simonsun347

Hi all I'm new to the forum, but I have some problems on my 99' A8 4.2L quattro that needs some help with.

Basically the car is idling rough, and when cruising it's constantly misfiring on one particular cylinder- cylinder 4. Car is running pretty rough on all speed. CEL is on, ran VAGCOM and came back with bunch of codes. Don't have the codes with me now but as far as I can remember it's

multi-cylinder misfire-intermittent
cylinder 4 misfire
O2 lambda sensor
MAF sensor
ABS wheel sensor
and something else

I'm not that handy with cars myself so I have a friend of mine took a look at it. He did:
new spark plug in cylinder 4-no good
new coil pack-no good, will return to shop
swapped the stage output module-still misfiring on cylinder 4

there's oil in there, so injector should be good.
there's no apparent leak in vacuum lines, is there a good way to thoroughly test them?

I looked around and have people reporting O2 sensor and MAF sensor will solve the problem, but in theory they should not cause a misfire on a cylinder right?

Is it electrical?--how do you test if it's the ECM?

Confused---what else could it be? Any suggestions?

THanks guys.

silverd2

Not definitive, but this sure seems to point to that injector. Electrical check:

Unplugged, put Ohm meter on the poles of the injector itself...should read 12-20 ohms.

With engine running (or during a vagcom injector output test), the injector plug (from ecm) should light up an LED test lamp...light hooked up between terminal #1 on plug and engine ground.

You can also trade that injector with another cylinder to see if the misfire moves there.

MOST IMPORTANT (and some otherwise knowledgeable people have gotten this wrong): Make sure you know which cylinder is indeed the correct #4.

simonsun347

Hi silverd2, Thanks for the prompt reply. I've read literally all related threads on this forum regarding my question but doesn't seem to fit my issue.

I have the VCDS-lite freeware version, not sure if I can run vagcom injector output test....but I will give it a try once I get back from weekend trip (not on this car obviously).

I thought Cylinder #4 is the passenger side closet to back of engine bay? (top left if looking standing in front of car?)

silverd2

Yep, correct cylinder.

Don't think the free VCDS will do the output test (that's what I have...useful for reading codes and setting soft coding, but lacking advanced features), so I'd think about using a standard volt ohm meter or switch the injector with another cylinder.

Misfire codes are not specific and can be caused by coil, plug, injector, stuck valve, airleak at injector seal, etc... Just sounds like you eliminated the spark part of the equation.

The injector would be next on my list.

simonsun347

ok. I will try to diagnose the injector part. Worse thought is broken valve...will prob mean end of life for engine unless spend $$$$ at a shop to fix it =(

silverd2

Yep, I was tryin not to depress you by suggesting a compression test, but that may be in order if there doesn't seem to be a problem with that injector.

simonsun347

UPDATE: FIXED!

so my friend took a look at the injector and it all looks fine, so he instead started to trace back from cylinder 4's coil pack on suspect that it's electrical. Traced back the stage output wire, all checks out fine, stage output module itself is almost brand new, pulled out the connector that's going into upper stage module ---voila! one of the connector pin is wide loose, pulled right off not even stuck in there (which prob explains why only cylinder 4 is misfiring). so he pushed it in, reconnected the pin, fired the car, and magic! cylinder 4 is no longer misfiring! engine sounds so much better now. Apparently the ECM fires the signal fine but never made it to stage which never made it to coil and thus did not spark.

Also cleaned the MAF sensor with carb cleaner, was getting dirty due to previous owner installing CAI with oil filter i think.

while I still have a minor water leak (maybe rad or AC) which causes engine coolant to drop like crazy, a O2 sensor issue, ABS wheel sensor and whatnot, the CEL light is gone and it actually drives like a v8! what a relief to hear it's not mechanical!