Nightmare diagnosing n54

[Msport335]

Did you test the cylinder more than once?

Yes, tested 3 times and outcome was 90psi each time

 

[Jeffman]

Did you try walnut blasting? Cylinder 6 is the toughest to get clean because it sits close to the fire wall. It’s my experience that it is difficult to angle the walnut blasting rod to get cylinder 6 really clean.

 

[Msport335]

Took it for a really hard drive yesterday and got it hot to see if it was carbon in hopes that some would come loose helping the valves to seat. No luck... idle is still lumpy and ran another compression check after that drive with the same outcome ...just a hair under 90psi in cyl6. I don't suspect its carbon since the meth keeps the intake head ports pretty clean. I guess as a last resort I can find the time to pull the intake manifold and have a look at the valves for carbon buildup. No codes thrown after the spirited drive either. When at a stop and Letting foot slowly off the brake, the car jerks forward in sync with the lumpy idle. No smoke whatsoever out the tailpipes , only from the oil cap when open and the catch can vta hose.