fuel leakdown

jabez · 12-08-2001, 09:33 AM

I just finished my son's 2056 that was a 74 1.8. I have retained the L-Jet. When cold, the fuel pressure maintains 39 psi without the car running and 30 with it running. When you shut the car off cold, the fuel rail will maintain 20 psi for several hours. If you warm up the car and shut it off, the fuel rail pressure drops to zero. I disconnected the cold start valve and all injectors, thinking one of them was leaking through, but got the same results. I narrowed it down to the discharge check valve in the fuel pump so I got a check valve from Grainger and installed it in the pump discharge. Same results. Could the fuel pressure regulator be letting it bleed through? Any other ideas?

As the handle implies, "Dazed and Confused".

GS

JP Noonan · 12-10-2001, 07:14 PM

Where is the pump located? If it is the stock location you may have gas boiling in the lines (vapor lock).

Will98D · 12-11-2001, 05:16 AM

I went through this on my BMW recently. Pinch off the inlet hose just before the fuel rail after stopping the pump/car, and observe the fuel pressure. Remove the clamp after you get your result and repeat the same thing on the return line that leaves the pressure regulator. On my 535, the pressure regulator was letting the system leak down after stopping. If you find it on your inlet side, maybe it's vapor lock, or perhaps the check valve from Grangier wasn't sufficient. I read on a BMW BBS that BMW has a check valve, p/n 16 149 068 988, and it's only about $4 from the dealer. If either of these tests didn't find the problem, I'd try pinching off both inlet and return hoses just after shutting down and see if there's a drop. If so, it may still be one of the injectors. Good luck.

jabez · 12-11-2001, 06:51 AM

JP - To answer your question, I have moved the 74 pump to the 75-76 position up front.

Will98D - Your procedure was my next step. If it is leaking down now through the inlet, that would mean the new check valve as well as the pump discharge check valve. I am gooing to assume nothing though.

I cranked it up last night and immediately shut it down cold and the pressure dropped to 0. I cranked it again and immediately shut it down and held 20 psi for about 1 hour. I cranked it again and immediately shut it down and it held 30 psi. I left that over night and had about 5 psi left this morning.

GS

Will98D · 12-11-2001, 07:55 AM

BTW: I also have a '74 1.8... you can save yourself the trouble of starting the engine every time by pushing on the air flow meter flap, which will start the fuel pump as long as the key is on.
As it dropped cold at least you are likely not looking at vapor lock, but keep an open mind as perhaps the first few times you started it last night the system was purging the vapor?

jabez · 12-11-2001, 08:08 AM

Will98D - Does your fuel pressure hold at 20 psi for 12 hours or so, or am I chasing ghosts? I have the L-Jet shop manual and the Bosch technical write up and it never says if pressure is supposed to be held in the fuel rail.

GS

Will98D · 12-11-2001, 11:27 AM

I haven't checked the 914 yet, but the BMW holds steady at 2.0+ bar for 2 hours. I don't plan on checking the 914 until next year (winter in MI).

Dave Bell · 12-11-2001, 11:56 AM

My 75 L-jet will hold fuel pressure for quite some time after shut off... never timed it though.

What I did do was was plumb in a small permenant fuel pressure guage in the engine compartment. Great for trouble shooting.

- Dave

Dave Bell · 12-11-2001, 05:13 PM

My 75 L-jet will hold fuel pressure for quite some time after shut off... never timed it though.

What I did do was was plumb in a small permenant fuel pressure guage in the engine compartment. Great for trouble shooting.

- Dave

jabez · 12-12-2001, 02:27 AM

Thanks for your help guys. I have installed the pressure gage and maybe that's my problem, I am over analyzing the problem.

I did use the AFS flap to operate the pump before the car was running and consistantly got a 20 psi fuel pressure after the pump was shut off. I didn't see the problem of the pressure falling to 0 psi until after the first break-in runs.

I am going to continue to test and if the problem continues, I'll replace the regulator.

Will98D - 2 bar = 30 psig? When mine holds it will only hold about 20 psig (1.4 bar) without the extra Grainger check valve, with it it will hold 2 bar.

GS