struggling with fix for P0303 misfire - 2011 s4 (Help/ideas appreciated!)

[666WS6]

Looking for any help or insight... really struggling to resolve my misfire issue.

I've had a misfire on cylinder 3 for 2 months. After the car sits overnight... it stumbles to start a little, then has rough idle, but it clears up after a minute of warming up. Always only cylinder 3. I figured it was a leaking SC intercooler, as I swapped coils and plugs without any success.

So I decided to tear into the engine, and do intercoolers, PCV, tstat and waterpump... and carbon cleaning. Cylinder three definitely had coolant leaking in for some time. It was bar far the worst. I also installed new valve cover gaskets as I had some oil in the spark plug tunnels.

I finished all the work, pulled and checked injectors as well... and put them back at random cylinders.

But now the car misfires on cylinder 3 all the time, and it seems worse.

Cylinder 3 pressure is above 150 as well.

Coils replaced, plugs replaced, injectors cleaned and installed at different cylinders, all valves carbon cleaned, PCV replaced. .... So what else could still be causing P0303 and rough idle???

[LowKeyLoki]

Tough to say without doing any data logging. If you have VCDS I’d monitor the fuel pressure after you turn off the car and watch if it drops from a leaking injector. If I had to guess I’d say that injector was bad and cylinder 3 had issues from possibly clogged cat…perhaps now that you moved the leaking injector it is causing issues idling along with cylinder 3 misfiring. My advice is do some data logging and scope the cats via the O2 sensor port see if they are clogged. Idk what mileage you are at but if you are close or above 100k miles I’d say go ahead replace all of the injectors.

[mys4.org]

Just to get it out there... what did you gap your spark plugs too?

[666WS6]

Gapped to 0.028.... and I actually tried 0.032 and 0.26 but didn't change the misfire.

- - - Updated - - -

I do have VCDS, but haven't used it enough to know how to watch fuel pressure. Good suggestion, I'll have to check into it.

[DrGrapeApe]

Don't forget to inspect the wiring and connectors for the injector and coil.

[666WS6]

I did a new trial on pass side bank to swap around injectors between 2 and 3, but the misfire jumped over to cylinder 4 (Drivers side bank which I didn't touch). I feel like I'm losing my mind... it always has been cylinder 3.

I also checked the fuel pressure after the car is off... and in 2-3 minutes, the pressure begins to drop. It never raises in pressure as it should from the engine heat... it holds initially but then drops after 2 minutes so I assume I have a leak... perhaps at the injectors.

Bottom line at this point... I just invested in new injectors. I've been e40 for 2 years so thought about it anyway.

I've had the SC off like 10 times now, so getting good at it. I'll do this one last time though... and if the misfire continues... may just have to take it to a shop.

[666WS6]

Just to close this one out... new injectors fixed all my issues. Not 100% sure why, perhaps maybe long term downside of e-40. But either way, all good now! What a relief.