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2003 GMC Code#P0300

Discussion in 'Chevy Trucks' started by EGLC, Dec 26, 2013.

  1. EGLC

    EGLC PlowSite.com Addict
    Messages: 1,733

    So my gmc has been flashing engine light and now its on...local shop can't seem to get it right was wondering if any of you could help.

    2003 GMC 2500HD Vortec - 86K miles
    CODE P0300

    Truck just got new spark plugs, wires, coils, fuel, oi & airl filters
    Last edited: Dec 26, 2013
  2. 2006Sierra1500

    2006Sierra1500 PlowSite.com Addict
    Messages: 1,758

    Possibly a valvetrain issue? Did you clean the MAF sensor?
  3. herbert02

    herbert02 Member
    Messages: 70

    I've had that code (random multiple misfire) on my 04 for at least 40,000 miles now. Dealership gave me the run around with possibilities, but really had no idea. I heard from a local shop that these trucks had a problem with the intake warping. They are plastic and if it gets hot enough can warp. My light only comes on sometimes over 60mph. Other than that it runs fine. So I quit worrying about it.
  4. rjigto4oje

    rjigto4oje PlowSite.com Addict
    Messages: 1,380

    Agreed a vacuum leak can cause a p0300 random misfire along with a tone of other things
  5. skostur79

    skostur79 Member
    from ne ohio
    Messages: 67

    do you actual feel a miss?

    some that I have worked on , would set false codes, show misfires even tho it wasn't

    have the shop do a crank shaft variation learn with the scan tool ,
  6. EGLC

    EGLC PlowSite.com Addict
    Messages: 1,733

    You can feel a miss sometimes, its not constant.

    btw injectors were checked, along with compression and fuel system...i will have them check the intake.

    @herbet; the original owner & i experienced those exact symptoms but now it has gotten slightly worse...

    I really only notice VERY SLIGHT misfire while idling....
  7. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,403

    I had a truck that did this once, but it only did it at idle. When you revved it or drove it everything went away. It was enough of a miss to show up on the modis as a #1 cylinder misfire.

    I tried swapping injectors, coils, plugs etc. But couldn't get it the miss to move. Valve springs weren't broken, Vacuum was steady, etc.

    Turns out the cam lobe was actually grinded down. Apparently it was somewhat common because gm had a bad casting? I don't believe that lol

    It went to 4 shops before it came to me - they told him to live with it :nono:
  8. herbert02

    herbert02 Member
    Messages: 70

    Yeah I would sink money into a mythical problem. One day it might not start or quit running. Then I'll find the culprit.
  9. chevboy167

    chevboy167 Member
    Messages: 96

    My 2000 1500 had the random missfire code as well when the intake gaskets started leaking. May also be one or both knock sensors (and/or sensor wiring harness) out spec just enough to set code. Its almost always the rear knock sensor that is crapped out from water/moisture down in the lifter valley cover where the sensors lay. I replaced intake gaskets, both sensors, wire harness and oil pressure sending unit cuz it was out in the open with intake off. Truck idled and ran better than when I got it! YMMV of course..... good luck.
  10. myzx6

    myzx6 Member
    Messages: 90

    +1 dealt with this scenario a few times now mostly on 6.0l
  11. 32vld

    32vld Senior Member
    from LI, NY
    Messages: 622

    2001 5.3 suburban. At 98,000 misfire code. Do not remember the code. I had a bad ignition coil.
  12. starspangled6.0

    starspangled6.0 Senior Member
    Messages: 270

    Same here. Crank relearn fixed it.
  13. ajracing81

    ajracing81 Member
    Messages: 78

    do the relearn!!!!