My 1989 300E has recently started having semi-serious problems. After I drive it for a while, it just shuts off and will not start until I let it sit for fifteen minutes. I have read several reviews on the same model and year: my "diagnosis" is that I need a new fuel pump relay. I just need to know if my diagnosis is correct before I attempt to resolve the problem. Please let me know if you believe I need a new fuel pump relay...if not, what could the problem be? Why or why not could it be the fuel pump relay? Thank you to whoever answers my question(s).

 

it could be the fuel filter... I would try a new one ...

 

the fuel pump relays on those would go and it sounds like one.find out where the fuse is for the fuel pump remove install a amp meter set to 10amp scale run the car the amps should be over 5 around 6-7amp constant then if car dies or will not start check the fuel pump just to be sure tapp on outer housing with a hammer( if you hear a buzzing noise start up and the car runs the fuel pumps are the problem replace and the filter they only last 70k anyways (pumps &filters) ). if nothing then it pretty good guess that relay is dropping power so not enough is getting to pump to run when the relay gets hot which was also common.
good luck!

 

I would also check the voltage regulator it is easy to do (and inexpensive)and fixed the same problem on my 89 300E. I do agree though it could also be thefuel pump relay.

 

I have replaced the fuel filter. The car will crank but will not start. I am not hearing anything from the fuel pump when I try to start the car. Could it need a new fuel pump?

 

I had this same problem on my E320 1999 and it turned out to be the crank shaft position senser. $150.00 and 5 min. to put it.
Good Luck