P0300 Code question

Discussion in 'Maintenance' started by Greg_P, Nov 23, 2016.

  1. Greg_P

    Greg_P New Member

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    My 2001 Silverado 1500 with the 4.8 liter is giving me a P0300 random misfire code. I am at 170K miles on the vehicle and the code comes up when only when the vehicle has been at a constant idle or speed for 5 minutes or so. So if I leave it idle for 5 minutes the truck will start to rough idle and hesitate and if I give it some gas, it smooths out and runs fine for another 5 minutes or so and then the issue repeats. The same is true if I am driving on the highway. If i put on the cruise control after the 5 minutes or so the saem thing happens. Speed up and it goes away. So far I replaced the plugs, wires and coil packs. I tested the EGR and found that the voltage being returned as low so I replaced that as well. Last thing I did was replace the fuel regulator. All of this had no major effect on the issue although the truck runs great when it runs smoothly and the gas mileage has definitely gone up so it was not wasted effort.

    I do not believe it is the fuel pump since I replaced the pump and all the fuel lines from the tank to the fuel rail about two months ago when the factory original pump gave out. I replaced all the lines since each part of the line I touched just crumbled and I figured that it was just cheaper to get the entire set from the dealer rather than piecing in sections that I broke trying to get them off.

    The other think that is funny is that when the truck is experiencing the issue, the Check Engine light flashes for about 15 seconds and then goes off. The only way I know the code that was put out was to use a code reader and it shows as a pending code.

    So does anyone have any ideas what else to check? I am currently on the road and the plan is to check ohms on the fuel injectors to see if any one of them is out of wack with the others. I am out of thoughts as to what else to do if it is not the injectors.
     
  2. 2005_Silverado

    2005_Silverado *

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    clogged cats maybe?
     
  3. Greg_P

    Greg_P New Member

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    Is there any way to test for that other than taking it off and looking through it?
     
  4. ron7000

    ron7000 GMFS Member

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    clogged cats would show other systems specifically P0420 P0430, not just a random misfire under those specific steady state low load conditions.

    for a 2001 with 170k miles showing only P0300 I think your best bet would be to put new front O2 sensors in first. If that doesn't cure it, consider a new MAF sensor. And if there is an idle air control valve that is separate on your throttle body, consider replacing that too.
    If it's only P0300 that's random misfire, which is the engine computer detecting an erratic rpm over time that it cannot pin down to a given cylinder. So it's NOT likely to be caused by a spark plug, wire, ignition coil, or intake/exhaust valve on a specific cylinder. It's something in common to all cylinders. And don't overlook the PCV system leaking extra air into the intake manifold, check for dry rotted cracked pcv hoses, and also excessive oil entering the intake from the PCV system.
     
  5. chile

    chile The Chilean wrench

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    Actually the SAE definition of a p0300 is a random or multiple misfire. With a good scan tool you can find the misfire monitors. I would start with condition of plugs and wires. More then likely it's probably a coil going bad.
     

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