I have a 2008 X3 (3.0si with the N52B30 engine) with about 65k miles on it. Recently, the car started exhibiting two symptoms: starting the car is a problem and once it does start it runs rough at idle. During starting, the starter cranks but no start for a long time (10-15 seconds). It sounds like no spark or no fuel. However after about 10-15 seconds it does fire up. Occasionally, after the prolonged startup I do get an intermittent 2A94 error code so I replaced the crankshaft sensor (not the camshaft sensor). However, no change in behavior. The starts are still excessively long. Engine temperature does not seem to matter. Then there is the rough idle. The rough engine RPM only happens at idle. When I connect my Enginuity scan tool, it confirms that the engine goes into Idle when the rough RPM occurs. As soon as I touch the throttle, the RPM stabilizes and it runs as smooth as ever at any RPM. I was able to capture the spark advance plot when the engine runs rough and it does correlate with the irregular RPM. It's all over the place from -7 to 32 degrees. Again, when any throttle is added, the spark advance levels off perfectly and the engine runs as smooth as butter. Here is the plot of the erratic spark advance values. I get no other error codes. I've looked for any obvious vacuum leaks but could find nothing. Any help as to what to go after would be greatly appreciated.
Start by removing the oil cap and checking for a center cone that goes through the filter. That sometimes get removed accidentally and will cause the fault you have and rough running at idle. Let me know what you find
Thanks for the tip. However, it looks like I might have to go back to the crankcase sensor. After doing some more troubleshooting, the crankcase sensor scope trace into the DME showed a flat line. I did not believe it at first so I pulled the "new" sensor back onto the bench and it behaves exactly as the old one does - no signal grounding when in proximity of any metal. This also confirmed that the old sensor was bad (which I should have checked before). I compared this behavior (on the scope and on the bench) to an intake camshaft sensor and that one works just as expected: nice square wave at the DME and a solid signal pull-down on the bench (BTW, I'm not sure why BMW decided to swap the power and signal leads between the camshaft and crankshaft sensor; I guess it keeps us on our toes). So much for this brand new Conti sensor, right out of a box - it appears to be junk. If another new sensor, which I plan to get from my local BMW, does not fix the problem then it will on to the oil filter housing. To be continued ...
Thanks for the feedback. First of all, the car is starting quickly and running smoothly again. It took another crankcase sensor to get the job done. In the process of replacing it (again), I did verify all of the wires and contact points. During the tests I was getting a solid battery supply to the sensor, but it is always possible that the actual contact at the sensor was not perfect. I did refresh the contact points by bending them pack with needle. As an additional update, it is worth mentioning that during the prolonged debug, sometimes the DME would throw error codes. These included the standard 2A94 crankshaft sensor code (again, somewhat misleading as I just put a new sensor in) and the lack of CAN engine speed error codes from the transmission (581E). The transmission would then go into limp mode at start up, but after driving a few yards it would revert to normal operation. This would also result in the yellow dashboard lights for ABS and 4x4 illuminating. Lesson learned here is not to trust a brand new sensor. Test it on a bench before you install it. Testing the hall-effect sensors is straight forward. Connect a 5V to 12V supply to the power pin (pin 1 on crankcase sensor, pin 3 on camshaft sensor), connect ground to pin 2, and connect a 10kohm pull-up resistor between the output signal (pin 3 on crankcase, pin 1 on camshaft) and your power supply (9V battery works just fine). When you wave any metal next to the sensor head, the signal pin will pull the output to ground. Otherwise the pin should be pulled up to your supply voltage. The original sensor I pulled out was completely broken - no grounding of the signal at all. The second sensor showed some functionality but it was intermittent and exhibiting lots of "sticking" of the output to ground. Again, for a 10 minutes test I could have saved myself a significant amount of time and avoiding installing a faulty sensor.
Glad it's all fixed first off. Yes the other faults will come when there is a crank sensor issue since the transmission module and DSC module don't get the engine speed message over CAN that they need. Very one off for getting a new part that is bad.
