Fuel Pressure Low Before Start - Runs Fine

[Rupert8]

Hello,

My 1992 Eddie Bauer Explorer has just begun to stumble when I am trying to start it, but only when it has sat for a few hours.

I tested the fuel pressure: with the ignition on (and the fuel pump running) the fuel pressure is 10PSI. When I get it running (after one or perhaps two attempts) it fires up fine and the pressure is then 30-35PSI.

When I remove the vacuum line from the pressure regulator, the PSI climbs to 45ish.

The system holds pressure at around 35PSI after I shut off the engine.

Does the initial low pressure after the car has sat for a while indicate that the fuel pump needs to be replaced?

I have serviced the car (spark plugs, filters etc. including fuel filter) recently.

Yet another thing going on this Explorer.... at least this forum is a great resource.

Many thanks for all the input.

 

[Roadrunner777]

I am going to tell you the problem with 100% confidence because I just fixed it on my explorer last week. On the fuel pump assembly, there is a return line that ends in the tank with a rubber tip. There is a slit in that tip that should be closed when the engine is not running. Yours is not sealing up. So, the fuel in the lines is draining back to the tank. The delay is the time it takes for the pump to refill the lines.

So, the pump is fine, you have a bad 25 cent rubber part on the pump assembly. In the trade, it is called a duckbill valve, and when you see it, you will know why. I have no idea where to find one at the retail level. My pump assembly was under warranty, so I just swapped the whole thing. Happy hunting!

 

[Rupert8]

Thanks for that information. This in interesting.

One question, though: when I switch on the ignition, shouldn't the fuel pump bring the pressure back up to a point where the engine starts if I give it enough time? No matter how long I leave it with the ignition on, the system only reaches around 10psi and the car won't start first time.

So on a normally-operating system, the fuel system would remain pressurized, even overnight?

Thanks again for the input....

 

[Roadrunner777]

The pump only runs for about one second when you turn the key to on. If you repeatedly turn the key off and on, it will eventually fill the lines and build up pressure. There is a contact on the ODB test port that, if wired to +12, run the pump continuously when the key is on.

Should there be pressure after it is left overnight? Good question! The answer should be... only as much as the duckbill valve will allow. This assumes the injectors are not leaking, of course.

How much pressure should that be? I don't know but I am willing to find out because I am curious about it now, and I am pondering writing a thread on the fuel system.

At this point, I feel my system is 'normal'. It starts immediately (less than 1/2 second), and I know my pressure is correct at idle. It fluctuates when I goose the throttle, so I know my regulator is working.

I will connect a gauge after I am done driving for the day and run it for a minute to take care of any pressure loss due to the gauge and connection. I'll record pressure drop after I shut it down, including a reading tomorrow morning. This should give myself and others a template to work from.

 

[Roadrunner777]

I spent some time today actually tinkering with the fuel system, and I need to correct myself... trying to catch every place I mentioned this. This problem has nothing to do with the duckbill check valve. The duckbill valve is downstream of the fuel pressure regulator, so it does not come into play here. That leaves three possibilities: The check valve inside the fuel pump body, leaking fuel injectors, or a problem with the fuel pressure regulator. Since replacing the fuel pump assembly fixed my problem, I'm going with that.

 

[Rupert8]

Thanks again. Useful information.

So does anyone know if 10PSI is an abnormally low fuel pressure after sitting all night then switching on the fuel pump with the ignition?

I have replaced the fuel pressure regulator and am leaning towards replacing the fuel pump but I don't want to dive into that project if I don't have to....

Thanks again.

 

[Roadrunner777]

Yes, that's low. Mine came right up to 30 PSI after resting overnight, and just turning the key to on.