I'm hoping someone here can save me a bit of money helping diagnose this problem with me.

Let me give you a brief history of the engine. I blew a hole in the short block in Dec '09. I had it into the dealer every month with a low oil problem but they couldn't find a thing. Well, one day, no warning (no low oil light), it blew. 6500$ later out of pocket I had a new short block and things were running fine. Twice in a year I got a blinking CEL and misfires but thought little of it as they seemed a bit too intermittent. Chalked it up to something weird. Then it started happening a lot more sometime in December.

Now it doesn't seem to stop. But how and when it happens seems to have /somewhat/ of a pattern.

It's always on cylinder 4.
It /seems/ to happen most when cold. It has happened at idle a few times but not always.
It mostly seems to happen between 2500-3000 rpms and is really bad (stays on a lot longer) in 6th gear. When I drop it into 5th and the rpms jump over 3-3.1krpms it seems to stop misfiring for most of the trip.

Last week oddly enough it didn't happen for a few days though, and of course, it was when i took it to the german auto mechanic here (won't take it to the dealer as they were horribly unjust in the engine repair). He did get it to happen once but all he did was repeat the same things I did. Which were:

Compression Test: 150psi accross the board he confirmed everything looked perfect ruling out rings and valves.
Swapped Fuel Injector, Plug, Coil with Cylinder 3: The misfire remained on cylinder 4.
Tried running it without the MAF, horrible performance but initially the misfires were less...then they ultimately came back in abundance. Reconnected, no change.

The mechanic is wanting to look at the intake manifold next which I'm tempted to just do myself, but I don't want to jump into it if I'm chasing a red herring. I was thinking maybe the wiring harnesses to either fuel injectors or coil but the misfire seems related to driving behavior and so i can't see that being the issue.

Any guidance would be greatly appreciated. I'll attach a CSV file which is a dump I took from VagCom where you can see the misfires and how they seem to relate to rpm's (generally) if you think it would be of any help. Already spent 600 dollars having someone diagnose it. I'm trying to really cut my costs if I can find the problem myself.

 

I beleive there is now a reflash for the 3.0L ECM for misfires, similar to the old flash for the 2.7 and 1.8 engines. If you haven't had that done yet i would have the dealer upload the new software before tearing into the motor, because you might end up just pulling your hair and not finding anything.

 

3.0L engines are the devil. Cylinders 4 and 5 are prone to coolant seeping thru the cylinder walls

 

Thanks guys for the feedback. A couple questions:

AudiTek: It seems Audi's are the devil. They lose every test comparison for performance and luxury against Mercedes and BMW and are always in need for expensive repairs. It's my painful addiction though cause I really love the cars.

Anyway, how can I diagnose this being the problem? What proof can I look for? Is this due to porosity in aluminum or is it a gasket leak? A compression test wouldn't have found this as a problem? What is generally the solution to this?

Could I perhaps find traces of coolant in the cylinders if I remove the plugs and try to siphon out whatever liquid may be there? This is intriguing because maybe 3 times out of the many many dozens of times the misfires occured, I got a misfire code on cylinder 5 also.

auditech79: I have seen your posts on so many misfire threads and ran across this idea from you before. If your suspicion is that it could be the ECM, can you tell me how I would know what version of software is on my ECM using Vag-Com? Maybe I've already got the latest? I'd like to be sure I need an update before going back to the dealership. I'm thinking maybe they did it when I got the shortblock rebuilt?

Thanks again for your suggestions.

 

Audi's aren't the devil, you've obviously haven't owned a mercedes benz LOL. I owned an old S500....lets not get into that.
Anyways, i wasn't aware of this so called cylinder wall coolant leakage when i worked at the dealer, it would seem to me that if this were a problem with all 3.0L engines it would have to be a TSB repair at minimum, or at least it would be a design flaw that audi should be responsible for fixing. Never the less, you should pull the spark plugs on the misfiring cylinders to see if there is any coolant in there.
As for the flash, i don't have the update software number. I was told recently by a 3.0L A6 owner that they just recently made the flash and it fixed his random misfire that happened on cylinder #4. He had torn the engine apart to the bear block and found nothing wrong with it, the flash eventually fixed it, very similar to your issue. I would assume since the flash is so new it hasn't been done to your ECM.
I am worried about this coolant cylinder wall issue, so please let us know if you DO find any coolant, because that would be very concerning and audi should be looking at a class action in the future if it is a design flaw.

