Desperate Need In Help For High AFR Idle Misfire

fs125

New Member
Oct 5, 2021
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Car is a 2009 BMW 335xi with about 80k miles.

A little back story, at the tail end of 2019 my car went down with a bad HPFP. As the car was already down and I'd have take it apart, I decided I'd install my mods and maintenance parts I'd been gathering. The misfire started pretty much after installing all the parts. Here's a list of those parts:

Maintenance:

  • Primary and secondary O2s
  • Map sensor on intake manifold
  • High Pressure rail sensor
  • Low pressure sensor with new line
  • HPFP OEM unit from FCP
  • Intake valves cleaned manually w/pick and GDI cleaner(not walnut blasted)
  • Intake manifold gaskets
  • Throttle body gasket
  • Injector 4 replaced, recoded. Car already had index 12 injectors

Mods:

  • VRSF 7 inch stepped intercooler
  • VRSF charge pipe with Tial BOV
  • 3.5 Bar TMAP n20 sensor
  • Dual BMS catch can
  • BMS DCI
  • NGK 97968 plugs gapped to .022
  • Bimmer life r8 coils conversion
  • PFS single shot bucketed pump
  • VRSF catless downpipes
  • CPE downpipe back exhaust
Car is running MHD Stage 2+ on ACN 91, and was already on Stage 2 MHD when I bought it.

Problem and symptoms

The car has a misfire, according to the DME, its on cylinder 4. The car basically misfires really violently at idle on cold starts, with the AFRs being really high like >20 on bank 2, and > 15 on bank 1. The fuel trims will also be pegged, sometimes hitting max at 34. I've noticed its linked with the weather, if its hot, its less severe with slight hiccups, but when its cooler (<70 deg) its really bad. The car will run like crap until its warmed up, and I can erase the misfire code without it coming back. When driving all warmed up, the car drives perfectly, and pulls hard. I don't like driving the car with this issue, so I rarely drive it, until I can figure it out.

I tried looking for a vacuum leak, and did a smoke machine test and couldn't find anything. I've replaced the cylinder 4 injector because I pulled the plug and it did have a fuel smell, but now I'm thinking it might of been related to the fuel trims being so high. As the weather is getting cooler, the problem is getting worse.

Grateful for any help, thanks!

Here's some data logs of the problem:
 

SLOWESTN54

Lieutenant
Feb 9, 2021
695
433
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22
B.C. Canada
Ride
2007 E92 335i / 2008 E60 535i
I have not, I'll try to get one done. What are your thoughts leaning towards?
engine tolerances cold and hot are very different, if your low on compression cold that's probably your misfire, and as metal heats it may properly seal and the misfire goes away.
 

Dumaurier7

Sergeant
May 19, 2020
356
117
0
If you AFRs ar high and fuel trims pegged high as well then the motor is not beig properly fueled, a lean motor will misfire too, go back and recheck all your fuel system mods and if necessary try re-installing some of the parts you changed out in your fuel system, you should also have the car scanned/codes read to see if there is any additional causal factors (sensor issues) being reported.

PS: why did you change your FP sensors? were they going bad?
 

fs125

New Member
Oct 5, 2021
4
0
0
If you AFRs ar high and fuel trims pegged high as well then the motor is not beig properly fueled, a lean motor will misfire too, go back and recheck all your fuel system mods and if necessary try re-installing some of the parts you changed out in your fuel system, you should also have the car scanned/codes read to see if there is any additional causal factors (sensor issues) being reported.

PS: why did you change your FP sensors? were they going bad?
Prior to my HPFP going out, the rail pressure readings were sporadic, so I changed the rail sensor to confirm that it was the pump. When I changed the pump, as I was already in the area, I changed the LP sensor as part of maintenance.

I'll take a look at the LPFP again, but all pressures seem to be in the right range. As per MHD, the only codes I get are 29D0 which is misfire for cylinder 4.

Thanks for your thoughts.
 

135boost

Corporal
Oct 28, 2017
177
1
97
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Ride
135is
tune is for 3.5 bar tmap, yes ? because if that is stock tmap tune, it will throw that off by that amount.
that struggle is sure from extreme lean, ignition limits is somewhere close to 17 afr anf you got there values
close to 26 on one bank what indicate 1 cylinder shut off and on other bank 17 what is on limit of ignite.

from start this engine run from "map" until it warm lambdas enough to switch over to closed loop.
if that tmap is ok selected in tune, then you have some basic value in tune very wrong.

check that tmap, what ever you use MHD, JB4 or something else..