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Thread: TBI Coil GM Diagnostic tests?

  1. #1
    EFI GearHead ! EagleMark's Avatar
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    TBI Coil GM Diagnostic tests?

    Anyone have the GM Test for TBI coil?

    My kids truck drove home and won't cold start? Has fuel, has weak spark/yellow. Checked timing #1 and rotor is pointing at 1 so timing chain did not slip. Plugs are not wet?

    Really the only thing I can see wrong is a yellow spark from coil wire or at spark plug. Not the sharp blue snap I'm used to?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  2. #2
    EFI GearHead ! EagleMark's Avatar
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    Found this on line and sure enough the coil failed 2 of three tests. Replaced the coil and before I even tried to start the truck I checked spark. BLUE! Snap, crackle type blue! Truck fired right up.

    I knew yellow spark was no good... but when you find yellow spark you replace and/or adjust points? So I was confused?
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    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  3. #3
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    For many years GM's HEI produced beautiful, sharp, blue sparks. They'd jump over an inch gap, too. But the spark I usually see on the Vortec engines is not so clear cut. There's a yellow or yellow-orange portion as well and I'm not sure why. But the spark's still got plenty of energy and it's more than happy to jump a 1" gap to your hand if you happen to to be in the shortest path to ground.

    Often the coil will pass those resistance tests but will have an internal hole in the insulation which allows spark to leak to ground instead of being forced through the plug. Sounds like yours had a good, old fashioned failure though.

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