Don't know much about your engine but the fuel injection system's the same as the 300E I think. If that's the case, sounds to me like the Idle Control Valve.

Most likely cause is a dodgy OVP relay, second most likely is the ICV is gummed up inside, third most likely is a bad/intermittent connection somewhere between the ECU to OPV to ICV, worst case scenario your ECU is cutting out power to the OPV/ICV circuit.

Do you have Anti Lock Brakes? Does the warning light ever come on? If so, its your OVP relay which runs on the same circuit.

Otherwise, you need to check power to the ICV while the engine is running. Should see about 6v constant across the terminals. You can, apparently, check the current as well but that's never worked for me. You can also hear the ICV click into running position when its powered up. If there's no current, the unit drops into limp home mode which allows an idle of around 550rpm, enough to idle but not enough to maintain idle on deceleration.

Try a search on ICV/OPV and wade through it. There's hundreds of posts on this one subject alone. Even some diagrams so you know where to find everything.

Good luck

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1987 300e manual 250,000 km (sold)