[redz06]

My 335i sedan with N55 engine occasionally goes into a reduced power mode and lodges fault codes P15DF, P0087, P0090 as well as various engine misfiring codes. Sometimes it shows the "Half Engine" warning light, other times not. Always, after I erase the fault codes, the engine runs normally, exhibiting boost and decent part throttle operation. Often, but not always, the lodging of codes and reduced power operation occurs just following a cold start.

Local BMW dealerships do not have a great reputation with me, so I have taken my BMW to two independent BMW shops in my area for diagnosis/repair. BMW parts are expensive, so one thing I wanted to avoid is just throwing parts at the problem. Both shops swore beforehand that they had excellent diagnostic capabilities beforehand and would not be throwing expensive parts at the problem.

The first shop claimed to use the same diagnostic software as the BMW dealerships use. The first shop charged me $165 and then admitted they could not pinpoint the issue, but recommended replacing the LPFP and regulator assemblies, since the HPFP had been replaced about 25k miles ago. Basically just throwing parts at it, since the LPFP and regulator were original, with 99k miles on them.

The second shop charged me $210 and admitted they could not pinpoint the issue. They recommended changing the HPFP and the HP rail pressure sensor, which sounded like throwing parts at the problem. Both shops said the LPFP and HPFP discharge pressures were normal. The second shop did run ISTA on my BMW to check the electric fuel pump, rail pressure sensor, and the HPFP and no faults were identified. ISTA did note that the test module does not include checks for load-dependent faults. ISTA said that if high pressure faults are encountered, the following fault causes may be present:

* obstruction due to fuel contamination
* damaged fuel lines

ISTA did also find the following fault which was lodged in DME memory: 002BEE - High Pressure Fuel, plausibility, cold start: Pressure too low.

I decided to take my BMW home and try to gather more data on the problem.
The next day I hooked up my OBDII reader to my BMW and did a cold start. Before the start the high pressure fuel rail pressure FRP) was 62 psi. After start, the FRP was only 75 psi and I hardly got out of the driveway before the half engine warning light appeared, so I turned around and went back to my garage. I shut the engine off and went into accessory mode so I could erase the codes. I then restarted the engine and the FRP went up to about 1300-1700 PSI until the idle went down to 727 RPM, where the FRP was 700 psi, on normal driving afterwards, there were no further codes and the FRP was 1300-1800 psi.

On the next day, I did another cold start and there were no codes and FRP was normal.

Another couple of days go by and I do another cold start, with normal FRPs. I drove the car 10 miles and stopped at a store. Did another start and no codes, normal FRPs. Then I decide to record data under full throttle starting around 3000 RPM. Here is my data recorded by my OBDII reader:

RPM Engine Load FRP
3133 22% 22 psi
3233 47 2078
3312 54 1933
3421 69 1722
3550 87 1395
3700 88 1408
3977 88 1358
4266 88 1222
4519 88 1105
4817 88 995
5510 67 1048
5530 24 75

At or near the last reading, the half engine warning appeared and a P0087 code lodged. I switched off the engine, erase the code and started the engine again without further codes and drove the BMW home gently.

I hope the presence of all of the data in one place will trigger more views and advice on how to address the root cause of the problem.