try draining 5 litres via the fuel filter. p.s. For the C4 with its front transfer box, is the fuel tank access under the battery?

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Ya, try to avoid stuffing anything down the tank filler neck. Lots of delicate stuff in there to get damaged. Do what DJ suggested first. IMHO,,, do not waste your time on 'what ifs'.. At this point, doing anything other than trying to locate the test port and put your advanced auto parts loaner test gauge on it is a waste of your time imo. Lets try and eliminate the fuel pump/fuel delivery first. Go from there... troubleshoot with a logical step by step process of elimination. JMO

Thanks Alcohol, I'll take your advice. We don't have those rental/loan shops over here in the UK - as far as I know. I might be able to borrow something from work, or just buy one for �20 or so. Re the test port - via the engine compartment, I presume, what if anything needs to be removed to get to it. The car is a cabriolet, if that makes a difference.

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If you can't get a fuel pressure test gauge, a refrigeration/AC manifold gauge set works good to test fuel pressure, just can't use the car134quick connect adapters. Have to use regular spin on refrigeration gauge hose ends with schrader depressor, used with almost all refrigerants r12/r22/r410/r402/r407/etc. Use low side gauge, hose should go right on the schrader valve test port... I've also got a cab 996.1 & 996.2, don't need to remove anything to access FPTP. As facing the engine from back of car, look to the right bank between the engine bay body and engine. Look a little farther in towards the right side front of engine, you should see the FuelPressurRegulator on the fuel rail hanging there on side of engine. Look a few inches down the fuel rail and you'll see the cap on the schrader valve FP test port. Clean off any dirt/debris on cap with squirt of brake clean and blowgun/rag, then unscrew the cap and screw on the hose. If using refrig gauge set, make sure manifold gauge low & hi side handles are shut tight. Turn on key to start position but don't start car. Read max fuel pressure. If pressure is within specs,, great. Next, start car and do the same reading watching gauge, if still good, rev engine too 2500rpm or so and keep at a steady rpm, read pressure. If all the above FP pressure are in spec, 1 last test to rule out a FP problem. Do a fuel volume test if all pressures are in spec. That will rule out the fuel pump/fuel delivery system, or not. Report back what you find.... Or ask a trusted shop to test FP for you if you don't feel confident doing this... JALFYI, if you want to remove fuel from tank, put a long hose on FuelPressureTestPort fitting, and other end of hose in 5 gallon gas can. in a few minutes it will be half full.

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I can almost guarantee that there is a burst hose inside the fuel tank. Access to the fuel pump is by removing the plastic "hat" located in the frunk. Then disconnect three fuel lines and one electrical fitting. Unscrew the big plastic nut and lift out the white piece enough to unplug the electrical connection and the duplex fuel lines that are connected on the bottom. Remove the rubber gasket. Reach inside the fuel tank and unplug the two jet pumps, two connections per jet pump. The jet pumps can stay inside the tank. Then reach inside the tank and disconnect the black corrugated hose from the fuel filler pipe - it simply pulls off. Turn the fuel pump 1/8 turn counter clockwise and pull the whole mess out of the tank. You are going to find a short black rubber hose that has a split in it. Replace this with SUBMERSIBLE fuel hose. You mechanic should be able to do this in an hour or so for about $10 in parts. The Porsche dealer will want to replace the entire fuel pump for about $600.

Is this the type of tester for the job? https://www.ebay.co.uk/itm/265214345356 So is the theory that the height position of that pipe is related to my half-tank full failure?

