Symptoms look like thermal instability of either the cam shaft position sensor or the crankshaft position sensor. Sometimes these sensors can fail by outputting reduced voltage with temperature increase so they appear to works fine at start up but fail when the engine reaches running temperature. I think the the cam sensor is Hall effect and the cam sensor magnetic:
2 pin socket (magnetic - sine wave output). One pin is 'ground' the other is
'signal'. A voltmeter set to 2 volts AC (note) should measure a signal in the 0.2 to 2 volts range on the 'signal' pin. 3
pin socket (Hall effect, magneto - square wave form output). One pin is 'reference' (5volts), one pin is
'ground' and the third pin is 'signal'.
A DC (note) voltage should be detectable in the 0.5 to 1.5 volt range from the
'signal' pin.
It is important that the voltage measurement
take place at start up when the engine is cold and again, 20 minutes later,
when the engine has fully warmed up to operating temperature.
These devices do not often fail and are expensive if purchased from the the Mercedes dealership, shop around on-line or try a breakers yard.
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