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Old 04-23-2011, 03:41 PM   #1
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have to prime the tbi before start?

I am having a issue with the car not starting unless I put a little fuel into the tbi then it starts and runs fine. What could cause this? Lifting the Hood and putting fuel in the intake every time I need to go somewhere is getting old. Thanks
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Old 04-23-2011, 09:02 PM   #2
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Re: have to prime the tbi before start?

The 2bbl. tbi units have a regulator that has a small bleed hole, so the fuel pressure will always drop to zero. But the fuel pump has a one way check valve that keep fuel in the lines going to the tbi unit. That one way valve is probably bad. I saw once that they had one way valves that you could add to the feed line, so you wouldn't have to drop the tank to get to the pump.
Have you tried cycling the key a couple of times to prime the system? To figure out the problem you'll need to get a GM tbi fuel pressure tester. Actron makes one, the sell 'em at autozone & other places.
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Old 04-23-2011, 09:18 PM   #3
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Re: have to prime the tbi before start?

OK I will definitely look into that thanks.
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Old 04-23-2011, 10:28 PM   #4
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Re: have to prime the tbi before start?

I saw in your "bringing it back from the dead" post that you had the pump out of the tank. Did you replace the pump? IMHO, if you went as far as to pull the pump, go ahead and replace it while youve got the tank dropped. Its worth 60 bucks or so for me not to have to drop that damn gas tank again..........
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Old 04-23-2011, 10:40 PM   #5
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Re: have to prime the tbi before start?

Fuel Pump Relay - check it and the connector for it. The fuel pump should "prime" for about 2 seconds when the key is turned to ON but car not started - if it doesn't, then the fuel pump relay or it's connector/wires are bad.

The reason it runs with you priming it, is because the Oil Pressure Sending Unit takes over as a back-up fuel pump circuit and runs the fuel pump - if the OPSU reads oil pressure, it will run the pump using a back-up circuit directly from the battery.
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Old 04-23-2011, 10:50 PM   #6
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Re: have to prime the tbi before start?

O I was under the impression that was the only one. Where is the fuel pump relay? Many wires were cut when I got this so that also could be my problem. I replaced the engine harness but under the steering wheel is another story
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Old 04-24-2011, 10:28 AM   #7
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Re: have to prime the tbi before start?

Fuel pump relay is under the hood, driver side, between brake booster and fender on the firewall. There would have been a bracket there, held to firewall with 2 10mm screws. The "inner" part of the bracket has 2 relays, one is fan, one is fp. The fan relay has a large black with red stripe wire, fuel pump relay is the other one.

Now, what's been hacked out in your car I have no clue - but that's where the fuel pump relay was when it left the factory.

The operation ... the ECM sends a signal to the fuel pump relay on a green/white wire, activating the relay. The ECM does this for 2 seconds when key first turned to ON to "prime" the injectors for you (since injectors don't 'hold' fuel like a carb does). Then, when you actually start the car, the inkectors have fuel to spray. Once car is started, the ECM is reading a constant ignition signal, and so it sends a constant signal to he fuel pump relay to amke it active always, as long as the ECM gets that ignition signal. When you turn car off, no ignition signal so ECM stops sending fuel pump relay signal.

GM wired in a backup system using the OPSU - in the event that the relay or ECM went bad, the OPSU was wired so that it has direct battery power to the fuel pump - and as soon as it senses oil pressure, it will turn the pump on and run it as long as it sees oil pressure. But, it takes 10, 20, 30 seconds of cranking sometimes for the OPSU to "sense" any oil pressure and start the pump. Most folks won't crank constantly for that long - but if you do, the pump should start to run and the car should start (assuming the OPSU fuel pump wiring and fuse is all good of course). It just takes the starter some time (10, 20, 30 seconds) to turn the engine over enough to make the oil pump start pumping enough oil for the OPSU to recognize the pressure.

This OPSU wiring for the fuel pump has it's own fuse as well - it's the one located by the battery in a small weatherpack - has one red wire in, one orange wire out - it's usually tucked in the fenderwell behind the battery.

Anyway - that's the factory setup - maybe all that will help you track down your issue and repair so you can stop manually fueling the TBI to start the car. Good Luck!
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Old 04-24-2011, 11:18 AM   #8
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Re: have to prime the tbi before start?

OK that relay is there and because the harness came from my other car I'm going to assume it still works but when it goes to the fire wall does the green wire with a white stripe change color after the firewall or should it be the same? Thanks a lot this is very good information.
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Old 04-24-2011, 11:20 AM   #9
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Re: have to prime the tbi before start?

Green/white wire runs to the pass side, and into the ECM harness to the ECM.

You should be able to "feel" the relay 'click' when the key is turned on. On your car, the fan relay and fuel pump relay should be identical size/shape, and thus interchangeable. Assuming the fan relay is good (does your fan work?), then mark the good one, and swap them - that will let you know if the fuel pum prelay itself is bad, or if there's a bad connector or wire to it.
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350 Carb 10:1 Dometops Mild Cam
Hedmann Headers Hi-Flo Cat/Aerochamber Muffler
700R4 B&M Shift Kit LS1 Disc Rear w/3.42 Posi
Poly Bushings Eibach Prokit Bilstein Shocks/Struts
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Old 04-24-2011, 11:46 AM   #10
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Re: have to prime the tbi before start?

Fan does not work. A ton of wires are cut under the dash. I ran a hot wire to the green and white wire and it fired right up. Both relays are good just the wiring to them are not.
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Old 04-24-2011, 11:52 AM   #11
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Re: have to prime the tbi before start?

Cool - then you at least know what to wire now.

On the fan, if it were me ... I'd just grab a seperate fan controller and wire that in - they are about $30+ at any auto store, and they probe the radiator fins rather than having a temp sensor in the block - so it makes them easy to wire in without using any of the existing (or in your case ... missing) factory fan wiring/relays/etc. They go up in price with features - some are set temps, some adjustable, some have provisions for manual switches, etc.

I run an expensive one - a Flex-a-lite I got from summit, about $90. But it has all the bells and whistles, and is fully adjustable. But cheaper less fancy units are available and work fine.
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350 Carb 10:1 Dometops Mild Cam
Hedmann Headers Hi-Flo Cat/Aerochamber Muffler
700R4 B&M Shift Kit LS1 Disc Rear w/3.42 Posi
Poly Bushings Eibach Prokit Bilstein Shocks/Struts
HOK Candy Teal on Silver Flake IROC Hood
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Old 04-24-2011, 11:52 AM
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