# 2003 GMC Code#P0300



## EGLC (Nov 8, 2007)

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


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## 2006Sierra1500 (Dec 28, 2011)

Possibly a valvetrain issue? Did you clean the MAF sensor?


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## herbert02 (Jan 18, 2011)

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.


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## rjigto4oje (Oct 27, 2010)

herbert02;1697664 said:


> 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.


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


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## skostur79 (Oct 20, 2012)

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 ,


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## EGLC (Nov 8, 2007)

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....


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## Whiffyspark (Dec 23, 2009)

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


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## herbert02 (Jan 18, 2011)

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


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## chevboy167 (Dec 7, 2011)

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.


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## myzx6 (Nov 17, 2013)

skostur79;1697712 said:


> 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 ,


+1 dealt with this scenario a few times now mostly on 6.0l


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## 32vld (Feb 4, 2011)

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


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## starspangled6.0 (Dec 3, 2013)

myzx6;1699738 said:


> +1 dealt with this scenario a few times now mostly on 6.0l


Same here. Crank relearn fixed it.


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## ajracing81 (Dec 19, 2010)

do the relearn!!!!


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