Misfire diagnosis

B6/7 S4/RS4
#1

So yesterday managed to do a few jobs on the S4, got inlet flapper actuator kit put on, valley pan gasket, oil filter o-rings, check valves and oil jet and pcv valve and sealed the rocker cover leak up

With the inlet off I looked down and all the valves were very clean, no soot or bulid up
All set up and running spot on

Now the problem is, I bought the car with an intermittent misfire, stupid I know but hey, the mistake is made now.
I’ve got misfire codes on cyclinders 1 and 3, last night I swapped the plugs and coil packs round yet still the same faults on 1 and 3
Cleared them and it was fine, was actually the fastest/quickest it’s been.

This morning the idle went a bit rough then the cel light came on, didn’t flash as it was solid. It still drives spot on.
Took it to my mech to check the codes and again 1 and 3 cylinders misfire

Kinda ruled out the chain tensioners for now as they’re quiet and no codes have come up regarding it.

My thinking at the min is pointing to the injector loom, wire connector to the injectors, the injectors themselves or a cracked seal on them, also read that the pre-cat/cats could be blocked, which could be possible as the previous owner hardly drove it and when I got it a lot of carbon came out of the back after some hard driving, this has stopped now

Does anyone have any ideas?

#2

Another point to make, the fault only comes up when the car is idling, when driving nothing appears and it still pulls hard but if left to idle it can sometimes start to shake a bit.

#3

Started this morning, air temp 4 deg C, very slight rattle for 1sec max, same as has always been. Car idled fine with zero missing. After a few mins driving and a bit of heat into the engine the misfire felt like it was coming back very slightly but only on idle, no light on at all though and pulled like it should.

Things to check/do on it this week;
Run a load of injector cleaner/petrol cleaner through it as pretty sure the previous owner used cheap petrol/gas (I only use 99ron in it)
Swap out 1/2 injector connectors to see if they’ve gone loose/bad

After that it’ll be to take out and swap/service/clean the injectors and see what happens
Check injector and coil pack look for splits in wires

Read online that someone else had a similar problem and it turned out to be a faulty ECU so that could be another option

#4

What do your fuel trims look like? If your injectors are not flowing properly you should see positive long term fuel trim numbers in one of the fields on group 32. Group 33 shows your short term fuel trim numbers.

#5

I have no idea, I’ll see if I can use my firend’s vag-com and check them

#6

Check your 02 sensors. I had a slight misfire at idle that would go away after a minute of sputtering at idle. I dont know if one of my 02’s were loose or dirty but we unplugged them and then plugged them back in and ever since the car has been fine.

#7

Would I be right in thinking the O2 sensor plugs are the green and brown plugs that are attactched to the bulk head?

Got to work this morning, about a 10-15min drive, and it was idling fine… I can’t work it out lol

#8

Sounds like them.

This is from a 2.7 but you get the idea

http://s882.photobucket.com/user/01A6Turbo/media/O2%20Replacement/6-O2-Connectors-Pass-Side.jpg.html?t=1265249181

Again i was similar. I would start the car up, it would idle rough until it began to warm up. Once warm the car would not idle rough again until i turned the car off and back on. I ordered a new 02 sensor thinking it was the issue. But all i needed to do was disconnect, clean and reconnect.