2008 BMW X3 3.0 Misfire

[broncos011958]

Hello there. New to the forum and new to being a BMW owner! We have a 2008 BMW that has a misfire that we have replaced the coils, spark plugs, run a compression test and everything is at 125. New injectors, as we were told by BMW that if all the other stuff was done, then the injectors has to be the problem, but it is still having the mis fire and showing a P0303 code even after all that has been done. Please, anyone that can help me figure this out would be much appreciated. I have worked on numerous vehicles and this is my first BMW and its getting pretty frustrating! I notice that the RPM's seem to be running too low, as I give it gas and get it up to the 1000 RPM's and it seems to run just fine. Thinking there might be a wiring issue??? Any feed back would be great. Thank you!

[dukedkt442]

While only #3 is misfiring, you probably have more than that. Look at you fuel trims. Use ISTA to operate your DISAs checking their operation. It's easy enough to check fuel pressure; ISTA can also shut off each injector one at a time to identify a soft or failing injector. Mine have 195k miles on them and are fine.

Your compression is low; N52 wants a minimum of 130 psi.

Another common problem is a failed ESS seal; this allows oil to entire the wiring/sensor, giving idling problems. This engine controls RPM by changing valve lift by way of the eccentric shaft and sensor; the throttle body is redundant. If the wiring harness for the sensor is oil soaked, you'll likely have to replace the sensor, which is under the cam cover.

With your presumed mileage for an '08, every gasket that is supposed to keep oil in is letting it out. This causes vacuum leaks, which no engine appreciates, especially N52. 8%+ fuel trims will cause an idle fluctuation with the N52, but won't trigger a p0171/174 until >20%+. Even something as silly as a bad oil cap seal will cause the engine to misfire or stall. An often overlooked source of unmetered air (vacuum leak) is the tube that runs to the solenoid in the trunk and then to the exhaust tip butterfly valve.

Troubleshooting/diagnosing an N52 isn't really any more difficult than a 350 Chevy, just much more expensive if the diagnostic procedure involves a parts cannon. Post back with your engine data.