Mercedes C240 Intermittent Starting Issue

I have an intermittent starting issue with my 1999 Mercedes C240. These cars have a starting procedure that means you have to turn and release the key, at which point it should fire up and the starter motor stop. However, on some occasions, the engine dies immediately, but after another two or three attempts, it will start and run OK. The problem began after the car broke down and was taken to the main dealer who diagnosed a faulty crank sensor.
I took it back to the main dealer, but the problem did not re-occur when the car was in their custody. I then took it to a local BMW/Mercedes specialist, suggesting it may be the crank sensor that was faulty. He was not convinced, but I have a gut feeling the crank sensor is to blame. While the car always fires up eventually, my fear is that one day it won’t start at all. John Measday

You do not mention if any repairs were carried out when the Mercedes broke down and was taken to the dealer. I am presuming that no work (other than diagnosis) was undertaken and, once it had cooled down, the engine started without further problems. Did your local specialist have the Mercedes Star system for diagnosis? This dedicated system can delve much deeper than any generic scanner. I would agree with you that if the main dealers have diagnosed the crank sensor as being at fault, then it is. The symptoms suggest a failing sensor, and although I cannot confirm that this is the problem, it is a likely possibility.
Another possibility would be the fuel pump relay. These also can develop intermittent problems due to poor internal connections. This can easily be checked by listening when the ignition is first turned on – before moving to the start position, the pump should be heard powering up for a couple of seconds. If no noise can be heard, this will indicate that the fuel pump or relay is at fault.

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