[RichardLong]

Just bought a 2010 335 which i bought broken (misfiring). Assumed based on a lot of reading that my problems of huge short term fuel trim values (+20%) on 1 bank and terrible smooth running values on cylinders 2, 4, 5 were becauce of failing injectors but decided anyway that the spark plugs needed to be changed. Upon changing the spark plugs my short term fuel trim values as well as smooth running values remained damn near perfect until i took the car for a rip where it was still fine until i turned the car off only 2 start it a few minutes later to the car running exactly like it did before like nothing had changed (misfiring at idle, and even more misfiring as the revs drop from revving it in neutral). The fuel trim values were back to shit and the smooth running values exactly like they were before. Anyone got any thoughts? any contribution is appreciated.

[iqraceworks]

The first thing I would do is physically check for leaky injectors. Let the car set overnight, pull the spark plugs in the morning...if any are wet, or smell like fuel...replace them.

If the injectors are all good, and plugs are new, next replace the coils.

If none of that doesn't solve the issues...do a leak down and compression test, the engine probably has a cracked ringland.

[RichardLong]

Originally Posted by iqraceworks

The first thing I would do is physically check for leaky injectors. Let the car set overnight, pull the spark plugs in the morning...if any are wet, or smell like fuel...replace them.

If the injectors are all good, and plugs are new, next replace the coils.

If none of that doesn't solve the issues...do a leak down and compression test, the engine probably has a cracked ringland.

Sweet will do, Thanks for the help.

[Jaronbwall]

iqraceworks Is a cracked ringland common on the N54?... Sounds like a Subaru problem.

[whyzee125]

This sounds like injectors to me. Your car will shut off a cylinder if it has a bad misfire so my guess is it's been hobbling along with a few bad injectors which fouled the plugs out with fuel. Putting new ones allowed it to run okay for a bit until the misfire came back but after they got fouled with fuel it shut them off again. A lot of guessing I know, but I don't think it's your coils.

[RichardLong]

Weirdly enough although i still believe i have injectors that should be replaced as the plugs did have fuel on them, the solution or atleast temporarily was pulling both VANOS solenoids out and drowning them in air flow sensor cleaner. The car started up and ran as though it never had a problem, 40 to 120 rip and all! Thanks for taking the time to help me out - the N54 lives on

[iqraceworks]

Originally Posted by Jaronbwall

iqraceworks Is a cracked ringland common on the N54?... Sounds like a Subaru problem.

It seems like every few weeks I see another post on the N54/335i Facebook groups about someone cracking a piston ringland on these motors. That's what happened to mine......and I was making sub-600hp, so nothing super crazy.

The factory ring gaps in these motors is around .009"-.012"...which is super tight for the higher boost levels (15psi+). My new forged pistons from Mahle came with paperwork saying I wanted to run .020"-.023" ring gap....so double what the factory gap is.

As the rings heat up under wide open throttle and boots, eventually the ends of the rings touch, and when they do.....something has to happen as they keep wanting to expand. Usually it cracks a ringland.

This is the same thing that happens to LS engines when guys put a bunch of boost to them on the factory pistons/rings. Search online, tons and tons of great into out there.

[Jaronbwall]

Very nice explanation! Appreciate the response. No plans for more power on my car so I'll take the safety card.