02 GMC 1500 Code PO301

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  • #12801 Reply


    I have a 02 GMC 1500 truck with a 5.3 that barely runs until warmed up. Has a 301 code (random misfire). If i park it inside it starts and runs fine, or if i plug it in it runs much better than without. I took it to GM and they threw about 600 bucks at it changing plugs, wires and a censor that i do not have details on. Still the same issue. The catalytic converter is new as well. Maybe the intake manifold gasket is shot? Please help.

    REPLY: Yeah, I would bet on the intake gasket leaking (vacuum leak) and causing your misfire. The code P0301 is not a random misfire code, that would be a P0300. The P0301 is telling you there is a misfire on #1 cylinder. The corners are where the intake gaskets usually suck air from so that is the most probable cause of your issue. This also happens mainly when cold, and after it warms up the o-ring material on the gaskets expand and seal up the leak which then makes the truck run fine. Get those gaskets replaced and you’ll be back in great running order.

  • #16196 Reply


    Thanks for the correction on the codes. It is in fact a 300 as opposed to my original claim of a 301. Is there any way to check if the manifold is cracked once i have it off? Maybe it would be visual? I hear people are going ahead and purchasing after market aluminum units instead of using the original plastic ones. What do you suggest? I will let you know how things work out after the repair.


  • #16197 Reply


    I’ve probably done 50 or more intake gasket sets to those engines and have yet to see a cracked intake manifold. I would take a look when you have it off for sure, just to check, but you’ll probably be fine keeping the original unit. I would definitely recommend using GM gaskets, and not any type of aftermarket set. I’ve seen the Fel-Pro gaskets leak almost immediately after replacement on some. Stick with the GM parts and you’ll never go wrong

  • #16025 Reply

    Lisa R Wiley

    After replacing front driver side caliper and bleeding breaks, the break petal goes to the floor.

  • #16033 Reply


    I must assume you know the proper way to bleed brakes. May be silly, but you have no idea how many people think they know how to bleed brakes, but do not.

    Crack open the bleeder on the caliper, let it gravity bleed for a few minutes. Close the bleeder.
    Press the brake pedal several times, hold it down, crack the bleeder.
    You should notice the brake pedal drop when the bleeder is opened.
    Close bleeder then release pedal.
    Do this several times.

    If the pedal never drops when opening the bleeder, you are not getting any pressure in that caliper. You may have a bad caliper.

  • #14037 Reply

    Brandon miller

    So I have a 07 gmc envoy with 4.2 vortex. My wife keeps up with all maintenance at the dealership. Drove it last week to do some running ran just fine. No misses, ac worked, had no loss of power and wasn’t hard to start. The next day she was going down the interstate and it just died. Got it towed back home and pulled the codes. It had mass air flow, and 3 missfires. I put new mass flow seansor on it and all coils and spark plugs plus cleaned throttle body. Reset with battery and jumped it to see if better. Won’t start unless foot is on the pedal and when it starts it really rough and dies when u let off. It threw another code of p0300, checked spark plugs, all good. But notice oil comming up from intake into throttle body. Figured it was from trying to keep starting it. So I changed map sensor, exhaust camshaft sensor, and cleaned crank selenoid. About to change battery and coolant switch hoping that’s it. Please if u can help let me know. Any ideas, but remember I don’t have tech tool to read real time. Thank u.

  • #14049 Reply

    Jimmy t

    Check that the PCV line )no actual valve) is not plugged or disconnected under the air cleaner box.

    Go through the drivers side wheel well and make sure the intake bolts are not loose. The rear ones are most common to come loose.

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