ChaseB

Alright, this will be a little lengthy, but any help would be very much appreciated.

I bought a 2002 Audi A4 Quattro, 3.0 150K miles.. It had some "minor" issues when I purchased it, but, like the idiot I am, I assumed I could work my way through them.

Initially, check engine light was always on; and it would occasionally flash. I was getting codes for MAF, as well as B1, S1 o2 sensor; and misfire in all cylinders. All 6 cylinders. Rather than just immediately jump into taking multi-meter readings, I went a head and changed all four o2 sensors. Check engine light when away.....for about 5 minutes. Came back with constant flashing anytime the car was running...so I shot all the wires between every o2 sensor and the ECM, found wires associated with B1 S1 shorted together (continuity to each other) while shaking the harness. Cut open part of the harness and found some wires with exposed conductor, and fixed them. While the harness was cut open, I also found broken wires for the MAF sensor, so I repaired those as well. It had to be the fix right? NO. The check engine light became intermittent, but the car started shaking (vibrating) violently; to the point where it is uncomfortable to ride in/drive the car. BUT, still getting misfire codes for all six cylinders.
Moved onto checking for a vacuum leak, no avail. All vacuum lines pressure checked/smoke tested good. No leaks. While playing with vacuum lines with car running, I noticed both Cats GLOWING red; the fuel smell was almost unbearable. I Immediately shut off the car, and started just taking things apart. I came across the wiring harness feeding the passenger side of the car completely roasted...after a few days of cutting/splicing/replacing wires, I fixed the entire harness. Even used a multimeter to check continuity for every single wire that I repaired, every reading was good. I thought this had to be the fix, it wasn't. While the car does seem to "run" better, there is still very violent shaking/vibrating; also still getting codes for misfire in Cyl. 1, 2, 3.. at this point, I'm lost. I don't know what else could be causing this?

All cylinders were checked for compression = good.
Fuel pressure check = Good.
Clogged cat check = good.

I'm leaning more towards the ECM potentially being burnt up due to the wire harness being shorted together, so I have a new (used) one coming in.
Is there anything I need to do for the new (used) ECM to be functional in the car? If the ECM doesn't fix it, does anybody have ideas on what could be causing an entire bank to misfire?

Any help would be greatly appreciated.

Thanks,

ChaseB

turbo510

I'm impressed with that tenacious trouble-shooting. Keep in mind that a properly-running 3.0 is very smooth and doesn't misfire.

First thing: before taking the original ECM out, get a VCDS scan tool (~ $200, well-worth it) and install the software on a laptop. Scan the installed ECM for fault codes, and save that. Then, install your 'new' ECM, clear any old codes, then run the car enough to store any new codes. Any that match what your original ECM had means those are legitimate problems, not the result of ECM damage. Then let us know, unless your solution is obvious and you get it fixed right away.

By the way, the super-rich fuel condition can be the result of a failed Coolant Temp Sensor, which are available inexpensively. I'd try that even before the ECU change.

ChaseB

Turbo510,
I've been an aircraft electrician for 10 years, so understanding electric principles really have helped me get through this. I really appreciate your input. I suppose I should have mentioned I did in fact change the coolant temp sensor while I was repairing the harness. It was one of those things that was in the area I was working, so I went a head and replaced it. I do need to get VCDS, but I would need a laptop that runs windows in order for it to work (currently only have a chromebook). I have a timing belt kit coming later this week so I plan on checking the VVT solenoids while I have the front end taken apart. The ECM came in, but due to the immobilizer, it wouldn't start. I'm assuming I will need to get the new ECM programmed via VCDS? Over the last couple days there are several other things I've checked, and have found nothing else. If the VVT solenoids, and timing check along with a new timing belt doesn't fix my problem, I will be completely and utterly lost.