Alright, I am going on two weeks now, and e90post.com has been worth its weight in sewage. Maybe that's generous because I imagine there's a market for sewage. Anyways, here's to hoping y'all can offer some insight.
Starting about two or three weeks ago, my E91 with N52 (328i manual) would take a long time to start. Cranking speed was okay, just no light-off. Then it would fire up, accompanied by a DSC light and eventually a CEL. Given the car's high mileage (140k) and I only had it for 20k miles, I wanted to knock out some of the known wear items first before really diagnosing since some of these could be culprits. I changed the battery (9 years old), water pump, thermostat, oil pan gasket, and VANOS solenoids. Slow starting persisted after these changes, and now I had a VANOS error. One of the solenoids did not go in nicely, and I could've damaged it with all the wiggling and prodding, so I swapped it with a used but good solenoid. Still slow to fire.
Next, I swapped the crank sensor with a used but good sensor, and I swapped the cam sensors at the same time (same donor motor for all used parts). Slow start has not since returned, but now it traded in a harsh stumble at around 3k rpm. I am pretty sure the stumble was new after the crank sensor, but it *has* always had a stumble off idle when engaging the clutch and shortly after. It could either be I created the problem when doing the crank sensor, or the problem got worse when the other issues were sorted and this stumble is now center stage. Prior to doing all the work, it could stall when pulling in my driveway, and this is still the case. It is not driver error. I thought it could be that the first time, but now it has consistently stalled or tried to stall when pulling in my driveway after experiencing the stumble. This is my main evidence that I did not create a new problem when doing the crank sensor. After pulling in the garage after experiencing the near-stall and stumbling, it idles like it always has. Then, out of nowhere, revs will flair (2k) and die.
When doing the crank sensor, I noticed the grain strap near the crank sensor was horribly frayed, so I swapped it. No change. I also removed the crank sensor, checked the plug and harness, cleaned it, and reinstalled it. Still stumbles.
I moved on to the valve cover gasket which had been leaking, and I replaced the valvetronic gasket, fuel injectors and rail (donor), eccentric shaft sensor (donor motor), and its gasket. Cams all look good, chain looked good, no sludge or buildup. All in all, it looked great for even half the miles. Put everything back together, and the valve cover leaks seem to be gone, but the stumble is still very present.
I took out the MAF, and it looked fine. Wires are a bit tighter than I'd like because of the AEM intake, but everything seems intact. I cleaned it anyway, no change. I drove with it unplugged, and it drove all sorts of awful. Reconnected the sensor, still stumbles and idle still dies.
Next, I removed the VANOS check valves / filters. They were clean, which I expected from the clean camshafts. I sprayed them out anyway with carb cleaner and am letting them dry. That's where I am currently. Any thoughts? One friend who is a BMW guru suggested it may be a fueling issue. I did noticed that I had no rail pressure when I pressed the relief valve on the rail, but it did dribble out of the hose when I disconnected that. A poor filter or pump could explain a lot but not the flair before dying at idle. That is, unless the ECU is doing something strange in response to a lean condition, but it is not reporting any fueling faults (or any other faults for that matter now). It would also be a remarkable coincidence to have such a major fuel deficiency after all these changes when fueling was never an issue before. I frequently floor it to redline, and it has never missed a beat under load.
VANOS adaptations can do strange things, according to the internet. I have not tried resetting those yet, but that's pretty much the only theory I have left that could fit this whole chain of events without relying too heavily on bad luck / coincidence.
