Hi im new to this 2008 gmc Sierra 1500 crew cab. 289k 4wd. ?

Hi im new to this, 

I have a 2008 gmc Sierra 1500. With 5.3 NA. What would cause 2 cylinders to randomly drop compression at once. About 30lbs each. I bought a preprogrammed computer. Drove 20 min ran better than ever since i got it (80k) an then just multiple cylinder miss fire. Thanks in advace. I have not checked any further. Also to add the afm was deleted in New computer as well as verified on dash the v8 mode stayed in v8. An the old computer does the same thing as the new. 

1 Answer

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  • Anonymous
    3 months ago

    Compression does not "randomly" drop.  Do a compression and leak down test.  Valves are likely shot.  If compression is good, you simply might have a random misfire.  Check fuel pressure and check for vacuum leaks.  Check for a sticking EGR valve.

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