My cousin's 300sd has a hot idle shake. When its cold or accelerating it, the engine is silky smooth. Its a 1983 300sd with a 1979 300sd motor in it. The motor has good compression, new glow plugs, valve lash set, popped/balanced injectors and all new mounts. In addition, the fuel filters are all clean as well as the tank screen plus it has a new primer pump.

So.... you can imagine our frustration when the car seems to 'lope' or shake when its warm. The idle is set properly at approximately 750rpm.

Now my question... Given that everything else has pretty much been looked over twice or thrice and replaced what could the culprit be????

Pardon my ignorance but did the w116 300sd 's come with a manual choke? I recall seeing some sort of throttle adjustment knob on the dash of one of those cars. Could the PO who swapped the 79 motor into the 83 overlook this issue? If so how could I go about fixing this. If not, what else can we try to address the hot idle shake?

BTW the motor has approx 250k on it.

Thanks!

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Hot rough idle is a classic sign of a rack damper bolt that needs adjustment or replacement. It is on the rear of the injection pump, look down between the oil filter housing and the IP. There is a lock nut and an adjusting bolt. It is usually adjusted on a warm engine.

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