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Fuel Injection Hot idle

Discussion in 'The Garage' started by k20, Aug 31, 2006.

  1. k20

    k20 3/4 ton status

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    Ok, had this problem ever since I put the motor in, oh, two years ago. But it never really mattered until now. Truck starts fine cold. If it turn it off pump gas, refire, its fine. But if the truck has sat between 2.5 & 4hrs, when I start it, it will fire up, then bog down, sometimes choke down, and others just idle extreeeeeeeemely low, like so low, I dont know how it doesnt choke down.

    Just so happens, now, I get out of class exactly 3hrs after I get to school, so I have to deal w/ this everyday, and pushin the brake w/ my left foot & resting my foot on the gas is getting old fast in our crowded lot. :doah:

    What could be the problem? Call it an 87 TBI 454 for diagnosis.
     
  2. loafer

    loafer 1/2 ton status

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    Have you tried checking fuel pressure when this happens? Specifically check your "resting" fuel pressure, or how long the system holds pressure after you shut it down. If the pressure drops too quickly, and the TB or fuel line is still hot than the fuel can boil and vapor lock. This is not very common, but with a custom plumbed fuel system it could very well be a problem.

    Your sig says you have MPI not TBI??? If it is MPI the fuel in the fuel rails could be boiling if the pressure drops. A friend of mine at work has a MPI plane, and this exact thing happens to him. He has to let the plane cool down before he can restart it as the fuel rail gets completely vapor locked.


     
  3. 1979jimmy350

    1979jimmy350 1/2 ton status

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    does the problem go away after you drive it a little? Do you have your fuel pump on a seprate switch or does your computer controal it?
     
  4. k20

    k20 3/4 ton status

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    The system will hold some pressure overnight. It will hold 45psi or so for 4 or 5 hours. It is MPFI, but for the sake of sensors & computers, its TBI. One of those custom deals that you have to decipher lol.

    And yes, the problem goes away after you drive it, some days itll do it for 30 secs, others it will do it for 3 or 4 mins.

    The fuel pump is on w/ the ignition/computer. I dont manually turn it on.
     
  5. loafer

    loafer 1/2 ton status

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    Ok, sounds like you can scratch vapor lock and fuel delivery off your list.

    Are you using an aftermarket ECU that is programable?
     
  6. k20

    k20 3/4 ton status

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    Nope, factory ECU, w. a reprogrammed chip.....Doesnt make any sense to me....I remember reading on here 3 or 4 months ago someone having this problem, now of course I cant find the post....argh...
     
  7. Blue85

    Blue85 Troll Premium Member

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    Can you read any information from the ECU? A common problem is that the air temp sensor is at an elevated temperature, so the ECU retards timing and cuts back on fuel to compensate. What temp sensor are you using? The open-nose air temp sensors are much faster acting than the closed-nosed ones. Switching to this one would make the problem go away faster, but wouldn't eliminate it. You could also relocate the air temp sensor somewhere further forward (air filter?) so that it measures the ambient air temperature as quickly as possible.

    Is this a standard TBI ECU? It probably doesn't use the IAC unless the engine is really cooled off. Maybe one of the TBI guys can comment on this, but it seems like you could get some cals that allow IAC at higher initial temps.
     
  8. 1979jimmy350

    1979jimmy350 1/2 ton status

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    truck TBI does not use a IAT sensor at all
     
  9. Blue85

    Blue85 Troll Premium Member

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    Really? How does it compensate for different temperatures outside?
     
  10. k20

    k20 3/4 ton status

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    It doesnt, I equate TBI to be a computer controlled carb lol.
     
  11. 1979jimmy350

    1979jimmy350 1/2 ton status

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    it ignores it, when i data log a TBI truck with datamaster the value is fixed at -40. But what makes it even weirder is that f-bodys that had TBI do run the air intake sensor.
     

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