'81 Euro 635csi loses fuel pressure overnight

[Vrmmm]

Gents, I am having an issue where the car loses fuel pressure overnight. It has to crank for 10 seconds or so when cold to build from 0 up to around 40psi fuel pressure, then it starts fine. It will maintain enough pressure to restart (within a few hours) pretty quickly, but after sitting overnight my temporary fuel gauge proves that the pressure does indeed go back to zero.

I recently had all 6 injectors rebuilt, but I do not know how to find or diagnose the cold start injector that I am thinking may be the culprit here? Any thoughts or suggestions?

Thanks in advance!

[RSheiman]

Sorry, duplicate post. You stated car and year in title........

[RSheiman]

Assume when you turn the key (without starting the car) you should hear a hum from pass rear, near the tank=fuel pump priming the line. The pumps should have a built in valve to prevent gas flow from draining back when the car is off. Unsure if that is the case with an 81 Euro pump (my 86 US version does). It does sound like the fuel is leaving the system and it has to go backwards.

If you want to cheat, buy a one way valve and install in the gas line near the firewall. See if that changes things. Valve should be high pressure ( most on Ebay are for carbed cars with low PSI). This is a easy test before spending more money and time replacing the fuel pump.

CS injector should have nothing to do with the issues you are having and is the last thing you want to play with due to it's horrible access. Reminds me of Boston roads.....you can see it but you can't get there from here.

[Vrmmm]

I had not thought of a one way valve. I will try that. Thanks Rob!

[sixerguy]

There could be a leak anywhere along the fuel route so systematically test for pressure at various segments. The O-rings on the fuel injectors or fuel pressure regulator could be leaking, the cold start could be leaking, as could the hard or rubber lines themselves. Also the fuel injectors may be leaking - not closing. As mentioned the main fuel pump has a check valve, but that valve can go bad. After pressurizing the system you can clamp off the rubber lines on the fuel rail for the delivery and return line to see if the fuel rail, injectors, fpr is holding pressure independent of the feed and return. While your testing - depending on age it might be a good time to replace all the rubber o-rings and feed/return lines in the engine bay. I just did mine and removed lines installed in the 90's.

[RSheiman]

We want follow-up so other members can learn, silence since 8/22. Problem solved or sold?

[Bert Poliakoff]

I have an 81 635 Euro with Moronic 1.0 When restoring the car, I put a new fuel pump in a new FPR and a new filter as well as the soft lines. I have had no such problem Oh and my gas tank is vented as original My thoughts go to the FPR