Misfire Diagnosis

B6/7 S4/RS4
#1

I’m back…

The used chain tensioner I had swapped in went out very quickly. Also, my fuel pump randomly stopped working leaving me stranded a few times. If I cycle the key it would eventually work. I replaced the pump.

I swapped out my Bank 2 chain tensioner and upon start up I have constant misfires on Cly 1&3 as well as the random misfire code. It idles extremely rough and eventually stalls. I can watch them rack up in measuring block 15. I touched nothing on that side of the engine.

I’ve done the following;

Swapped around plugs, coils, and injectors
Compression test looks okay - 120ish cold
Unplugged MAF
Unplugged Front O2’s
Wires are all good from coils and injectors to ECU

I hooked up a gauge to test fuel pressure and it was 0. So my attention quickly turned to this. Not sure if it is the cause for my misfires but I need fuel pressure before I can move on.

The pump is new (Delphi from RockAuto but came with Audi rings and VDO stamped on it so I lucked out there).
Fuel pump relay is new
Fuel filter is about a year old (KYG Audi)

Shot all the wires and all I found was the ECU not grounding the relay 100% of the time. I’ve seen many posts about this grounding issue but nobody has followed up with a fix. As far as I know the pump should prime every time the ignition is cycled.

I pulled the cover off the relay to manually trigger the pump. I started testing pressure from the pump on and discovered that the fuel pressure regulator was dumping all pressure back into the tank from the forward center fuel line on the filter. I did some digging and found nothing like this mentioned anywhere. A new filter is on order and I’ll be able to continue troubleshooting in a couple days.

I’ll report back with any progress. I think I’m on the right track but feel free to keep me between the lines.

#2

Following to see what you find out!

#3

Get a pressure gauge at the rail start there.

This is what can happen even with a brand new fuel filter. Keep in mind the fuel filter has the regulator in it.

Also keep in mind the gauge is a cheap ass hobo freight gauge just used to test pressure at the rail. It was just bought for this job so they didn’t spend big money on it.

Bad fuel pressure at rail
http://audirevolution.net/videos/106/rs4-fuel-low-pressure-test-fail.

Good pressure at the rail key on.
http://audirevolution.net/videos/111/rs4-fuel-pressure-test-good

Any ways. You can see you can have crap fuel pressure and the car will start and run. But low fuel pressure will be a big issue and cause missiffes.

#4

I now have fuel pressure. Guess the new pump smoked the old filter, who knows.

Unfortunately, I still have misfires on 1, 3, & 4. the plugs are soaked and smell of fuel so it’s Ignition. No breaks in the wires to the ecu so I’m not sure where to go from here. It ran fine before I swapped the bank 2 tensioner. Any sensors I could have damaged cutting ignition? or is my ecu smoked? I have been trying to wrap my head around swapping ecu’s but can’t find any clear instructions. There’s little talk about the immobilizer portion.

#5

I have noticed my throttle body sounds like it’s smacking at idle. I have no codes for it and done an adaptation. No change. I also tried looking at my fuel trim and I’m getting all zeros. Seems strange to me. I know I’ve been clearing codes like crazy but I assumed it would read after a few minutes but it’s not. Another red flag is my MAF sensor is reading about 10gps at idle. I’m pretty sure that’s over double what it should. I cleaned it a few months ago and it ran fine after. So has my TB or MAF sensor gone out but not bad enough to throw a code?

#6

Sounds like you have a lot going on. It might make sense to stick an inspection camera down the inlet past the MAF to the throttle body and see if you can see it moving.

Also block 001 will give you instant fuel trim adjustments after the car is out of the warm up phase. LTFT data can take a while to show up.

The ECU has the drivers for the coil packs so if the ECU is bad or has a bad driver the coil packs wouldn’t get a good spark.

If you have a JHM tune you can send the ECU to JHM they will clone your ecu to a new one.

#7

Sent a camera in the vacuum elbow hole and the throttle body flap is fluttering. Took the intake tube off and the flap has some play.

It idles terrible so I only let it run for a couple minutes. Still reading zeros for trim data in block 001. MAF is showing flow from 9-12.

I’ve got a new MAF sensor coming today and a used throttle body showing up tomorrow.

#8

No change with a new MAF sensor. Going to pull TB off to give it a look.

#9

Yeah the MAF is reading high due to the throttle body being open too far from commanded. that in turn is causing the cars fuel calculation to be out of step. The car corrects but doing so will cause the car to run rough.

I didn’t go back and re read but did you see if the misfires went away if you unplugged the MAF