Dying E38 Fuel Pump?

[thadbafundo]

Hey everyone,

I’ve had the SES light on my e38 (2001 740i sport) for a while now. A few months ago it threw codes for P0171 and P0174, indicating that the engine is running lean on both banks. After scanning the fuel trims, they run at around 3-4% at idle, but spike to 25% under load. My mechanic found air leaks, but after fixing them the code returned, and he noticed a hesitation in throttle response. He then checked the MAF, O2 sensor, and even replaced the fuel filter. The codes remained. After replacing the fuel filter, two new codes appeared: P1158 and P1160, both BMW specific codes about fuel trims. Fast forward a few months, now the car can’t make it past 4,000 RPM. It’s still powerful, but definitely lost some torque. Last time I drove it it stalled right before turning into the driveway. I even noticed some slight surging when parked and idling, could this issue be a dying fuel pump? Will replacing the fuel pump fix it? Any help is greatly appreciated.

[Mayorchuck]

Did he replace the MAF? This sounds like a dying or aftermarket MAF issue, although the 4k limit can also be clogged cats. A back pressure test would confirm that. Mine rattled pretty good when it happened to me.

[thadbafundo]

Thanks for the response. He only tested the MAF, which looks to be the original Bosch sensor. Should I clean it and see if anything changes, or would cleaning a dying MAF do nothing? I could go ahead and order a new sensor, but I would love to make sure that it's the issue.

[Mayorchuck]

Can't hurt anything by cleaning the MAF. I've seen them really dirty and even oily because a guy was using one of those oil soaked air filters. You really need a better diagnostic. P codes can be helpful at times not great for diagnosing something like this. Something like INPA would be helpful here. You can try unplugging the MAF to see if it runs any better in default mode. I can give you my opinion based on past experiences, but no way to tell you exactly what is going on with the car. If the DME is not getting proper readings from the MAF, it may think there is less air coming in that there actually is and adjusts the fuel accordingly. The MAF does not always throw a code on this car.

[thadbafundo]

Turns out my mechanic just swapped the MAF from another car that he had there. I cleaned it, and now the SES light is no longer on. It still struggles at high RPMs, and scored a 55% on a volumetric efficiency test that I ran. Bosch is OEM for this car, correct?

[Mayorchuck]

Yes. Bosch is the way to go with the MAF.