2000 Jimmy fuel pump problems

fuel pump not working in neighbors jimmy, diagnosed fuel pump not working, relay picking and no pressure. Installed new pump, drove it around 4 days and then problem reoccurred. noticed that when you turn on key the fuel gauge pegged out as if the sending unit circuit was open. relay picking, getting power at plug for about a second when key switch is turned on. Installed another fuel pump. Still not starting, do not hear fuel pump coming on. what gives? sending unit is reading correct now, how long should i get power at pump to prime, what would cause the power to come on and then shut off so fast, maybe oil pressure fuel cut off??

Oil pressure switch doesn't cut off fuel pressure in these trucks like that. Pump should immediately build 60PSI+ in the fuel system with the key on/engine off. Get a pressure guage and test it. Could be another issue with fuel system leaking down somewhere lse.

Do you mean, oil pressure does not cut off fuel flow ever or just not on start up? and pressure test shows 0psi, pump is now not coming on. I believe I am dealing with a new problem with second and third pump than i had with 1st pump. I am positive that this vehicle has a oil pressure drop fuel cutoff switch, just not sure when it engages and what disengages the fuel pump relay to allow for the oil pressure cutoff switch to then be the only path of current to fuel pump. Does this happen during startup? Or does this happen after startup, giving the oil pressure time to stabilize. I have manual on vehicle and I can clearly see this in the wiring schematics. And in response to your Idea of pressure leaking down, I'm aware of the problem this vehicle has with the spider getting hairline cracks in them causing pressure to bleed off, but I,m sure that this is not the case here. Thank you very much for your response, please keep them coming.

Oil pressure does not do anything untill oil pressure come up and even then it is not a shut off but a saftery switch to keep fuel pump running if the fuel pump relay fails.
You should get a 2 second fuel pump prime up key on.

Then on engine cranking the fuel pump should run while cranking.
And fuel pressure should jump to 65 lbs. pressure.
Check the fuel pump for 12 volts power on the gray fuel pump feed wire.
And for good ground to fuel pump.
That gauge acting up can be a ground problem
Also 12 volts on orange feed to fuel pump relay.
Key on 2 second prime up or engine cranking.
Post back if fuel pump is getting 12 volts on gray wire key on for 2 seconds and 12 volts on engine cranking.

Also confirm good spark when cranking.
No spark = no fuel pump kick in.

What brand of fuel pump are you feeding it?
If a Airtex or aftermarket dump it in the trash can and get a Delphi oem type fuel pump.

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.

Another possibility

pull the carpet back on the drivers front. the wiring harness is right under your feet
you rest your right heel on it while driving. i have seen more than one rubbed thru
doing stupid stuff intermitently. it has a plastic cover on the top but the bottom rests
on the floor. fuel pump, rear lights, rear wiper, air bag module are on that harness.

Everyone thank you for you help. another problem solved. Intermittent ground to the pump. Installed a local ground to chassis and problem is now solved. thanks a million

You are welcome and thanks for posting back the fix.

To help other people that may have same problem.
Tell us how you found the bad ground?

I tested for the bad ground by testing for voltage across the positive and negative of the fuel pump plug, no voltage present, then I tested with the positive(red) lead of my meter in the plug and the negative lead(black) to the chassis. Now I could see the voltage was present on the positive lead, which lead me to the conclusion that I was not getting a reading on the first test because of the absence of a ground in the plug.

Thanks for posting back the test and fix.
Good luck