 

So I finally went to the dealer after not having any of my calls returned. They said there was no ECM update for my VIN, and also did a slightly more generic search for my model engine. They said the ECM code for the A6 and A4 are different even though the engine's are both 3.0's. Little dissappointing as it would have been a great quick fix.

The tec had heard of coolant leaking into the cylinder's however (in fact he told me he had just fixed one where he pulled the plug pressurized the cylinder and coolant started coming out). I'll pull the plug and check the cylinder ASAP to check for coolant. I'm a little concerned that because it's intermittent in nature it may not be there all the time but it can't hurt to check.

I'm going to try and take the intake manifold off just to make sure things seem ok under there. Maybe a gasket leak around 4 there?

After that the only other thing I can think of that was recommended was to check for worn cams...not really sure how involved that is but we'll see.

Thanks again and I'll keep you posted. I see so many misfire posts with no resolution...let's hope I can and that the answer was "Traded it in for an SL500"

 

So they only have a flash for the A6 and not the A4? Thats just stupid.....i forgot were talking about germans for a moment...
That is very concerning that the tech has heard about the coolant issue, its now the 5th person thats heard/seen the issue. This can't be good and it most definately isn't what i would have expected from audi's engineering department, i hope thats not whats happening to you.

 

So here's the current status. I ended up pulling off the intake manifold, and after breaking a few things (crank case ventilation hose which evidently breaks all the time), I got it of and could take a look at it and the injection ports for each cylinder. They all looked fine with the exception of a piece of plastic I found in port 6...not the problem cylinder :-(. None the less it was very odd but had no idea how it got there or what it was.

After asking an audi tech from the german repair place he had told me to look at the variable length drive shaft in the intake manifold. I took it out and to my surprise it was broken. Basically, it was sheared in half from the point of contact to the outside mechanism that spins it. Now, on one hand, I'm kinda happy that I found something definitely broken because that could obviously be causing a problem and it would be coincident with the idea that it only happens at certain RPMs and under load and driving style etc (I believe that thing spins based on load conditions and other factors to give the best performance).

On the other hand, I'm a little nervous because I don't see why that would cause a problem on just cylinder 4... I have to fix this, it's clearly a problem, but will it fix the misfire? I don't know. I have pics I've posted but when the parts come in tomorrow and I get it back together (hopefully) I'll let you know what happens. I also purchased the large radiator fan and the new crank case ventilation pump/hose assembly. 470$ for the intake drive mechanism (would have been 1090$ for the whole intake manifold...a piece of plastic), 300$ for the fan, 170$ for the pump/hose. I REALLY hope this is it.

After showing the tech he didn't think there would be much likely hood of bad valves or a blown head gasket (oil looked ok), but there was water condensation in the crank case ventilation hose and I'm hoping it truly is just condensation (i heard it to be the case from multiple techs).

Because I can only upload 5 at a time, I'll post the rest of the pics in another post. But for this one:
1833: Inside crankcase ventilation hose, a frothy mix of water and oil it seems...opinions?
1850: Cylinder 1
1851: Cylinder 2
1856: Cylinder 3
1857: Cylinder 4

 

1858: Cylinder 5
1863: Cylinder 6
1877: The broken drive train that inserts into the front of the intake manifold. This I believe should be one unit. You can see it is missing a few rings (you can see two of them sheared off above it. And the spring that fits between the front vacuum motor and the tube has gotten all chewed up and found little bits of of the metal in the gunk inside the manifold.
1881: A little disconcerting, what looks like olive oil is water in the little compartment below the intake manifold (this was under the black cover under the manifold).

 

1890: The car as it sits till saturday waiting for the drive and fan. Gonna have to get a new dip stick, stupid thing is so curvy it kinks and wears all the time. Anyone know of a metal version that won't?

1891: This is the little piece of plastic I found just laying there, lodged in the intake port of cylinder 6 pictured in the previous post!

Let me know what you guys think...that intake drive could be the problem I'm hoping. It is clearly broken...but why cylinder 4 only. The heads seem ok, I drained the oil, no trace of coolant or murky white stuff.

If this doesn't fix it I'm at a loss. I pulled the plug on 4 and looked as best as I could inside but couldn't find any evidence of coolant or liquid. However, they head of the piston was shiny and clean while 5 and 6 had the typical burnt look from combustion...