BMW E38 uneven idle
The E38 idle fluctuates or the car stalls when stopping, what causes it?
The most common causes are a vacuum leak, a broken diaphragm in the crankcase vent valve, a stuck idle valve or a faulty mass air flow sensor (MAF). The fault often shows on a cold engine. First read the fault codes and check the vacuum hoses and the accordion hose.
How do you find an E38 vacuum leak, and where should you look?
Leak points include the intake manifold gaskets and the accordion hose between the mass air flow sensor and the intake manifold, whose inside can crumble even if the outside looks intact. You find the leak by spraying brake cleaner on the suspected spots: when the revs rise, the leak is there.
When is the fault in the crankcase vent, the idle valve or the mass air flow sensor?
The crankcase vent valve at the end of the M60 intake manifold is a known weak point, because its diaphragm deteriorates. A stuck idle valve can drop the revs very low and stall the engine when cold; cleaning may be enough. You can try cleaning the mass air flow sensor or swapping in a borrowed one.
Can a sensor cause uneven idle in an E38?
Yes. A crankshaft or camshaft position sensor can make the engine hunt and stall, and the fault is often revealed by fault codes. After replacing the sensor, running usually evens out. After the replacement it is worth reading the fault codes again and resetting the adaptations.