Are you talking about the CLK? If so, I thought the newer models have a different ignition switch which allows you to just tweak the key to the start position and then let go, and the engine will automatically crank for as long as it needs then stop. Could this be what's going on? It's kind of an unusual thing the first time I drove a car with it (a 2001 ML I was looking at.)

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Steve
'93 400E