My 2002 3.0 Quattro is still misfiring. It's not enough to throw a code for misfire but the car will put on PO421 Bank 1 Warm up Cat efficiency below threshold after a decent amount of idling (misfire is worst on idle). The car stumbles at mid rev range slightly and feels restricted.

So far I've done the following:
Replaced all coils with latest genuine revision.
New Plugs
Replaced inlet manifold (2 position intake shaft had cracked up and stopped working)
New OEM MAF
replaced PCV and hoses (just blanked the pcv system out entirely to rule it out)
EVAP system seems to be working correctly, (i blanked it and the ECU threw codes for insufficient flow)

What else should I be looking at?

Does the 3.0 have an EGR system/Valve? If so where is it?

I've been trying to solve this for months now and I'm at the point where I want to get rid of the car if i can't solve it!

 

Is the catalityc converter overheating and glowing red?

 

I haven't checked that yet, are you thinking that the cat may be blocked/damaged?

 

If it is blocked it could cause a really serious issue.

 

Is a blocked/damaged cat going to give me an idle misfire?

Does the 3.0 have an EGR valve? I'm still thinking there might be a vacuum leak somewhere.

 

I don't know about the EGR valve, I only heard about it on the 1.8Ts. You say that only one code is being thrown?

 

Yeah just the P0421, I think that's a symptom of the misfire though.

On idle the car misses every few revolutions, like a vacuum leak would but I can't find a vacuum leak! It's hessitant when you nail it and feels very flat and slightly jerky when you hold it at a constant speed.

Would you mind posting a pic of your vaccum line setup from above the thottle body? I'm sure somebody might have put them all back wrong before me and there is no info to go by!

Any other ideas on causes of misfires with this engine? Or anywhere I can find more info on the 3.0 engine?

 

This is what happened. A misfire developed from having a bad coil/s, that took out a cat converter, replaced the coils but the misfire still persists because the exhaust flow is restricted. Thats my best guess. If you had a vacuum leak there would be a fault code for it. There are EGR valves on the 3.0L, but they are called combi valves, and they are bolted on the back of each cylinder head. They have a little shiny top, but i don't think thats your problem.

 

^^ This. If you had alot of misfires before then you stand a good chance of clogging the cat on that bank due to too much unburnt gas getting through the combustion chamber of the cylinder missing. The cat is plugged and that whole bank is starting to miss.

 

Ok so a cat that's been subject to unburnt fuel from a misfire for a period of time will block up? There doesn't seem to be a rattle from it so I don't think it's begun to break up into bits.

The misfire is heavy on idle really, on probing the exhausts the bank 2 side comes out fine on all emissions, bank 1 shows high O2 output, i take it from an incomplete burn i.e. misfire.

Should I be checking other things like compression on that bank before I take the plunge on a cat? Or are the symptoms pretty conclusive?

I really would appreciate all help on this as it's been making me lose sleep for weeks!

Thanks