This is a problem that has been thoroughly discussed. Try a search, like on "300E AND idle".

Air flow sensor, idle air valve, temp sensor, lambda system, coil or other ignition issue, injectors, EHA, ECU, flakey wiring and more have all been suggested and investigated as causes. Some, including me, never were able to solve the problem.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe