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  1. #1

    Default 2001 Seirra 1500 5.3 Rough Idle

    Hi I have a 2001 GMC 1500 With the 5.3. I have a constant rough idle, that doesn't seem to change weather engine is hot or cold. My LT fuel trim is up to 17 and drops to about 9 above idle with light acceleration. Misfire monitor shows misfires on all cylinders and does not seem to favor any one cylinder. Misfire count dosen't get high enough to turn on the Mil, and no DTC's are stored. I have tried cleaning the injectors twice with the Motorvac. I also replaced the intake plenum gaskets. Injector balance test showed a 2 psi variance, but was inaccurate due to the engine being hot (Heat soaking). Also cleaned the Heat element in the MAF. Fuel pressure at idle I believe is about 50-55psi. Exhaust was chopped and glass packs were installed by previous owner, but cats were untouched. O2 Sensors appear to be working properly and cycling from high to low. Any help would be greatly appreciated.


  2. #2


    change your spark plugs and your wires. That will cause a miss fire and change the fuel filter. That should help.
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  3. #3


    ya plugs and wires are the next area, you have checked pretty much the fuel side and if everything checks back as good and functional then check your ignition. its very simple for you since the 5.3's have coil packs. the only thing since you have a constant misfire on all cylinders are plugs. lemme know what happens

  4. #4


    Well i fixed it. The plate that the EGR mounts to was cracked between the two hoses allowing exhaust to flow into the intake even though the EGR valve was closed. Causing my misfire and the elevated fuel trim. Also my map had been reading at 49 kPa at idle. I replaced the EGR Tubes/plate. My fuel trims are down to 3% at idle and my Map is up to 39kPa at idle, and of course my misfire is gone.

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