Polo35m

Hey guys I've got a high mileage 08 S550 that has developed a starting problem when cold. The car will not start until after three or four attempts. I put a screw-on type pressure gauge on the fuel rail to monitor the pressure and sure enough overnight the pressure drops to zero. As soon as you press the start button, it immediately shoots up to 55, but doesn't always start until three or four attempts. Then it is fine for the rest of the day. So my question is, does the fuel pump itself have a check which maintains pressure or is there a check valve somewhere in line that I should be looking for? I'm considering just replacing the fuel pump on Principle as the car does have 160,000 miles. Clearly I don't think this is a problem with the pump not working or the relay for the ECM or the DAM as the pump is kicking on when the start button is pushed. Again, the problem seems to be it is losing pressure overnight. Any help greatly appreciated.

Plutoe

Before you waste your money---look at the various fuel pressure specs for the car in MB WIS, if not you will struggle with your FUYAA

Polo35m

Well unfortunately I'm not subscribed to Wis but I think I'm just going to replace the fuel pump...it's only $45. Another MBZ site indicated that the fuel pump itself does have an internal check valve so that sounds like a pretty inexpensive easy try. By the way what does FUYAA stand for?

tusabes

Polo35m

Thank you. Yes, you are correct and I replaced that last year. It was actually clean as new but it had the tiny micro crack in the top resulting in fuel smell that is common with these cars. If the fuel pump doesn't do it I'll change that next but the fuel pump is just so easy I think it's stupid not to.