GM TBI gurus, projunktion swap - Pirate4x4.Com : 4x4 and Off-Road Forum

halfgassed · 11-09-2007, 03:28 AM

quick explanation:
What I have is a 87 chevy 3/4ton, this is my plow truck/ work truck. The truck had a TBI 350 in it till I blew it up (pulling my boat). SO I decided to put a 454 I had sittin in the garage in its place. I still wanted to keep the truck fuelie so I installed a Holley Projection 4 barrel fuelie system. Seeing the truck is already a fuelie I figured I wouldn't have to hook up the holley fuel pump and I could just run off the factory chevy in-tank pumps. I removed the factory TBI pieces but I left the wiring harness in place in case I decide to go back.

Heres the problem I came on last night
-When I turn the key into the on position the stock fuel pumps (either of the two tanks) don't turn on.

My question
-Is there some sort of sensor/hookup that sends a "feedback" signal to the factory computer that tells the fuel pump to kick on ? ANd this needs to be hooked up in order to get fuel?

ANy info related to this would be appreciated
Thanks
Matt

gottagofast · 11-09-2007, 07:19 AM

first you shoulda kept the gm tbi.... second the computer controls the pump via the fuel pump relay.....wire up the pump the way the holly instructions tell you too.... You may well need the holly pump too, personally I'd go back too the gm stuff... All you need is the proper throttle body and computer chips for the 454

Badburn · 11-09-2007, 07:43 AM

I am far from an expert on this, but I will try. I believe there is a fuel pump relay that recieves a signal from the computer when you turn on the key. There is an oil pressure switch on mine, The oil pressure switch turns off the fuel pump when there is no oil pressure. Not sure if yours is the same, but mine is a 1990 3/4 ton suburban 454. Hope this helps.

Bubba_Jeep · 11-09-2007, 08:37 AM

The GM ECM turns on the fuel pump for two seconds, via the fuel pump relay, when you turn the "key" on, but don't start the engine.
That's test "one" to determine circuit functionality.
If that's a go, then the ECM again, via the fuel pump relay, turns the fuel pump on while the engine is cranking (ECM senses this), AND the ECM is receiving ignition pulses.
It's possible for the relay to be bad/incorrectly wired, and still have fuel pump run; however, the pump would not come on untill the oil pressure builds high enough to close the pressure switch (part of the oil pressure sensing switch). Again, the ECM needs to sense ignition pulses at the same time.

gottagofast · 11-09-2007, 10:27 AM

is there an echo in here?

11-09-2007, 03:28 AM	#1 (permalink)
halfgassed Registered User Join Date: Oct 2005 Member # 54792 Location: marquette michigan Posts: 19	GM TBI gurus, projunktion swap quick explanation: What I have is a 87 chevy 3/4ton, this is my plow truck/ work truck. The truck had a TBI 350 in it till I blew it up (pulling my boat). SO I decided to put a 454 I had sittin in the garage in its place. I still wanted to keep the truck fuelie so I installed a Holley Projection 4 barrel fuelie system. Seeing the truck is already a fuelie I figured I wouldn't have to hook up the holley fuel pump and I could just run off the factory chevy in-tank pumps. I removed the factory TBI pieces but I left the wiring harness in place in case I decide to go back. Heres the problem I came on last night -When I turn the key into the on position the stock fuel pumps (either of the two tanks) don't turn on. My question -Is there some sort of sensor/hookup that sends a "feedback" signal to the factory computer that tells the fuel pump to kick on ? ANd this needs to be hooked up in order to get fuel? ANy info related to this would be appreciated Thanks Matt

11-09-2007, 07:19 AM	#2 (permalink)
gottagofast Registered User Join Date: Apr 2003 Member # 19031 Location: louisville KY Posts: 1,078	first you shoulda kept the gm tbi.... second the computer controls the pump via the fuel pump relay.....wire up the pump the way the holly instructions tell you too.... You may well need the holly pump too, personally I'd go back too the gm stuff... All you need is the proper throttle body and computer chips for the 454 __________________ Kevin [URL="http://www.pirate4x4.com/forum/showthread.php?t=567624"]My build thread[/URL] Killed the S-10 n2 jeeps now

11-09-2007, 07:43 AM	#3 (permalink)
Badburn Registered User Join Date: Feb 2004 Member # 27377 Location: Shingle Springs, CA Posts: 251	I am far from an expert on this, but I will try. I believe there is a fuel pump relay that recieves a signal from the computer when you turn on the key. There is an oil pressure switch on mine, The oil pressure switch turns off the fuel pump when there is no oil pressure. Not sure if yours is the same, but mine is a 1990 3/4 ton suburban 454. Hope this helps. __________________ 78 Blazer, Dana 60/14bolt, crossover steer, front and rear ARB's, beadlocked 39.5 Irok's, 465/205, smallblock 400, TBI. "...........and that was without a single drop of rum!"- Captain Jack Sparrow

11-09-2007, 08:37 AM	#4 (permalink)
Bubba_Jeep Registered User Join Date: Oct 2004 Member # 37184 Location: Santa Barbara, CA Posts: 1,323	The GM ECM turns on the fuel pump for two seconds, via the fuel pump relay, when you turn the "key" on, but don't start the engine. That's test "one" to determine circuit functionality. If that's a go, then the ECM again, via the fuel pump relay, turns the fuel pump on while the engine is cranking (ECM senses this), AND the ECM is receiving ignition pulses. It's possible for the relay to be bad/incorrectly wired, and still have fuel pump run; however, the pump would not come on untill the oil pressure builds high enough to close the pressure switch (part of the oil pressure sensing switch). Again, the ECM needs to sense ignition pulses at the same time. __________________ 88XJ, Chevy 5.7L, Edelbrock MPFI, NV4500, Atlas, D44's, high steer, ARB's, 7" custom semi long-arm 3 or 4-link lift, BFG 33x12.5-15's.

11-09-2007, 10:27 AM	#5 (permalink)
gottagofast Registered User Join Date: Apr 2003 Member # 19031 Location: louisville KY Posts: 1,078	is there an echo in here? __________________ Kevin [URL="http://www.pirate4x4.com/forum/showthread.php?t=567624"]My build thread[/URL] Killed the S-10 n2 jeeps now

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