I`ve had this C240 for two years and its never failed to start until about 3 weeks ago.It takes a good 5 to 6 seconds hanging on the starter motor.It tries to fire and eventually does but struggles.It sounds like a carb engine trying to start with the choke not working.I have put a new Pierburg fuel pump,a fuel injection water temp sensor and a crank sensor on.Before this starting problem appeared it had already had a new catalytic converter,mass airflow sensor,two lambda sensors,12 new plugs,fuel filter,and a compression test.This was to get the engine to pass the emissions test.This problem came on gradually over a week or two from about 3 out of ten starts to virtually every time.when it finally starts and you switch off and start again its fine,but leave for an hour or so its back to grinding away again.Any ideas please.

 

sounds more like sticky fuel pump relay,relay is found in the trunck next to the spare wheel its like squre block in black......