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  1. #1

    Default C1500 Backfires Seems affected by Air Temperature?

    1991 C1500 with 5.7l/350 TBI, 154k miles. Runs fine when air temps are low but in the afternoons it backfires through the exhaust and has loss of power under load like accelleration and going uphill. I've replaced the alternator, battery, coil, plugs and wires. New distributor about 6 months ago and radiator, thermostat and temp sending unit recently. It's rather aggravating and I can't seem to figure it out. Any help would be appreciated.


  2. #2


    a backfire is usually caused by an air leak in the exhaust system. check around make sure nothing is rusting out

    05 silverado shortbed

  3. #3
    Sr. Apprentice dannys99's Avatar
    Join Date
    Mar 2010
    Northern California
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    How about you fuel filter and pump. Or your lines, are any lines near a heat source. From my experiences, backfires are usually due to a lean fuel mix or retarded timing. Maybe some kind of vapor lock or plugged injector. When the engine is cold the computer makes the mix rich and when it warms up it leans it out a bit Maybe the advance in your distributor is not working correctly.
    "If I were lie'n I'd be cry'n and I aint shed a tear"

  4. #4
    Master Mechanic daddytech's Avatar
    Join Date
    Mar 2008
    Portland TN


    sounds like a timing issue to me. a 1991 you can still time them with the timing light check your timing. that truck has the ODB1 connector so unless you have an old connector the only way you have to tell if the computer is throwing a code for something is to jumper the upper two far right jumpers to get the codes and count the flashes. remember there is a single and double flash at first (code 12 showing the computer is functioning) and after the 2 code 12's the lights will flash any other codes. if nothing else flashes other than code 12 check your timing and run some seafoam through the Vacuum line on the power brake booster and keep reving a little till the smoke stops coming out and see if that helps any.

    that stuff is like gold cleans all the cabon and varnish out of hour cylinders and helps clean out your Catalytic converter too.

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