Hi!



I have a wierd problem that I hope someone knows the answer to.



First of, I have done my research and tried finding an answer by myself. I do also consider myself an okay mechanic(not my trade tho). Have fiddled around with cars and motorcycles for many years and have several engine rebuilds in the book. But the electrical side has always done my head in.



I have a 2001 C230k sports coupe with the M111 engine.



And as it started to get cold here in Denmark I noticed a pattern where it would have trouble starting. The more cold, the harder it was to start. But it didn’t seem like a normal cold engine problem. Tried fixing the parts that normally intervened with temp. But still the same.

After some problem solving I found out that there was no fuel pressure. Until there suddenly was. For no specific reason.

I also found out that the ECU didn’t allow the fuel pump relay to have a ground connection.

As soon as I make a ground for the relay before starting, it fires right up. Regardless of temperature. As soon as the car has “learnt” that it is running. I can remove the ground and the car runs absolutely fine. Also if I was to turn it off again. It only appears with a cold engine. As soon as it is running there is no problem what so ever.



So it kind a seems like it needs to know that the fuel pump relay want ground to run. But “forgets” after sitting for some ours.



Could it be a software problem of some kind, that the ECU does not allow ground?



I have a icarsoft MB2 reader but it doesn’t seem to give any information of parts that could cause the problem.



So far I have fixed:

-Spark plugs

-Cam sensor

-MAF

-Fuel filter

-Fuel pressure valve

-checked for vacuum leaks

-tried using engine start spray. Doesn’t help.



Best regards Jacob.

 

Have you tried replacing the battery?

Anything unusual when the car is cranking?

If the battery is confirmed to be in good condition, try swapping out the relay in question. Hopefully there is an identical one somewhere in the same fuse box.