[fvelasquez]

Hey guys, I'm hoping someone can shed some light on this issue. I have a '13 X3 F25 with N20 engine with 143k miles which has had a timing chain kit upgrade, and recently I've replaced and coded the EKPM module. In the recent past, I've had several cylinder misfire errors and other accompanying errors, most notably "11AC02 Fuel high pressure, plausibility, cold start: Pressure too low". I am hesitant to start throwing parts at it to see if this gets fixed, so before I do that I am looking for some sound advice.

After the EKPM replacement, I was driving my car when I suddenly experienced misfires and the car almost stalling completely so I had to return home quickly for fear of the car dying on me on the street. One main symptom that occured is that the car suddenly started throwing a lot of smoke through the exhaust and the smell of gasoline was pretty strong. I let the car sit for a few days and just today I started diagnosing it with ISTA. From the 11AC02 code it would seem that there may be a problem with the fuel rail pressure sensor or the HPFP, so I ran some diagnostics and live data and sure enough the rail pressure was showing at 1.44MPa and cylinder 4 was throwing a lot of misfires. The rest of the cylinders seemed to be ok at that point since at least there weren't any misfires reported. These values were taking justa few short minutes after starting the car.

After a few minutes of the engine running rough, the car suddenly stabilized itself and no further misfires were reported and also the the rail pressure went up and started to match the setpoint value. I took the car for a spin and everything responded normally, so I'm at a loss here as to whether I can still pinpoint this behavior to a faulty HPFP and/or injectors/coils/spark plugs. As I said, I'm hesitant to start throwing parts at it if I can't be certain of who the culprit is. Because of today's resulting behavior, I am inclined to say that perhaps it's a sensor's fault, because at the moment the engine started warming up, the problems went away, the only thing is that I'm not sure which sensor may be failing on me under these conditions. I think it is relevant to mention that I scanned the codes of the car immediately after the initial incident a few days ago and got these two codes along with a bunch of cylinder misfires codes:

12AB04 Oxygen sensor after catalytic converter, electrical: line disconnection
11A002 Fuel high pressure, plausibility: Pressure too low

I have yet to see those codes appear again after I cleared them all and started diagnosing the car on that occasion; the only relatable one is the 11AC02 code mentioned above.

Going back a bit further, after I installed the replacement EKPM and made sure that no codes related to that issue showed up again, I was using the car normally without any problems. I wanted to test if everything worked OK regardless of the conditions I put the car through, so I started using the A/C to check if it held up. Lo and behold, not even 5 minutes into me turning it on and the car started roughing and stalling. I am aware that for some time now, I have had a loud vibration sound whenever I turn up the A/C to more than level 3 intensity and from what I've read, if the AC compressor locks up then this can cause a stall in the car, so I thought that the origin of my problems lied there, but to this date I haven't had any codes relating to A/C, aside from a failed AUC sensor. I tried my AC today on the successful run and everything went smoothly, although I didn't crank it all the way up this time.

I will really appreciate any insights on this issue before I start swapping and changing parts out. For more context, I haven't yet checked the MAF sensor or the air filter, which I plan to do soon in case it's related to that. I also captured a bunch of running values for various items in the DME module with ISTA in case there's any relevant information there that I can share with the audience.

Thanks in advance for any help you guys can provide me!

[Hellshalfacre]

Hello, your symptoms point to a fuel delivery issue. Here's why: firstly, the recurring Low Fuel pressure code and cylinder 4 misfire (4 being the last cylinder on the rail) are good indications that the engine isn't seeing the fuel pressure it is expecting.
Having an engine bog down when turing on the AC compressor is a CLASSIC fuel restriction symptom, and when combined with the first two symptoms further makes me think that you are dealing with either a fuel delivery problem, a restriction to fuel flow problem, or an improper fuel relief issue, namely into cyl4.

Pull the #4 cylinder plug. If it's covered in fuel, then that injector could be stuck partially open which would drop your fuel pressure, and cause smoke, fuel smell etc. If it's clean and you've kept up with regular maintenance (plugs, coils etc) I would suspect the fuel pump as the next possible cause.

Why is it only doing it when cold? Many fuel pumps begin to fail the opposite way and show symptoms only when hot. This is why I think it's worth checking the plug for fuel first as Injector solenoids can properly seat when warmed up. Good luck to you.

Edit: it's probably worth checking all plugs for fuel fouling. Any one injector that is having issues could potentially impact the 4th cylinder. If possible, pull the plugs shortly after running to catch liquid fuel before it evaporates. Be careful with a hot engine, safety etc.

[fvelasquez]

Thanks for your reply, those are some great insights you gave me.

I wanted to give you an update on where I am right now with the car:

I'm planning on swapping out two parts shortly, the MAF sensor and the belt tensioner.

The MAF sensor because I performed the test where you have the engine running and you unplug the sensor and the car should stall if the sensor is OK, but if it isn't then the car keeps running still, which was exactly what happened to me.

The belt tensioner because I came to find out that is the real culprit behind the strong sound and vibration whenever I crank up the A/C; I never really noticed that until I got to check the car with sufficient patience and time.

I will also be changing all air filters and resetting adaptations and from then on I will start testing everything again more thoroughly. I want to get those things out of the way so that I can rule out a faulty sensor and the A/C compressor and focus on the next components, which would be the spark plugs, coils, and of course the injectors. If all those pass the tests, then I guess I'll be looking to swap the HPFP.

The car has been running fine all these days, although I haven/t been using the A/C at all for fear of the issue arising again, but at least I have been driving it under several conditions and I've yet to experience any noticeable issues, so I don't want to jinx it for the time being.

I will update the thread once I swap the parts out and do some more testing with it, hopefully I can bring good news to anyone else experiencing something similar or at least more information to share here.