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My BMW will not rev beyond 4,000 revs. I replaced the cam position sensor after a diagnosis indicated this was the problem, but the problem remains. (I think it was the cam position sensor, but it may have been the crankshaft position sensor if there is such a thing).
I asked this question a month or two ago, but unfortunately have inadvertently deleted your reply. Sorry for the inconvenience.
The crankcase breather is could be blocked. Also, check the rubber air hoses from the airflow meter to the throttle body look for air leaks hissing noises under the bonnet, or the airflow meter itself.
It may be a process of elimination remember the fundamentals what is your engine needs and why.