Misfire under boost with no codes

UN54FE

New Member
Feb 28, 2022
18
24
0
Brooklyn, NY
Ride
BMW 335is
Okay guys so my car has a persistent misfire under load at 4500 rpm or higher when im running 23+ psi. Anything under 23 psi the car runs fine. I am running out of ideas here so i will rely on you guys to help. Here’s the rundown on the car:

- built motor as of september 2022 (piston/rod/sleeve/crankhub). Will throw it out that the head has been refreshed by the shop also (new valve stem seals, machine work, & such). Car has frankenturbos, all the bolt ons, & all the fueling (twin 450 lpfp, reflex, 750cc injectors)

Here are the things ive tried to remedy this:

- New spark plugs ~600 miles old (ngk 97506 0.018 gap. Also tried 0.020 & 0.022)

- brand new eldor coil packs ~ 1000 miles ago

- i sent out the di injectors to get tested & cleaned while the motor was being built, all passed testing

- ppifpr injector bore fix kit (fantastic kit btw)

The car also keeps throwing a fuel mixture code. At this point i am considering doing o2’s, maybe cleaning the port injectors, & doing a leak down test to prepare myself for the possibility that there may be an issue with the head. What do you guys suggest?

important to note again that the misfire only happens when the car is making 23+ psi.
Here is a log: https://datazap.me/u/isslow/full-e8...d_qeJl08lbgs0O-pIcpFqMcK9of347nNd038Isl7adtUo
 

UN54FE

New Member
Feb 28, 2022
18
24
0
Brooklyn, NY
Ride
BMW 335is
For me it was false knock at first, then it was a case of i needed better coils.
My question is if i had false knock, i would see timing corrections in the log wouldnt i? Im not really an expert at reading these things lol i am trying to learn as much as i can to keep my car in good shape
 

carabuser

Lieutenant
Oct 2, 2019
849
1
745
0
UK
Ride
Z4 35i & 335i
Misfire and knock are two different things. You'll see corrections if you are getting knock but misfire detection will just put you into limp or cut injection.

You might also have knock related issues depending on the tune. If you have adjusted knock ignition correction sensitivity in the tune but not adjusted the knock related fuel cuts to match then you'll also have a scenario where you'll get fuel cut but no corrections which can seem like a misfire.

You won't see corrections if the coils/plugs are igniting the mixture fully, you'll just get misfire codes because of the uneven crankshaft rotation and spikes in O2 readings.
 
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UN54FE

New Member
Feb 28, 2022
18
24
0
Brooklyn, NY
Ride
BMW 335is
Misfire and knock are two different things. You'll see corrections if you are getting knock but misfire detection will just put you into limp or cut injection.

You might also have knock related issues depending on the tune. If you have adjusted knock ignition correction sensitivity in the tune but not adjusted the knock related fuel cuts to match then you'll also have a scenario where you'll get fuel cut but no corrections which can seem like a misfire.

You won't see corrections if the coils/plugs are igniting the mixture fully, you'll just get misfire codes because of the uneven crankshaft rotation and spikes in O2 readings.
This is real interesting. Okay i follow. So i have another question now. I just ran the codes again and it seems like now i have bad rear o2’s. Those control fuel trims correct?
 

carabuser

Lieutenant
Oct 2, 2019
849
1
745
0
UK
Ride
Z4 35i & 335i
Rear O2s (narrowband) calibrate the Front O2s (wideband). Fronts are responsible for trims but if rears are bad then you'll get inaccurate fronts.
 
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