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  1. #1

    Default 1990 GMC Sierra 1500 Running Real Bad

    5.7 L, 100K miles. Bought this truck last week. Idles great, but no power.
    Check engine light was on, indicating bad throttle sensor and knock sensor.
    Replaced o2 sensor, EGR Valve, knock sensor and throttle sensor. Has new gas tank and fuel pump. replaced fuel filter. No more check engine light, but still no power. EGR solenoid checks out. Diagnostic computer doesn't find anything out of ordinary. Checked for vacuum leaks. Has new exhaust including catalytic converter on back. New distrib. cap and plug wires.

    Later, found leaking gasket around back side of throttle body. Replaced gasket. Still running bad.
    Tested fuel pressure on engine side of filter. Reads 17 PSI.
    Check-Engine light comes on once in a while. Code indicates engine is running lean.

    Any ideas what is still causing lack of power? Runs like only using half of the cylinders.

    Thanks in advance for the suggestions.

  2. #2


    mine was running bad and just quit on me. I changed the fuel filter and cap and rotor, plugs and wireset and it's like a new truck
    90 Stepside 4X4
    98 Cadillac Eldorado ETC
    07 Pontiac Solstice GXP

  3. #3


    make sure you gapped the plugs properly. Shhould be at .035". it's not much, but it's a start. A guy on gmt400 gapped his at .045" because he was told it was better performance wise, and he said it ran like complete crap.

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