EFI: Fuel Pressure Drops Quickly

[Jambi]

I just got done putting a newly rebuilt EFI 351W in my Bronco and, while it starts and idles ok, my fuel pressure drops to zero almost as soon as the key is turned off. When I turn the key to "On" the gauge measures 38-40, but as soon as the pump clicks off, the pressure immediately drops to zero. Also, while the engine is idling, pressure maintains at 25psi and I know it should be closer to the 35 - 40 psi range.

I have a brand new in-tank high pressure fuel pump (from BC Broncos). I'm also using a Fox Body mustang fuel rail with brand new pressure regulator, and the BC Broncos braided fuel line kit with pressure gauge. I have the pressure gauge positioned right before the fuel rail.

I checked the fuel pressure regulator and there is no gasoline present.

I checked the resistance of each injector (twice) and I'm getting 16.2 ohms. I also checked for a leaky fuel injector by removing the the rail with the injectors seated in the rail, then pressurized the rail to see if any fuel was leaking out of any of the injectors, and there was no fuel leaking.

What else can I check to determine why I'm not getting adequate pressure while the engine is running, and why the pressure drops to zero immediately.

 

[jim3326]

Sounds like a vacuum leak to the regulator. But I wouldn't think it would lose pressure when shut off. Losing pressure would be the check valve. So My best guess would be the regulator.

 

[McLeod]

pinch off the fuel return line and see if the bleed off slows down or stops

 

[Broncobowsher]

Pinch the return line, pressure should spike. Shut down the engine. If the pressure stays for a little bit you can shut down the engine first then pinch. Or even pinch and cycle the fuel pump. If the pressure stays, you have a pressure regulator that is bleeding pressure. May be new, doesn't mean it is any good.

Same trick for the supply line. If the gauge is on the pump side of the pinch, and the pressure drops, the check valve in the pump isn't holding.

The only thing left is injectors dribbling fuel out.

Keep in mind the volume of fuel needed to hold pressure in the fuel system is very little. Just the volume of the expansion of the fuel lines. maybe 1cc of fluid, 2 if you have a lot of rubber line. So not a lot of fuel past anything will drop pressure.

Fuel pressure on the older port style EFI is 42.5 PSI pressure drop. That is what the vacuum reference is for. Maintain a steady pressure drop through the injector. That will maintain a predictable flow rate that the computer is programed for. So 10" of vacuum is about -5PSI, so 37.5PSI indicated. And 20" of vacuum is about -10PSI so 32.5 PSI indicated on a pressure gauge, but still the 42.5 pressure drop across the injector.

And don't rule out the gauge may not be accurate either.

 

[Jambi]

Since my fuel lines are braided coming out of the fuel tank, stainless steel running along the frame, then braided to the fuel rail, what's the best way to pinch the return line?

 

[Broncobowsher]

Make a rubber jumper. Disconnect the braided, insert the rubber, pinch the rubber,

 

[nvrstuk]

Just make sure the rubber one is the EFI rated fuel hose... hate to blow the line and spray gas everywhere... not as easy to work with for a simple bypass but a great idea and easy for you to check.

 

[DirtDonk]

...with brand new pressure regulator

Hate to say it, but that was my first thought as soon as I read this. Bad regulator right out of the box. Happens all the time.
Of course it can be a check valve in a fuel pump too. Not unheard of either.

No guarantee of course, but it is a common problem.

Paul

 

[jmhend]

I agree with Paul and the others here.

I had a similar issue but mine was the pump. I had the pump mounted to the rear crossmember above the pickup. The pump was a pusher not a puller so it was dropping pressure whil it was trying to get fuel.

Good luck troubleshooting.