I have a rebuilt 2007 MCS auto, which starts fine, and runs great once warmed up. Between start up and warm, it has a lumpy idle. The revs dont vary much (maybe 30 rpm) but when I watch ignition timing with my scan tool it bounces from -12 to +16 very quickly. If I rev it to 2k and hold it there it is totally smooth and runs great. The engine was rebuilt less than 5k ago, and it has done this since then. Ive changed or tried man different things including: new HPFP, new vanos solenoid, cleaned maf sensor, cleaned throttle body, new plugs, new coils, injectors we new with the rebuild. Fueling looks good - a/f ratio and lamda are good, fuel trims are good - near 0. Ive checked for vacuum leak and smoke tested - no leaks. Ive tried different known good upstream o2 sensor, which made no difference.

Seems like it has to be a sensor issue that is giving a strange signal to the ecu until its finally warm (to 120 f or so). What am I missing?

 

Any BMW fault codes?

Use a scan tool to read engine coolant temperatures, and also try bleeding potential air trapped in the cooling system.

 

No codes at all, and running temp is normal. It only behaves like this when cool, up to about 130-140f. I may try swapping out the downstream o2 sensor with a know working unit to see if that brings any change.