does anyone know how to bypass the oil pressure sensor to actuate the fuel pump? or maybe just wire the fuel pump direct somewere? i'm havina a problem with starting. have to crank it 20 times till the oil pressure builds then it starts and i'm tired of buying 50 dollar switches at gm. also as the oil gets more used i have to dump gas in the throttle body to get it to start:confused:

It sounds like your fuel pump relay is not working.

The fuel pump relay is what should be supplying power to the fuel pump for startup. When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through one of the oil pressure sensors.

When you turn the key on without cranking the engine, do you hear the fuel pump run for a few seconds? It should.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse.

- The key is turned on engine not cranking – relay applied for 2 to 20 seconds, depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.

The key must be turned off for a certain amount of time (usually about 30 seconds) before it will prime the system again when the key is turned back on.

The relay is, under the hood, on the firewall, on the passenger side of the center.

Here is a illustration if the link works. It's for a 1992 and #3 is the fuel pump relay, but many trucks do not have three relays in a row. Your plastic cover that covers this area (if it's still there) will also tell which relay is for the fuel pump.

http://z.about.com/d/autorepair/1/0/5/O/fuel_pump_relay-860.jpg

Source:
http://autorepair.about.com/library/illustrations/bl433lib.htm

Outstanding post ChevyTech

Thanks Mike

I wonder if duffy has seen it yet.

i just read your reply and i also read in the manuel that i should unplug the fuel pump relay while its running and if it shuts off then its the oil pressure sensor. i have found or at least it appears to be....if it don't start in the morning i crank it till the oil pressure builds then shut the key off for 20 or 30 seconds and it starts. i appreciate your help chevy tech. do i need to change the fuel pump relay or as the book says the oil pressure switch?

i just read your reply and i also read in the manuel that i should unplug the fuel pump relay while its running and if it shuts off then its the oil pressure sensor? If you pull the fuel pump relay while the engine is running it should keep running. If it dies the oil sender or its wiring is bad or the engine lacks oil pressure.

If you unplug the oil sender while it is running it should keep running also. If it dies the fuel pump relay or it circuit is bad.

Keep in mind there is a redundant circuit (two circuits) supplying power to the fuel pump when the truck is driving down the road.
1 – The oil pressure sender
2 – The fuel pump relay

i have found or at least it appears to be....if it don't start in the morning i crank it till the oil pressure builds then shut the key off for 20 or 30 seconds and it starts. ?
I have a couple thoughts here:
1 – IF you stop cranking the ECM will de-energize the fuel pump relay. Then you crank it again and the relay is applied again. If the contacts in the relay are burned, de-energizing and re-energizing the relay may get the contacts to make a connection.

2 – Cranking it until it gets oil pressure is what you have to do if the fuel pump relay has failed so if you crank the engine enough the engine should start.

3 – Sometimes when the fuel pump is failing the currant flow through them go up considerably. This tends to, damage many of the parts of both of the fuel pumps redundant circuits, melt and damage the connection at the fuel pump, fuel tank sender and wire harness. Also when the pumps are failing, often if you hit the bottom of the tank it will help get the pump to start pumping.

So one is never positive what is wrong until they have the problem fixed.

i appreciate your help chevy tech. do i need to change the fuel pump relay or as the book says the oil pressure switch?

If you turn the key to the run position (not cranking, just to run) the fuel pump should run for a few seconds. If it does not run, the fuel pump relay circuit is not doing it job. Usually it is the relay that goes bad.

If fuel pump relay and its circuit are working correctly, the oil sender or its circuit could fail and you would not know it, unless you test it, because the truck should still run.

This oil pressure sender also supplys the oil pressure guage with it's readings so if it were bad it would show up on the guage....right?

The oil gauge and the power for the fuel pump use the same sender but they are separate in the sender. The gauge uses one of the wires and the metal housing (ground) for the other connection. Power for the fuel pump uses the other two wires. So the gauge can work ok while the portion for powering the fuel pump does not.

well i'm starting to think its low oil pressure because it runs 60 cold and 15 hot. could be a little loose on the bottom but if i can figure out how to bypass this fuel pump thing it's gonna run till it explodes

15 PSI is not to low if the sender is not failing. I believe the threshold pressure is 4 PSI to close the switch to make the fuel pump run. 15 PSI is not so low that the engine will fail, it could run many more years.




Edit:
If you turn the key to the run position (not cranking, just to run) the fuel pump should run for a few seconds.If you turn the key to the run position (not cranking, just to run) does the fuel pump run for a few seconds??

yes...i had the pump pressure checked. i don't seem to have a problem with this untill the summer months if that means anything. i get aggrivated and unplug the fuel pump relay 4 or 5 times with the engine stalling everytime but then it seems i'm ok for a week or so. if i replace the pressure switch it seems to help for a couple months then starts over

if i replace the pressure switch it seems to help for a couple months then starts over


3 – Sometimes when the fuel pump is failing the currant flow through them (edit)will go up considerably. This tends to, damage many of the parts of both of the fuel pumps redundant circuits, melt and damage the connection at the fuel pump, fuel tank sender and wire harness.

The problems with the circuit could easily be caused by the fuel pump. If you have the equipment, you could test the currant draw.

How old, how many miles are on the pump? Is it a GM pump? Is it Original?

it is the same pump thats been in it as long as i've had it and thats 6 years. there's 175000 miles on the truck and the reason i bought a new pump is i thought it was bad but the mechanic said it was fine. are you saying a circuit in the pump it's self could be bad? i wonder why it doesn't do it all the time them? it seems it does it after the oil gets hot

are you saying a circuit in the pump it's self could be bad? Yes, This does happen to TBI trucks. And if you replace the pump take a close look at the fuel tank sending unit when you have it out. The connectors sometimes get damaged.

i wonder why it doesn't do it all the time them? High currant flow can cause the contacts in the relay and the sender to get burned and pitted. Then when they close sometime they will make a good connection and sometimes they will not.

it seems it does it after the oil gets hot And the connects in the parts are hot (burning pitting) from running a long enough to get the engine hot with high currant flow taking it's toll on the parts.


I can only give you possibilities, for what might be causing the problem, and tell you what I have seen happen to trucks like yours.

Think of an old distributor with points. As the points get more and more pitted the engine runs worse and worse until they are so burned they will not allow currant to flow through them at all.

well i finally sent this thing to the local garage at 8 am...they called at 9 and said there was a short between the fuel relay and the pcm and it would take an hour to trace.called me at 10 and said it was the pcm,i explained i'd changed it not long ago but they insisted that was the problem so i bought a pcm and handed it to them at 11am. never hear nothing till 530(and they close at 5). the bill was 120 bucks for 2 hours work and on the bill i noticed they changed the fuel pump relay...at no cost. kinda makes me think i bought a pcm for nothing but at least it starts

Thanks for the update.

My guess is the fuel pump relay was the problem.

Hopefully it will give you no more problems.

thanks for all the help chevy tech. i appreciate it.