Pathart500

All,

I'm at my wits end with this misfire on cylinder 4. It started happening at random times, but is now pretty consistent when under load after the engine has warmed up.

Codes:
- P0304 Cylinder 4 misfire

Things I have done:
1) switched coils. Misfire stayed in cyl 4.
2) switched plugs. Misfire stayed in cyl 4.
3) changed coil wires (brittle and broken). Misfire stayed in cyl 4.
4) switched injectors. Misfire stayed in cyl 4.
- After I changed the injector, the car misfired for a day, then the misfire went away for an entire week. I was ecstatic to have my car back in running order!!!! THEN.......

Codes:
- P0304 Cylinder 4 misfire
- P0420 Catalyst system efficiency below threshold
- P0411 SAI system incorrect flow

Things I have done:
1) Park the car
2) Hold back tears of frustration
3) Post on this forum


HELP!!!!!

westicals

Since you're geting the P0420 code and P0411 codes look that route.. Maybe check your 02 sensors ( 10 min job to unscrew them and inspect), my cat broke internally and melted my downstream 02 sensor... Also check for vacuum leaks..

Prospeeder

Ok look at the intake runner on #4. See how at the start of it theres a large vacuum line to the suction jet pump thing and the brake booster. If there is a large air leak there, cylinder 4 will be leaned out to the point of misfire. I see it all the time when the jetpump is broken, breather parts broken, check valves broken. Ect. Makes sense because you said when its cold it doesnt seem to do it. The engines much richer when its cold so it will compensate.

Pathart500

Prospeeder, Do me a favor and find a picture and mark up what/where you are talking about. I have looked for air leaks, but may have over looked the area you are referring to.

Prospeeder

Brake booster, vacuum check valve? What part is this? A4 B6 Cabriolet 1.8t

What i see is that part the suction jet pump broken just like that and sucking air .Also Just look down around that area, you will see 2-3 check valves that can break also.

lyleswk

You have done everything right so far for a misfire. I searched around for sthe thread on this, but couldn't find it. So, will go from memory.

After putting coil pack, spark plugs, etc off the table, the next step is vacuum leaks or cat system issues. Some cylinders are more prone to aim you in one direction or another. If I remember correctly (and this is mostly due to the codes you have), cylinder 4 misfire is more susceptible if you have an SAI issue for some reason. I would look for lose ducting to your SAI or SAI not working. The resoning is that if the ducting is lose, you are creating a low pressure in the exhaust side; if pump itself is dead, then an overpressure, both of which can cause misfires. Would ignore your cat code until you eliminate the SAI system.

I will keep looking for the thread that I am thinking of.

doubleblackUrS4

Simplistic, I know, but have you checked compression in the #4 cylinder? Any excess oil use or other goofy symptoms? I always rule out all the simple explanations first - there are some great ideas on this thread. Good luck!