Fuel Delivery Problem

[GMC Hauler]

I recently put in a new lift pump, because under full load, it would only have 1/2# of pressure.

I have a fuel pressure gage in the cab.

Before I put the new lift pump in, I would see 8 Lbs key on, engine off, 4 lbs idiling, and 1/2 to 3 lbs driving.

When I put the new lift pump in, I would see within 2 seconds of turinig the key to on, engine off 13#. I would see 8# idiling, and 2-6# driving.

Last sunday, fuel pressure dropped to 0-1 driving. I figured that the lift pump was a defect, and obtained another one. An external gage connected to the drain line agreed with the gage in the cab.

I changed the pump today, and saw the same results.

I took the filter out, and it was dirty but I wasnt surprised. What shocked me was when I flushed the drain line from the fuel filter assembly, there was sand and small gravel like particles in it (looks to be NO metal)!

I changed the filter. The max I saw at first startup was 8 key on, engine off, 6 idiling. So I went for a ride. Pressure dropped to about 0 over a couple block range. I took it home, and checked the drain line and fuel filter... clean.

Now I turn the key to on, engine off, and have the red connector to power the pump connected to a 12 volt source and pressure very slowly and eratically makes its way up to 11-12 lbs.

An inspection inside the tank revealed it was absolutely clean. Any advice???

[ 03-01-2005, 07:58 PM: Message edited by: GMC Hauler ]

[rjwest]

Sounds like a restriction somewhere

I see you have a 96, There should be a Jumper Connector by the left hood hinge to test pump ( Jumper to 12 Volts to operate pump ). It's in the wire bundle.

When pump comes on does the sound change as pressure builds up. If it " Clacks loudly, it is not getting fuel. ( line restriction to pump, Dirty sock, etc )

If " No Clack " I would guess restriction on pressure side of pump.

How did you check tank ( drop it and inspect )?

Do you get a good flow of fuel out of the drain .


My pressue gage is " tapped In " by a "T" just before pump inlet. Pressure comes up within 1-2 seconds.

Did you bleed the Fuel pressue gage at Indicator (
Assuming mech Gage ).?

[GMC Hauler]

It does sound slightly different for a few seconds and then gets quieter.

I dropped the tank and looked inside. Absolutely clean.

I did get some flow from the drain.

I did a pressure test of all the lines with air... no leaks.

My gage is an electrical with the sender connected with a tee between the filter and the injection pump.

My wife told me the other day the suburban had a problem with crusing ( shudder while light crusing). Since I had just noticed the pressure go down to 0 the day before, I figure it is a fuel delivery problem...
Now I am thinking there is a clogged or bent line between the pump and the fuel filter assy..... I will have to test every line I guess...

[charliepeterson]

A restricted fuel re-turn line can be a problem with delivery also but rare. A failing oil pressure sender can make the lift pump quit as well.

[GMC Hauler]

The oil pressure switch is fairly new. I have bypassed the OPS by connecting to the red conector in the harness by the underhood fuse center, and with the key on and engine off, after the fuel pump and glow plugs go off (via the computer, the pump stays on because I am powering it from the red connector), get the same wierd readings.

i can feel the pump running when i touch it while the engine is runnng.

Is there a screen in the fuel filter manager assembly that could get clogged? :confused:

[charliepeterson]

When you pull the fuel filter up and out the post in the middle, which is the delivery to the injection pump, is supposed to have a very small screen on it. I bet 99% of us don't have it. When removing the filter it can get stuck to the filter and you may not even see it. The new filter won't have this with it. I'm pretty sure this is a Stanadyne part number and not a GM number.

[rjwest]

Screen in inlet to pump. See Note from Qwestav

Take out Fuel solenoid ( to get socket on inlet nuts.) Remove Hose, Take out large Nut,
Screen is under nut, comes out with nut...

[JohnC]

Get a measuring container of one or two pints. Run the engine at idle. Put the water drain line into the container and open the valve. If you get a pint in less than 15 seconds the pump is delivering more than 30 gallons per hour. As long as you have positive pressure you should be good to go. The injection pump has its own internal pump and pressure regulator. As long as the inlet to the injection pump is under positive pressure, there will be no fuel delivery problems.

There are 2 lift pumps, depending on the year. I forget which is which, but they differ in pressure capability. Both deliver (IIRC) 30 gallons/hour. I think the electronic pump uses a 6 psi lift pump.

[GMC Hauler]

I think I have it fixed..... :confused:

I checked the tank, the seals at the lift pump, and all fuel line. Nothing...

I connected a gage to the drain again, and ran the lift pump and also watched my in cab gage.... It turns out that the sending unit for the in cab gage is bad. Pressure tracks on the in cab gage with the temporary gage for about 10 seconds. Then the in gage cab pressure drops slowly to zero. I am guessing there is a hole in the diaphram and it is equalizing. So I never had a lift pump problem.

The thing that led me down this path was the fact that my wife told me the other day the Sub had a couple of "hickups" while driving. I figured since there was no "indicated" fuel pressure at the pump, it could have been the cause of the hickups. There are no codes stored in the computer. The best I can guess is that since I didn't bleed the fuel filter assembly (I had a bad experience with one filter (after I bled one filter, shut the air bleed valve and verified no leaks, it leaked fuel all over my hot running engine a few days later.)) I had air in the system, and this was causing it to "hickup" down the road. Anyone verify this?

[rjwest]

Sounds good, I went with a Mech gage, 1/8 inch line, reacts " quickly " to fuel pressure.

Need to bleed air at gage on install.