I have a 1991 Mercedes 350SD with 253k miles on it. It starts runs and idles great while cold. When the engine comes up to operating temp it idles with a lumpy rock when in drive or neutral. It will shake the whole car. If I bring the RPM up to 900-1000 with the throttle it goes away. The car has full power when you mash the gas and runs perfectly smooth down the interstate. It does not seem to burn any oil over the last 3k miles. When I got it, it had no power as it had been sitting for quite awhile. The fuel tank was steam cleaned, injector nozzles replaced, transmission rebuilt and fluid changes done. The low power was traced down to the boost controller which was repaired. Everything runs great except this idle issue when hot. Any ideas? I thought maybe engine mounts but wouldn't it idle rough when cold too? Any ideas?

When the idle is lumpy, pull the electrical connector at the round thing at the aft end of the IP. If the idle doesn't get worse or the engine doesn't stall, there's a problem with the idle control circuit (ELR). Maybe a connection point fails as temperature increases.

Sixto
MB-less