Sounds like the cat to me. When my coils went out (3 at once) my cat was GLOWING red...replaced the coils and a few days later I got another CEL, but not flashing. Brought it back to the dealer and the cat was replaced under the emissions warranty. Too much gas poured into it and messed everything up. If youre <80k, free cat!

 

Almost forgot about that, if you are under 80k its replaceable under federal law for free. You don't need to hear a rattle for them to be plugged, take it out and look at the inside and you will see how it can melt together and not rattle.

 

Thanks for the replies.

I'm in the UK so no cat replacement over here.

The blocked cat idea doesn't seem to fit with what I'd expect from a blocked cat. Once the car is up higher in the rev range it performs well and feels smoother.

The misfire and stumbling comes steadily on idle, constant missing but no misfire codes. Then from lower rpm's as you accelerate the car feels un easy and bogs down, eventually it picks up.

When you let off to change down or deccelerate in gear then reapply power the car stumbles and can jerk for a second before it picks up.

I've looked at the FPR, the car runs exactly the same with it connected or not. No fuel leaking from vaccum pipe connection though. Should it make a big difference to the way the car drives with it connected to vac or not?

I'm going to performa a vac check on the manifold this weekend to rule out a leak, I'll then check the compressions to try and rule out a head gasket.

Would the P0421 suggesting the cat is no good (probably a result of the misfiring chucking loads of fuel through it) cause a misfire?

 

just a quick thought- try cleaning the mass air flow sensor. not sure if a dirty/bad one would cause this code but it could cause the symptoms you are having

 

Already fitted a new MAF unfortunately :-(

 

FAIL! The UK sucks LOL....JK.
If you leave the FPR vacuum line it will run rich as all hell. You're thinking too much into this, if you have a vag com you can easilly see what the fuel trim is in data block 32, which would indicate if there was a vacuum leak or not. Having a fault code for a cat won't freak out the ECM into creating a misfire. You can also watch live misfire data in block 14,15, and 16 which would make it easier to diagnose. If you don't have a vag com i would suggest investing in one. I still think you have a partially clogged cat, especially if its only missing on one bank. If you had a vacuum leak or a bad MAF you would have several random multiple misfires all over the engine, and it would hardly run at all.

 

Ok, so I've checked the vacuum and the compressions just to try and rule everything else out.

The Vacuum was at a steady 22/23inHg from the manifold.

Compressions were;

Bank1 - Cyl 1 180psi, cyl 2 173psi, cyl 3 187psi,

Bank 2 - cyl 4 173psi, cyl 5 163psi, cyl 6 170psi.

Any opinions on those results?

 

...sounds like someone needs a HFC!

 

No closer to solving the misfire. Any second opinions on the cat causing a misfire?

Pre cat O2 sensors?