Hello everyone,
I've got an manual R56, as the title suggests, with an N18 engine. Bought used from dealership, at the time I got a great deal on it because the clutch was going. Got the clutch repaired and it drove like a dream, however it only took a week or two for something else to come up.

Was getting on the highway so I gave it a bit of juice, throwing it in a lower gear to hit highway speeds, and got a nasty misfire. Blinking CEL, limp mode light, and of course it ran awful after that. I pulled over, restarted the car, and it ran fine. Testing to see if it happened again I hit the gas pretty hard from stand still, found that it only showed up in third gear and higher will it is at 40%+ throttle. Checked codes, misfire on 2, 3, and 4 with a P0300 random misfire. No trouble will driving like a really safe person, where is the fun in that?

Shortly after my catless downpipe came in, so I threw it in and gave it a test drive. Now, I used to not have misfires at first and second gear, but now if I get on it quick it loses all power and "stutters," doesn't not feel like a normal misfire. Codes being thrown are the normal O2 sensors due to decat as well as the new misfires, same codes as the old, but I am not sure if that would cause these misfires. My R53 did not need anything special after the decat, ran just fine (just had to pull out the o2 sensor from the exhaust and tie it somewhere safe). Bit of a hillbilly move I know, but it worked for me before.

These new low RPM misfires arrived RIGHT after the new cat was put in, so they must be correlated. I've also put in a cold air intake and can hear the turbo wind up just fine, so that doesn't seem to be it (I hope), no trouble after the new intake.
Checked spark plugs, they are all fine. Didn't even bother with coils because the misfire on all cylinders makes that sort of hard, never heard of all coils going bad either. No rough idle, so I would think injectors but I ran some techron either way, didn't make a difference. Now, I read that the HPFP causes problems and I am at 111k miles so I'd not be surprised, but I'd think I would be getting more problems with a bad HPFP. For another thing, I have an N18 and, while the N14's have a nice $150 tag on them, the N18's cost $500 on a good day. Don;t want to plug and play until it works, I don't make enough money.

Other than the above mentioned, car is stock. Always 93 grade fuel, oil is all good. Everything seems fine, aside from of course the misfire. Anyone have any ideas?

 

My first guess would be the plugs. Are they OEM? Are they gapped right? Since they are pretty cheap comparatively, I would probably replace the plugs anyway just to see. When my plugs get tired, I get misfires under hard acceleration. On a 2012 you should be using Beru 12ZR-6SPP2-1, that was the factory plug in 2012. I would only use OEM unless it’s tuned, then you will ant to use NGK 1422’s

 

Spark plugs are good, really good looking actually. Gap is a near perfect .66mm, OEM plugs. Not sure if it was tuned by the previous owner

I should also add that if I slowly put the gas to the floor there is no misfire at all.

 

Have you scanned the codes?

 

The HPFP presents usually on cold starts. It will immediately misfire on start-up and run rough or die briefly then be ok. That’s the most common c but it could be choking a little bit under heavy loads. If you can monitor your fuel rail pressure it will tell you. Either using a scan tool or something like dash command.

 

Scanned the codes a few times, CEL is just mad about the decat with a three codes for the lower o2 sensor in addition to the P0300 and misfire on 2, 3, and 4. Found a decently priced N18 HPFP but that won't get here for a few weeks. While it is on its way I'll see if I can check the pressure. Failing turbo wouldn't cause trouble like this? Not a decat?

 

If anyone was curious, it was a completely clogged fuel filter. Fixed nearly all issues, still one thing wrong but that is in another post.