Ignition switch/fuel pump relationship on TBI?

[TylerT]

A couple of weeks ago I started a thread about my fuel pump problem, and now it's happened again. Last time I tried to start it and it kept cranking even after I took the key out. When I got it to stop cranking, I switched the key on to listen for the fuel pump and it was not turning on. I checked it every half hour or so and nothing. The next morning, it fired right up, and it has everyday until yesterday. I went to start it and it cranked, although it didn't keep cranking, it stopped when I turned the key. I left it alone and came back in a couple of hours and listened for the fuel pump. Then my wacko father shows up and he's just a crankin away and having me pour gas into the throttle body. I left it parked in the commissarry parking lot overnight and I just went and picked it up. It started right up like there was never a problem..

So I am stumped..why would the fuel pump not work until several hours of sitting after a failed start attemp? Why is the fuel pump even shutting off like that? Thanks a million if anybody can help. I'm thankful that I didn't have to work on that thing in a grocery store parking lot.

 

[mouse]

The fuel pump should only run for 1-2 seconds initially, and then turn off. The fuel pump relay controls the pump, giving it a short pump to start, then when oil pressure comes up, the oil pressure switch allows the pump to run. You should have a loose wire hanging from the fuel pump relay that will allow you to test the pump.

 

[TylerT]

Thanks Mouse, got it. That's what I mean when I listen for the pump to come on, that 2 second 'humm' after you turn the key. But I'm still having trouble figuring out what would cause the pump to not come on for a period of time, and then be perfectly fine after sitting for a few hours. Would it be a problem with the fuel pump relay..where is that located anyway? On a 92 Suburban..Thanks

 

[mouse]

The oil pressure sensor allows it to run after oil pressure builds up - more than 4 psi. Maybe it has a leak internally. Do you have a guage? If so, and it reads normal oil pressure, you could probably rule that out.

Then I would look at fuel filter, and maybe the sock in the tank.

 

[HarryH3]

On a '92 the fuel pump relay should be under that black plastic cover near the top of the firewall, on the passenger side. If you look close, it's even labeled. Just remove the cover and you can then access the relay.

There are actually TWO things that can trigger the relay. First is the ECM. When the key is first switched "ON", the ECM will turn on the relay for 2 seconds. However, while the engine is being cranked (or is running), the ECM is getting ignition pulses from the distributor circuit. As long as it sees those, it keeps the relay turned on.

Second is the oil pressure switch. The oil pressure switch is simply there as a failsafe, to keep the rig running if the primary circuit fails.

The symptom you're describing is the classic failure mode for the electric fuel pump. Sometimes it works, sometimes it doesn't. A new pump and filter sock will run about $50. Dropping the tank to swap it totally sucks.