READ THE DESCRIPTION FOR ALL THE INFO.....

Car is a 2006 BMW 330i. Same engine (N52) as the 328i
Issue and symptoms:
First symptom: The car was hesitated upon starting. Only on cold starts. Smelled like it was running rich.
It was intermittent and did not happen on every cold start. Then about 2 weeks later, it would also show similar signs mid-day after having driven the car.
Gas mileage went down by about 20%.
NEVER got a check engine light or indication that there was something wrong.
The battery was only 7 months old at the time and the alternator was fine, the car was only 3.5 years old and I was the first owner.

During the 3 weeks of these systems I was taking it to 3 different dealers trying to diagnose the problem.

Verdict....
Eccentric shaft sensor had failed. I took it to 3 SoCal BMW dealers. Each wanted to take apart the engine and charge an upfront $1,200 to diagnose it. On the 3rd try I went to Sterling BMW where I purchased the car new. They found the fault! The OBD2 (onboard diagnostics) did not indicate a problem. Likely why the other dealers couldn't figure out what the issues was (they were lazy relied on computer for answers).

I had the car fixed for about $550 at Sterling BMW and fixed all the issues.

Unfortunately what i did not know at the time was since this sensor manages the engine timing and so on it made my car run rich to point where it clogged up my O2 sensors in the exhaust and ruined all 4 of my catalytic converters. That was estimated to be a $4,000 repair and at that point I traded the car in 8 months later.

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