B8 A5 2.0t misfires

[BobG999999]

Getting P0303 (Cylinder 3 misfires) that happen at random intervals. Not at idle, they crop up while driving. Some times when accelerating, other times when cruising. Swapped #3 coil with #1, and #3 plug with #2. Still cylinder 3 misfiring.

I'm pretty sure it's not carbon as I've read in several threads. I rebuilt for the oil ring foul up about 18 months ago, had the head re-done by a shop with some new valves, so it was totally clean. Haven't even done 10,000 miles since then (working from home).

Driving the other day I got the stumbling, then CEL, then EPC and I chugged home to see what I could see. Cleared codes and it runs good. After driving a bit, it will randomly start misfiring again. Tonight while checking, got a flashing CEL with a misfire, then it stopped and the misfire went away.

These "random" problems are always the "most" fun to find. Can't predict when they will occur, no specific set of circumstances.

Any help appreciated.

[eskimo0101]

Time for a compression test and see if cylinder 3 is showing different than it's neighbors.

[T_Glass_27]

If you haven't replaced plugs and coil packs recently I would probably just go ahead and start there by replacing them all

[BobG999999]

Thanks to both of you for your replies.

I will now have to do a compression test. I hadn't done it as it was less than 10,000 miles since I rebuilt with the VW rings to solve that problem. It had been running good since then.

Also got new coil packs and plugs at that time.

When it's not missing it "used" to run real strong, so it didn't seem like there was a loss of compression in #3.

So, thinking that moving the coil & plug to #1 and #2, ruled out the compression due to recent rebuild, it's not carbon build-up (taking the intake manifold off I could see the valves were clean ... had the head rebuilt when I did the rings). To me and my immediate group, that left fuel. I just replaced all four injectors and I'm getting the same damn result.

So ... I can do a compression test, to hopefully rule that out. After that, I'm stumped.

[BobG999999]

Okay ... compression test:
1 - 150
2 - 150
3 - 140
4 - 150

My understanding is that less than 10% difference between cylinders is okay.

Checked out all of the coil pins and the connectors, they all look okay.

As I mentioned in the first post, I already swapped #3 coil and plug, the misfire stayed with #3.

Any other thoughts?