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Quick TBI Question

Discussion in 'The Garage' started by SkysTheLimit, Feb 15, 2007.

  1. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    Does the stock fuel pump on a TBI truck run at anytime without the motor actually running? Ie, key in run position but not having the motor started?

    Also, it's not controlled by the computer or anything right?

    Thanks
     
  2. George_Pimpdaddy

    George_Pimpdaddy 1/2 ton status

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    Not controlled by the ecm i beleive and its on with key on before motor starts
     
  3. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    cool. thanks for the quick reply!
     
  4. JEBSR

    JEBSR 1/2 ton status

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    When you initialy turn the key to the run position it will run for a few seconds to prime and then shut off. It runs when you are holding the ign switch in the start position, and then continue to run as long as the engine is running. It is controlled by the Fuel Pump Relay. There are several thing tied into the relay that send signals to start the pump, the relay is where it gets it's power from though.
     
  5. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    Hmm.. I'm planning out my install of a pressure regulator since I've gone carb during my engine swap. Part of installing the regulator is to test it by running the pump to build pressure so you can adjust it where you want. I guess as long as the ignition stuff is unhooked and the starter is unhooked, i can turn the key to start and monitor the pressure.

    Anyone used the Aeromotive regulator #13204 with the stock EFI pump and have some tips?
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    The ECM controls the fuel pump prime, (what you hear when you turn the key to run before start) and is what controls fuel pump relay operation normally.

    If the relay fails, the oil pressure switch takes over.

    Since you've added more info, you can force the relay to run the pump constantly by giving the red wire that hangs off the relay by itself, 12V. This is for testing only of course, not a way to rig it.
     
  7. JEBSR

    JEBSR 1/2 ton status

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    It will only run for a few seconds though. And I am not sure, but if the fuseable links aren't connected to the starter you may not get power. Hook everything up and then you can manually jump the relay to get constant KOEO power. Look in any repair manual and the troubleshooting or electrical section will be able to show you what two wires to jump.
     
  8. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    It also runs for several seconds after shut off. For what purpose I'm not sure. I suspect to keep pressure up momentarily for an immediate restart...but just guessing.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    I'd have to check more closely, but I'm sure I never hear mine run.

    Some (all?) of the 454's have that hot fuel relay in them, it and what you describe may be something to keep the fuel cycling just long enough to prevent "vapor lock" in a hot environment for a restart as you said.
     
  10. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    Given that I don't have an ECM anymore, how should I wire the pump to operate? Hook the relay wire to a switched 12V source?

    EDIT: FWIW, I never heard my pump run in all the years I owned the truck.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    If the relay has failed, the oil pressure switch takes over, so the engine is turning before the pump even starts to work, so you'd perhaps never hear it if that was the case. You'd have long starts as well.

    My pump is pretty loud (I believe it's 75PSI) and the TPI system operates right about 43PSI, so it's going to tend to be louder than TBI pumps anyways. But I have no problem hearing it when the engine is running. It's a distinct sound.

    I'd wire it with a relay, and use an accessory terminal off the fuse block to run it. Probably won't hurt to have it running whenever the key is in run or start, just another power draw if the engine isn't running. With the later fuse panel I assume you have, there should be some accessory terminals that are hot in key in run/start only, and not in accessory.

    Fuse the wire from the fuse panel to the relay.
     
  12. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    Makes sense Dorian. It used to have to crank a few seconds before it'd fire. It was otherwise fine.

    I'll go ahead and wire it up to the fuse block then. Another excuse to go ahead and start re-wiring the truck...

    It really doesn't even end does it? :crazy:

    Thanks for the all replies and info.
     
  13. Cricket

    Cricket 3/4 ton status

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    Yep.

    My current gerator pump is on the frame rail and I hear everything it does.
     
  14. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Thinking about it, my 7747 would run for 2 to 3 secs maybe? But the EBL runs it for perhaps 6 or 7 secs maybe more.
     
  15. Cricket

    Cricket 3/4 ton status

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    3 seconds to prime for the 7747 sounds right. The only time it ran longer was during -18 degree weather. I believe my fuel lines froze because the pump sounded horrible. That may be the big advantage for in tank pumps - cold weather starts.
     

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