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Not familiar with that particular kit, but if it says it screws on a schrader test port, Ya, that kit should do it. Looks like it has plenty of adapters. You'll probably find exactly what theprf suggested in his above post about burst hoses in the tank connections to fuel pump, seen that posted here before. Probably still a good idea to check fuel pump pressure before you tear it all apart, as its a very easy/fast test, just to get a baseline. Suppose pressure could be affected by where the split in the line is located and tank fuel level.? Bottom line, it still has to come apart regardless of FP. Would like to hear theprf's opinion on 'once you're in there' if he would recommend changing out a 25 year old pump and all hoses,? or just do the fuel line repair and call it a day...? He has experience with the same tank in your c4 and knows the best way to proceed for sure... jmo

I would not bother changing the fuel pump unless there are issues with fueling at maximum engine load. In other words, running lean at the torque peak. The accordion hoses seem to last forever. I think, perhaps, the latest fuel pump revision may have all accordion hoses. If so that may be a valid reason to spend all the extra money to change it out. And, yes, the location of the split and how much fuel in the tank can let the engine run slightly. I have had this line split at least 3 times. Once the car would idle but any throttle made the engine die immediately. The second time it would drive up to around 35 mph but no faster. All the other times it was running flawlessly then suddenly quit and could not be restarted. I have replaced this fuel line on the side of the highway and twice at the track.

All this ^^^^ is why i said check the pressure. It will tell u if u have to open up the tank. In a c4 its very east yo get to the pump and sender. My gt3 pump burst the hose too because it has a secondary pick up and its a similar design as a turbo pump. On the new pump that hose is accordian vinyl. If u do have a burst hose u have to get "submersable" hose. Standard line will not last and even the correct stuff fails eventually. Amazon does sell the accordian vinyl for cheap and i think its 8mm but u will want to verify. Its what i used to repair my old pump and its now my spare. My pump...died on track and would not restart.

Thanks for the very clear explanation of where the test point is - I have now located it: As you can see, it looks fairly nasty in there. The tester is on order too, should arrive in a day or so. Do I read that I need a replacement cap? Can I find those on ebay?

Cap is reusable no need to buy another.

My tester just arrived. It tells me 45PSI with the ignition on, and 47PSI with the engine running. I don't have much time to do more tonight, but tomorrow I thought I would... Use the bleed button and tube on the gauge to bleed out 5 litres of fuel - more than I topped up with. Then see what the gauge says, and if it still starts. Let me know what you think guys,

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Good to know 45-47psi... Porsche 996 Essential Companion book gives 3.8 bar +/- 0.2 bar or 55psi +/- 3psi. regardless, at this point I'd do exactly as theprf suggested. Time to get ur hand wet....

Hello again guys, I need some more advice... Today I thought I would bleed out some fuel (via the gauge) to see if the pressure went bad with less fuel in the tank. When I switched in the ignition there was no pressure, actually maybe 10psi. Thinking I hadn't screwed the Schrader in enough - it is quite stiff - I switched off, then tightened, then switched on again. I saw pressure - the 45-47 that I had seen previously. I then pressed the bleed button and pressure went to zero, and stayed there when I released the bleed. I switched the ignition on and off again a few times, but no pressure. I then used my scanner to activate 'Fuel pump relay final stage' With that I heard the pump run and I now see 50psi on the gauge - a little higher. What is going on? Should the pump run all the time? Does it switch off when up to pressure? Is there something wrong with that pressure switch - if there is one? More info... I tried to start the car with the zero pressure and it started, and the pressure went up to the 45-47. With it running I could bleed fuel out and the gauge would return to pressure. When I switched off, bled the pressure to zero, and turned ignition on again - I saw the usual pressure. When I bled that, it did not return. So I guess the pump runs for a short time when ignition on, but does not keep the pump running - makes sense I suppose. I'll get on with my tank level thing using the scanner.

OK. So with the pump relay activated by the scanner, I bled fuel into a can. After what must be a few litres, there was a whining from the pump, and the pressure started fluctuating - sometimes down to 20psi. So I think that proves that, as you say, I need to get my hands into the tank. I could carry on bleeding and see if pressure goes to zero. I could pour the fuel back into the tank and see if it recovers. However, I think I'll just get on with it.

Well. The talk access is not under the battery tray, it is a bit forward under the cover that hides the 4WD stuff. Anyway, I removed the lid and had a look inside. I see the black pipe, which looks OK, although I haven't withdrawn the whole assembly yet. What I do see is no fuel. The tank is dry. I guess that means the float assy / fuel indicator? Let me know what you think. Did I hear that someone had the float arm get caught on that mess of pipes in there?

