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Fuel Pump keeps running...

Discussion in 'The Garage' started by MTBLAZER89, Jan 3, 2007.

  1. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    I finished replacing the pump in my 89 Crew cab. When I switched the tank to that side and turned the key I heard the pump click on and start running, but it didnt stop after a few seconds like the other side does. What controls this? and how do I fix it. The truck starts and drives fine with the new pump. i just want to make sure something else isnt wrong and is going to cause this to fail again because it was NO fun replacing.

    Thanks
     
  2. HarryH3

    HarryH3 1 ton status Author

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    The factory service manuals make mention of some sort of fuel pump module on the 1-ton models that runs the fuel pump for 20 seconds after the engine is shut off. Could this be what's keeping yours going?
     
  3. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    I didnt notice if it keeps going after it is shut off. As soon as you turn the key on it starts to hum, but doesnt stop..the passenger side runs for about 5-6 seconds.
     
  4. 79k20350

    79k20350 3/4 ton status

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    when you turn the key to run it should start up until it builds pressure... If it doenst stop the pump isnt holding pressure...
     
  5. HarryH3

    HarryH3 1 ton status Author

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    That's true for some of the low pressure aftermarket pumps that are used to feed a carb. They shut off when the float bowl is full and the needle shuts off fuel flow. But TBI pumps run continuously, IF the engine is running. TBI and MPFI systems use a regulator at the engine to control pressure, so there is no need to switch the pump on and off all the time to control pressure. The sequence the ECM uses is:

    Key switched on, but engine not running or being cranked - pump will run for 2 seconds then shut off. The pump will start back up when the key goes to the start position. It will also start again if the key is swithced off and then back on, but will only run for 2 seconds again if there is no attempt to start the engine.

    Any time the ECM is getting a reference signal from the distributor (meaning that the engine is running), then the ECM is commanding the fuel pump relay to be on.

    When the key is switched off, the ECM commands the fuel pump relay to turn off. The exception to this is some 1-tons which have an additional fuel module that keeps the pump running for an additional 20 seconds, then lets the pump turn off.

    As a fail safe, there is also an oil pressure switch that bypasses the fuel pump relay and sends power directly to the fuel pump when the oil pressure exceeds 2 to 4 PSI. This is so that an engine with a failed fuel pump relay can still run. It takes more cranking, to build up oil pressure first, but at least you don't have to walk home. :cool1:
     
  6. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    I will check tonight and see if it keeps running after I shut the truck off.


    Anybody else?
     

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