Fuel Problems.. tbi - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chevy4ever · 02-06-2009, 01:19 AM

Ok guys. It started about 10 days ago. I had let the truck sit for about 10 minutes and when i went to start it back up it just cranked and cranked and would not fire up. I was in the parking lot of a auto parts so i went and got a can of starting fluid and it started right up. That was the first time i have ever had problems with starting. Well from there it now happens every time the truck sits for awhile. its 350 tbi 90 burb. Ok, basically this is whats going on... from what i know and have been told when the key is on position it should engage the fuel pump for 5 seconds to prime and then shut off. Then after the oil pressure builds up to a certain point ( i think around 30 psi) it re-engages the fuel pump. Well its not getting that first signal to prime. So as i try and start the truck it doesnt have fuel right away.. it has to wait till the oil pressure builds up and that can take 45 seconds. Its not the relay.. ive checked it with a known working one. Replaced 18 yr old fuel pump and did a new fuel filter. I have a friend who swears its the computer behind the glove box. That the same thing happened to his jimmy. Its no longer sold thru dealership but the local junk yard has it for 30. Any ideas guys on if this is the way to go next? Any info would be great.

Storm Chaser · 02-06-2009, 09:31 AM

Well, if it's not the relay and it's obviously not the pump (since it does start with help), it would seem that the signal to prime is the problem. That can either be a bad ECM or a wire issue. Before buying an ECM, trace the wire to make sure it didn't get corroded in half or broke/cut/pinched. Also check the plug at the pump and at the relay for corroded terminals.

I can't remember which wire it is (a Haynes manual will help), but a quick way to check out the wiring would be at the relay plug- feed 12 volts to the terminal that goes to the pump. If the pump activates this way, you know everything from the relay connector to the pump is good.

ChevyTech · 02-07-2009, 08:36 PM

If the fuel pump relay is bad the computer should have stored a trouble code 54.

A code 54 happens when the computer does not see power on the circuit from the fuel relay to the fuel pump, when the computer is sending power to the relay to apply the relay.

Have you checked the ECM for codes?

If you have unplugged the fuel relay and turned the key on there could be a false code set by doing this.

The ECM controls the fuel pump relay.

The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 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 before it will prime the system again when turned back on.

When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender. This circuit is independent of the fuel pump relay.

It only take a few pounds of oil pressure to supply power for the fuel pump through the oil sender/switch.

02-06-2009, 01:19 AM	#1
chevy4ever Registered User Join Date: Aug 2002 Location: Corpus Christi, Tx Posts: 156	Fuel Problems.. tbi Ok guys. It started about 10 days ago. I had let the truck sit for about 10 minutes and when i went to start it back up it just cranked and cranked and would not fire up. I was in the parking lot of a auto parts so i went and got a can of starting fluid and it started right up. That was the first time i have ever had problems with starting. Well from there it now happens every time the truck sits for awhile. its 350 tbi 90 burb. Ok, basically this is whats going on... from what i know and have been told when the key is on position it should engage the fuel pump for 5 seconds to prime and then shut off. Then after the oil pressure builds up to a certain point ( i think around 30 psi) it re-engages the fuel pump. Well its not getting that first signal to prime. So as i try and start the truck it doesnt have fuel right away.. it has to wait till the oil pressure builds up and that can take 45 seconds. Its not the relay.. ive checked it with a known working one. Replaced 18 yr old fuel pump and did a new fuel filter. I have a friend who swears its the computer behind the glove box. That the same thing happened to his jimmy. Its no longer sold thru dealership but the local junk yard has it for 30. Any ideas guys on if this is the way to go next? Any info would be great. __________________ email is matthew.solar1984@gmail.com 1980 Malibu Classic 2door(sold) 8-( 2001 Silverado single cab 4.3 V6(stolen and the totalled) 1979 GMC sierra swb stepside 350(sold) 1990 chevy suburban 4x4 350 2001 tahoe 4x4 2001 suburban 4x4

02-06-2009, 09:31 AM	#2
Storm Chaser Registered User Join Date: Oct 2006 Location: In the Indiana Corn Posts: 449	Re: Fuel Problems.. tbi Well, if it's not the relay and it's obviously not the pump (since it does start with help), it would seem that the signal to prime is the problem. That can either be a bad ECM or a wire issue. Before buying an ECM, trace the wire to make sure it didn't get corroded in half or broke/cut/pinched. Also check the plug at the pump and at the relay for corroded terminals. I can't remember which wire it is (a Haynes manual will help), but a quick way to check out the wiring would be at the relay plug- feed 12 volts to the terminal that goes to the pump. If the pump activates this way, you know everything from the relay connector to the pump is good. __________________ 1988 R2500 Crew Cab Immediate plans- get it running! Future plans- Rear disc swap & Hydroboost Dually conversion with shackle flip-n-switch 4x4 conversion with 52" springs Maybe some new body panels Who needs a car when they make crew cab trucks?

02-07-2009, 08:36 PM	#3
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: Fuel Problems.. tbi If the fuel pump relay is bad the computer should have stored a trouble code 54. A code 54 happens when the computer does not see power on the circuit from the fuel relay to the fuel pump, when the computer is sending power to the relay to apply the relay. Have you checked the ECM for codes? If you have unplugged the fuel relay and turned the key on there could be a false code set by doing this. The ECM controls the fuel pump relay. The truck has a fuel pump relay that supplies power to the fuel pump when: - Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking) - The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 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 before it will prime the system again when turned back on. When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender. This circuit is independent of the fuel pump relay. It only take a few pounds of oil pressure to supply power for the fuel pump through the oil sender/switch.



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