Fuel Pressure Not Maintaining

[Dannk]

With the key in run, engine off, my fuel pressure drops from 55 to 0 within about 15-20 seconds. Causing hard starts but truck runs great...If I time it just right, truck fires right up (i've become pretty good at it). I just replaced the filter tonight, but don't think it solved the problem. Regulator??? How do I check the regulator to verify it's faulty?

If I listen closely near the gas tank, I can hear what sounds like fuel returning to the tank as the pressure is dropping (while engine is still off).

I know this is simple, but just don't know where to look!!!!

[Dannk]

Please Help!!

[rlith]

You don't mention what year/engine you have... but... If it's a CPI (92-95) Vin W, Pull your upper plenum, look for signs of washing...(where one side is cleaner than the other) If the passenger side is washed, it's your CPI unit, if the drivers side is washed, it's your nut kit... If they are both equally dirty than it's the check valve on the fuel pump... It's messed up... This can happen sometimes. When you pressurize the system, the check valve is supposed to keep that pressure for a while, when the check valve go bad, no pressure can be maintained without the assistance of the fuel pump. So it takes a bit extra to start it... Replace the fuel pump and sending unit, you'll be fine

[Jeremy-WI]

Originally Posted by Dannk

With the key in run, engine off, my fuel pressure drops from 55 to 0 within about 15-20 seconds. Causing hard starts but truck runs great...If I time it just right, truck fires right up (i've become pretty good at it). I just replaced the filter tonight, but don't think it solved the problem. Regulator??? How do I check the regulator to verify it's faulty?

If I listen closely near the gas tank, I can hear what sounds like fuel returning to the tank as the pressure is dropping (while engine is still off).

I know this is simple, but just don't know where to look!!!!

It could be a faulty regulator or fuel pump dampener and they're both about $70. The fuel system test says to pinch the return line to the tank where it is plastic hose and see if the pressure still drops but not to let the fuel pressure exceed 75 psi.

According to the manual, the only way to single it down to the regulator is to drop the tank and inspect the fuel pulse dampener(no cracks or not partially disconnected) and then continue the test. But if you had the right fittings to do a pressure test at the fuel filter you could diagnose whether it was a problem in the tank or the pressure regulator

[Dannk]

It's a 96 4.3L Vortec in a S-10 SS Pickup...

[Dannk]

IF I pinch the return line AFTER the regulator, and it still leaks down - it would be a bad regulator?? If I pinch the return line after the regulator and it does not leak down, it would be the fuel pump dampener??? Correct??

[Jeremy-WI]

There are two ways the fuel pressure regulator could leak down pressure, bad spring= leak back through return line, bad seal= leak into intake

Without an inspection of the in tank pump/dampener or a fitting to hook pressure guage to fuel filter fitting you can't really rule out the pulse dampener/fuel pump assembly. It usually easier to pull the bed off to check the fuel pump assembly compared to dropping the fuel tank

[Dannk]

Originally Posted by Jeremy-WI

There are two ways the fuel pressure regulator could leak down pressure, bad spring= leak back through return line, bad seal= leak into intake

Without an inspection of the in tank pump/dampener or a fitting to hook pressure guage to fuel filter fitting you can't really rule out the pulse dampener/fuel pump assembly. It usually easier to pull the bed off to check the fuel pump assembly compared to dropping the fuel tank

I am definitely going to try this. And let me say, thanks very much for the help.

One more question: Can I use my pressure gage on the fitting on the supply line to detect a bad regulator if I pinch the return line (why do I have to put the gage on the filter)?

I wish I had a circuit diagram of the fuel system!!

[Jeremy-WI]

If you put the gauge on the filter you can find out if it is the pump assembly(Fuel pump or pulse dampener) that is leaking down by eliminating the pressure regulator from the test. If the presure holds with a meter on the fuel filter connection you would be able to eliminate those components and the only thing left really is the regulator. The fuel you hear running into the tank could be coming in through the return line from regulator or coming back through the supply line due to a cracked pulsator. What circuit diagram do you want electrical?

[mjohn]

again, is it a Wseries...Vortec engine...4.3?
the 8th VIN Number is a W?
Does it start perfect after you have been running the engine...but, if let to set...for 15min. start hard again? Poor gas milage?

[Jeremy-WI]

Sorry, you're right, was thinking about another thread. If you pinch the return line and the pressure holds, its the regulator

[Dannk]

Originally Posted by mjohn

again, is it a Wseries...Vortec engine...4.3?
the 8th VIN Number is a W?
Does it start perfect after you have been running the engine...but, if let to set...for 15min. start hard again? Poor gas milage?

It's a 1996 4.3L Vortec, CSEFI, Wseries.

As soon as the fuel pump stops, pressure falls to 0psi within about 20 seconds - so no, it will not start perfect anytime UNLESS I time it just right when I know the fuel pressure us up (turning key on for 2 seconds and just after I hear fuel pump stop - START ENGINE).

Gas mileage is the same as it has always been, no black smoke, no MIL, runs great.

I am just about ready to replace the stinkin regulator unless anyone has any other ideas?????

[rlith]

it's either the injector unit, or a faulty check valve in the tank...Both can cause this... If you're driving fine AFTER it starts, then most likely its the check valve in the sending unit...

[Jeremy-WI]

If crimping the return line keeps the fuel pressure from falling, replace the fuel pressure regulator

[Dannk]

Originally Posted by Jeremy-WI

If crimping the return line keeps the fuel pressure from falling, replace the fuel pressure regulator

OKAY, just crimped the return line after system pressure built to 62 psi. As soon as pump shut off, pressure still dropped. Must be bleeding back into the tank via the supply line - back through the faulty check valve...RIGHT???

NOW, I just have to develop fittings to get my pressure gage to fit on my supply line before the filter. This will prove the check valve theory - thank you rlith!


May be easier to put an aftermarket in-line check valve in than to drop the tank!!!!

Any ideas??

Will keep posting until this thing is fixed...Thanks again fellas.