OK, answering my own post here but have identified the Fuel Pump Relay as the cause of the 4000 rpm rev limit. Removed the relay, wired the terminals to activate fuel pump and went out for a test. Engine revved quite happily above 4000 rpm.

My problem is that I can't get the correct old style relay. I ordered one through my local parts shop using the number 002 545 27 05 but when the relay arrived, discovered it was exactly the same as the one I'm using right now. Its not a Bosch part but made in Hungary.

Have tried Parts Shop but there's no photo of the relay listed under the number 002 545 27 05 and I'm afraid I'll only get the same Hungarian relay again. Can anyone help me with finding out how to tell if is the the correct relay or even how to disable the rev limiter in the relay I'm using now?

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1987 300e manual 250,000 km (sold)