Just recently my 300CD stopped shutting down when I turn the key off, so I have to open the hood each time and shut it off using the throttle stop. Can anyone tell me how to diagnose/fix this problem?

 

I experienced a similar problem with my 1995 E300 a few years ago. When I took it to the dealer's they ran some tests and told me the vacuum pump had gone bad. That was replaced, and the problem went away. Other problemsI experienced with the engine not shutting off were hard steering and hard brakes (almost like no power steering, and no power brakes). I hope this helps. My regular mechanic was away at the time, but when I mentioned the symptoms to him after having it repaired, he told me it was probably the vacuum pump. If he had been around at the time, I would not have had to pay the additional $180 or so for the diagnostic tests.

 

Look around the engine oil filter canister for broken, disconnected vacuum hoses. Trace each hose in that area to confirm no split hoses or dangling hoses. This engine requires vacuum be applied to shut off the fuel. No vacuum, no shutting off of fuel unless the throttle lever is pressed to shut the fuel off. Also, please add the year and type of Mercedes to your signature and/or profile.