The tank has a lower level on each side. A jet pump is installed in each lower level to suck the fuel out and place the fuel into the fuel pump. When the tank is below about 1/2 there is no fuel in the top section. When the fuel level goes below 1/2 tank the float no longer reads the fuel level. Instead the instruments guess how much fuel is left by using the fuel consumption rate. To get the fuel gauge to read the actual fuel level the tank must be at least 1/2 full so fuel is visible around the fuel pump. It is possible you are just low on fuel and the gauge is not reading. You can check this by filling the tank a little more than 1/2 way, watching the level through the opening. Then refit the white cap and fuel lines to re-test the fuel pressure. Since you have done so much disassembly I would disconnect the two jet pumps and remove the main pump to inspect the lines. You can also easily remove the jet pumps to inspect their lines. A leak anywhere in the lines will cause low fuel pressure.

Thanks theprf, your picture in one of your posts above - showing the whole pump assy out of the car, does that show these jet pumps, or do these remain in the tank? i,e, do you have a pic/diagram. I will take it all out and have a good inspect. p.s. I've seen videos of people popping off those pipes to withdraw the stuff - is it obvious which pipe goes where - or should I carefully mark them when I take them off? pps, I found this marvellous drawing from ksteen13 (from a Turbo though) https://cimg5.ibsrv.net/gimg/rennlis...94b09e13d2.jpg ..and this vid shows the pick-ups out of the car, which I found useful: You Tube

I dont understand what the issue is.....Pull the sender up, unclip the hose and heness connector, remove sender. Reach in, twist pump and remove. Its not complicated or difficult and takes less then a minute to do. Just trying to motivated u to fix your car so u can drive it.

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The fuel pump will only build pressure for a short time, enough to start the car, but will not continue to run if the engine isn�t running. This is a safety feature.

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All the AWD have the split level gas tank to clear the awd. The 2wd access is under battery tray, the AWD access is in front of battery under that dome cover The split level awd tank necessitates 2 transfer pumps, 1 in each side well of the tank. These transfer pumps feed the main pump. If you're concerned with the float level sensor not working correctly, once you take out the assembly you can plug it back in and move the arm up and down slowly while someone watches the fuel gauge. This should confirm its working ok. iirr, on awd tanks the full throw from bottom to top will only go from full to half tank. The last half tank level is computed somehow as theprf explained. Pull this apart already, don't over think this. Take pictures of hose layout. Once you have it apart it will all make sense and be a lot more straight forward. If you're worried about getting your hand wet with fuel, borrow the wife's dishwashing gloves, she'll appreciate the disinfection qualities of the fuel dipped gloves... jmo

Hello all, you will be please to know that I have got(ten) up off my thin **** and removed the unit. In-twist-out as we used to say in the war. See attached pic. I can see no damage to the pipes/hoses around the pump, nor around the two venturi pump ends.

The fuel sender is giving me variable results - more tests later.

Carefully inspect that rubber hose. That one is original. Look for cracks etc. Gently flex it to make sure it has no cracks. I had the fuel gauge float stop working. I posted about it here: https://rennlist.com/forums/996-turb...l#post18877453

Hello all, I have carefully inspected all the hoses and pipes and all look good - including the ones which go down to the 'side-tanks'. In any case, the tank is fully empty. I took a look at the fuel gauge with my resistance meter. As I moved the float, there were some odd readings along the way, but I guess the float will be bobbing around most of the time, giving an average reading. I took the float off, inspected the wiper and cleaned the contacts. There are a number of test pads on the ceramic PCB which holds the printed resistors. Here I measured 10R, 78R, 142R and 205R at the end. If anyone else has a float assy available (for a C4), maybe they could check. My PCB says VDO and has the number 493 on it. Anyway I wired it up to the car and (according to my daughter) it read from 1/2 tank to full tank as I moved the float. I have put everything back in the car now - ready to fill up and monitor the mileage vs gauge reading carefully - with a spare can of fuel in the frunk. If anyone has any other ideas, please let me know.