1993 VW Corrado VR6 misfire

Hi everyone,

so this problem has been going on for quite a while, and it's getting worse, there's a misfire in the lower RPM range. I've replaced the plugs, wires, cap, rotor, cleaned the ISV. Tested fuel pressure, coil resistance, mass airflow. None of the sensor readings on VAG-COM jump out as obviously wrong. A reputable VW mechanic in seattle had the car for 3 weeks and couldn't find anything wrong, so this is really driving me nuts at this point.

car: 1993 Vr6 Corrado 3.0L VSR, OBD1, not chipped, EGR delete. 30k-ish miles on the engine since a rebuild.
symptoms:

• When warm, at 1.5k to 3k RPM, it is obviously misfiring, driving is super sluggish in the low RPM range. when i hook a vacuum gauge up to it, the needle bounces all over in that low RPM range.
• when cold, if you rev above 3000rpm, and then let off the gas, it dies and seems to have flooded itself and will take quite a bit of cranking to restart.
• a small amount of white smoke in the exhaust at any temp, smells very rich, though o2 sensor says everything is fine
• Idle is also recently being a problem - if you put the clutch in at higher RPM and take your foot off the gas, the car will sometimes die, but restart quickly when you crank it.

Here is an album of waveforms. The first 6 are the voltage and amperage of each injector. The last is the inductance on the primary ignition.

This primary ignition waveform doesn't make a ton of sense to me; maybe my probe is garbage?


I've been watching a lot of the scannerdanner videos on youtube, and they are an amazing resource, and I've learned a lot, but haven't yet found the problem.

Anyone have any ideas on what I should try next?

Try a cylinder leak down test?

I once had a problem similar to yours. Vehicle will misfire at idle n low rpm but run smooth at higher rpms. Problem was a broken valve spring.
A cylinder leak test should point you in the correct direction, whether its mechanical or electrical.

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I agree that engine mechanical should be checked before proceeding on a rebuilt engine especially since your vacation gauge is bouncing a compression test and a leak down test is in order and you could also check running compression if you don't have a leak down tester

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Cool, I'll try a running compression test tomorrow and post the results. Thanks for the replies.

Alright, so these numbers seem fairly low. I tried another gauge and it was reading about 8psi higher than the one i measured with all across the board, so that might have something to do with it. Either way, nothing here jumps out at me.



I don't have a leakdown tester, though I could maybe borrow one, as this test is pretty inconclusive.

I also took some pictures of the spark plugs while I had them out. Seem fine.

There aren't long term and short term fuel trims like on a OBDII car, but there are adaption values, which don't seem far out.


It's worth mentioning that the car has aftermarket cams, but if a valve spring was shot, I feel like it would show up in a compression test?

Anyone have any thoughts on this data? Or thoughts on what I should try next? I'm really stumped :sick:

According to my knowledge, I always do a dry then wet cranking compression test first, then follow that by a running test and then matching the cranking values to running values, if needed.
If Im not mistaken, your idle values seem somewhat ok, the snap throttle values should be around 80% of the cranking values.

I did notice abit of oil on the plugs, or is that fuel?
According to the pics, it seems as if the plugs are not sealing as there is no wear or any sign of mating between the cylinder head and plug crush washers.
And, if you compare the threads on cyl1 to the other 5 cylinders, the plugs on the other 5 cylinders doesnt seem to be threaded in full.

You could be losing compression through the plug threads, with the engine idling you would be able to hear a compression leak though.. Have that checked out and let us know

Plug no2 has the stained appearance you sometimes see with a head gasket issue but the symptoms suggest to me the valve clearances might be too close it is a fairly old engine valve seat recession is a possible issue.

Doesnt this vehicle have a hydraulic valvetrain?

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Ash03 wrote: Doesnt this vehicle have a hydraulic valvetrain?

Yes it does. This is the AAA (engine code) 2.8L Vr6 with a distributor, but rebuilt with 3.0L.

Ok, so I did a dry/wet cranking compression test. It looks alright, and the snap throttle values should be at about 120psi which they were



One thing that was interesting was that when I put the plugs back in and started it up, I saw a puff of oil getting burned off which was expected, but it was coming out from around the exhaust manifold. I'm wondering if maybe an exhaust leak is making the o2 sensor read lean. And, If it's a small leak, that may explain why the symptom only appears in the 1.5k to 3k RPM range, because the proportion of air sucked in via an exhaust leak is greater in the lower ranges.

I'm going to look for ways to test this...if anyone has any thoughts on how to do so or sees an issue with this theory, I'd love to hear them.

You would be able to hear an exhaust leak though.. Did u not spill any oil while doing the wet test?

Easiest way to check for a leak on the exhaust, get some sort of fuel hose, or garden hose around 1meter long, hold one end of the pipe at the suspected leaking area and the other end NEAR your ear, you will be able to hear air blowing if leaking. (Do not pipe the pipe into the ear as it will damage your hearing)

I am kind of stumped with this issue, if I do think of any other tests that can be done I will let u know..
If u do sort the problem out please post the cause.

Thanks for the suggestion, but, couldn't find an exhaust leak. I tried the garden hose thing and then tried putting some seafoam in it (just to get smoke to make a potential leak more visible) but couldn't hear or see a leak.

Sorry for the delayed reply, for some reason my phone does not notify when I get a reply.

After some thought, if the car is smelling rich and the o2 sensor says everything is ok, the sensor is clearly lying.

The white smoke is possibly raw fuel, Im heading towards leaky injectors, that kind of explains your wet plugs (plugs normally black on overfueling symptoms though)

I saw a video on how Paul tests for bad injectors, do u have that tool? Im not sure what its called - it pulses the injectors.

Another way u could check if injectors are leaking (not so accurate), hook up a fuel pressure gauge, let the system build pressure, leave it for some time and check if the pressure drops.

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