Rough/Hard Idle at Cold Start, fine after ~1min

Discussion in 'General Chevy & GM Tech Questions' started by MightyMax, Oct 7, 2010.

  1. MightyMax

    MightyMax Rockstar 100 Posts

    '91 Silverado, 5.7l v8
    Recently (within the last week) when the truck has sat over night, I turn it on, and it is very rough and knocking....I give it gas and after about 1 min, it will settle down.
    I start it while warm, and it is fine. Only does it when cold.

    This video this guy did of his truck
    Is what it mine sounds like when cold, when he goes to the side of the truck w/ the camera.

    His repair turned out to be the MAP sensor tubing/hose.
    I will check mine...

    Just wondering if there is any other ideas??

    I have recently done a rebuild on the TBI, so I don't believe it is a leaky injector.
  2. silveradotrailblazer

    silveradotrailblazer Epic Member 5+ Years ROTM Winner 5000 Posts

    Check the IdleAircontrol valve.
  3. MightyMax

    MightyMax Rockstar 100 Posts

    He is new.
    I changed him out w/ my last batch of problems I had.

    Starting/ignition system is new (plugs, distributor, cap, rotor, etc..)
    IAC is new
    TPS is new
    TBI has been rebuilt and cleaned.
    ECTS is new
    Fuel Pump is new
    Fuel Pump Relay is new

    MAP sensor has not been changed.
  4. MightyMax

    MightyMax Rockstar 100 Posts

    Hmm...well this problem went away as quickly as it came..
    So no more rough/knocking start up.

    But another problem.
    I am driving down the road last night, cruise control on, set at 50 MPH.
    Engine just shuts off. Dead.
    I put it in Neutral and started it back up...fired and was fine.
    Have not done anything with it today.

    Now I have had issue where I would have to disconnect the wire harness from the Fuel Pump sending unit, located near the tank at the truck frame. Because no fuel was being sent to the TBI.
    Disconnect the wires and reconnect them and it has been fine.
    Had to do it twice......before I did that though, it was doing a similar thing like it did last night.
    Where I would idle or be driving and it would just cut off.
    So maybe the issue is just creeping back up again, and I will have to do the whole disconnect/reconnect of the wiring harness.

    But my question is could the Oil Pump or Oil Pressure Switch be playing into the problem?

    I have read that once a vehicle is started, the Fuel Pump relay is out of the picture, and control of the Fuel Pump is controlled by the Oil Pressure switch.

    My Oil Pressure guage shows the truck between 40 - 60 while driving.
    When I sit at a light and idle, it will drop to between 20 - 30....this is once the engine is warmed up.

    So any ideas??

    I really would like to get this thing straightened out, I hate driving at 50mph and the engine cuts off.
    I just need a better understaning about how the fuel pump system works, at startup and while running.

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