1999 GMC seirra misfire


Asked by Apr 12, 2016 at 02:09 PM about the 1999 GMC Sierra 1500 SL Standard Cab LB

Question type: Maintenance & Repair

I replaced my cap rotar and wires and plugs
and my truck is still misfiring but if I put it to
the floor it stops or if its just idoling

2 Answers


I'd be sure the coil pack has proper grounding...1999 tells me there is some corrosion on the connection of the Black cable to the chassis ground...and to the engine...them cables must pull 40 amps...also if nearly 10...your battery will no longer have the same potential and will not charge up very easily anymore...so battery gettin' thru is our problem....put all NEW clean stuff in there and you will be happy!~


Provided that your tune up went perfect, (and that the vehicle acted the same way before your effort) you need to locate the cylinder that has the "Misfire." Please consider that the "Misfire," could in fact be a faulty/clogged injector, as these will commonly seem like an ignition issue. Once again you need to locate that particular cylinder...then pull the plug on that one...inspect for lean or rich condition. You should be able to determine where the problem lies. Now, don't forget that you may have a cylinder which is low on compression (that's why the truck feels ok when you get to higher revs...it carries that cylinder with the momentum of the others). Could be a bent/burnt valve...and I hope not...but something to consider.

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