Through experience, I would also include electrical control of the ICV and/or FPR as a suspect.

Check the connections to the ICV and make sure they are good, open up the FPR and check the soldered connectors are OK, they quite often need resoldering. Then do the same with the ECU, it suffers from the same solder problem. That's where I finally tracked down and fixed my idle woes.

Mind you, I also had to replace cracked hoses, a sticking valve and all the usual suspects along the way. Every time I fixed one bit, it only improved part of the overall problem, which made it incredibly frustrating.

good luck

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1987 300e manual 250,000 km (sold)