fuel pressure fast leak down

Tell me what you guys think ...

1997 Jimmy 4.3 - the truck is a little sluggish to start up. It takes a few seconds before starting.

Battery is new but I charged it just to be sure. Serp belt is new. Fuel filter is new. Distributor cap is new and inside of it looks fine. When turning the Ign key, the inital sound of the fuel pump whining/priming sounds normal.

Here's my fuel pressure test: Cold in the morning: Ign ON, Eng OFF:

1st turn of Ign: 40 psi then leaking down to 20 quickly.
2nd turn: 62 then leaking down to 20 quickly
3rd turn: same as 2nd
4th turn: same as 2nd

Startup and Idle: 62 psi then down quickly to 55

While driving: stays at 55. Will fluctuate btwn 55 and 64 when revving or sustained throttle.

There is no pressure or spray at the Schraeder valve after the truck sits for awhile.

So far the truck still drives. It's just been sluggish to start over the last 10 days. I haven't noticed it long enough to monitor fuel consumption/mileage ...

Fuel pressure regulator? o-rings?

Any help would be appreciated. Thanks.

I went through the same thing last fall.

Mine ended up being a leak in the small plastic hose that runs from the pump itself to the top of the plastic cap on the fuel pump assembly. But mine didn't hold pressure at all.

With yours holding some pressure, I'd guess pressure regulator , under the manifold on '97s.
To be sure, you'll need to test the reg. with a gauge tool. I made mine for ~$25.00. It's a shut-off valve installed right after the fuel filter, once the pump primes (not starting the motor), shut the valve off and look at your schrader valve gauge. If pressure holds, regulator is good. Open the valve, if pressure releases, it's in the pump assembly.

Here's the link
(Thanx Old Master for the tip/link).
http://www.automotiveforums.com/vbulletin/showthread.php?t=925619&highlight=fuel+pump


Good Luck.

Here's a few thoughts.

To see if the fuel pressure is the starting trouble turn key on twice or more before starting of course.

The pressure leak is either inside the manifold or going back into the tank (if we assume it isn't leaking out onto the floor) To differentiate between the two , repeatedly turn key on every so often for an hr or so. After this start vehicle as normal and if the leak is inside manifold then the truck will start very rich, and have black smoke coming out of pipe. If it still starts normal then the leak can be assumed to be going back to fuel tank and is pump or regulator perhaps allowing it to return thru the return line.

Most likely the check valve in the fuel pump is leaking, but the only way to know for sure is to install the shut off valve that Mike mentioned and run the test. Typically if the fuel pressure regulator or an injector is leaking, the pressure will drop to zero. Let us know what you find.

Thanks Mike2004 and Old Master for your help. I made the valve tool. The manifold holds its pressure when I close the valve. When I open the valve the pressure bleeds down fast. It looks like something on the tank and pump side ... I haven't dropped the tank yet.

Old Master, is it possible to repair/replace a check valve without replacing the whole pump? Check out this link ... what about an inline check valve like the one on this link ... ?? ever tried one?

http://www.redhillsupply.com/Fuel-Line-Check-Valve-for-38-Lines.htm

If the check valve is leaking back you need to replace the hole pump moduel to fix that part.

A pump leak back/down usually does not affect the pump running pressure.
But is a pain on cold start up.

I would check full fuel pump pressure first.

A good pump will produce 95-105 lbs full pressure.

If not you will need another fuel pump anyway.

If you need a fuel pump only get a Delphi OEM type pump.

Guiide lines for a fuel pump test.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

Thanks Mike2004 and Old Master for your help. I made the valve tool. The manifold holds its pressure when I close the valve. When I open the valve the pressure bleeds down fast. It looks like something on the tank and pump side ... I haven't dropped the tank yet.

Old Master, is it possible to repair/replace a check valve without replacing the whole pump? Check out this link ... what about an inline check valve like the one on this link ... ?? ever tried one?

http://www.redhillsupply.com/Fuel-Line-Check-Valve-for-38-Lines.htm


Your diagnosis is correct, something in the tank is allowing the pressure to bleed off. It could be the check valve in the fuel pump, or the flex line from the pump to the module housing. In your first post you show a prime pressure of 62psi and that is within factory specs. If the flex line is leaking, the pump will not be able to build maximum fuel pressure, (75+psi). Maximum fuel pressure will continue to decrease as the size of the leak increases. I've never tried an external check valve but whatever is leaking in the tank, the external valve will stop the leakdown. The only question I would have, is how much the check valve will resrict fuel flow volume. Keep us posted.