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I have had a P0300 " random misfire " code issue for a few years now. At first it would only do it on a long trip - about 180 miles into it there would be a slight stumble to the engine, and the CEL would display. Usually went out after another 100 miles. Very intermittent and as such not much cause for concern.
Fast forward a couple years. The frequency of this has become once every 50 miles or so. Okay, some work with the diagnostic tool and still no definitive direction to go.
And this year. It has gotten to the point that it does it on every short trip. Along with some bad stumbling, flashing Check Engine Light, and lack of acceleration for many seconds.
Have done new plugs and wires at 130,000 miles. The plugs were due but all were still serviceable. NO difference at all.
Finally came to the opinion that there was actually nothing wrong with either fuel OR ignition. In fact, my opinion is that there is in fact NO MISFIRE AT ALL being detected.
Rather there is something going on that the poor ECM computer is confused on, and is simply lost and INTERPRETS AS A MISFIRE.
Enter the crankshaft sensor. There is actually nothing wrong with the sensor, but my theory is that rust scale builds up on the engine block - under the sensor lip - ever so slightly forcing the sensor out of calibration with its reluctor wheel on the crank. Ever had a wheel sensor act up from scale under it on the spindle hub surface? Same thing.
So with that in mind, here is my cure.
Remove the starter. There are only two bolts and you can leave the wires connected if you are careful. Slide it forwards and then down between the block and the transmission cooler lines. The Crankshaft position sensor is hidden behind the starter, no way you can access it with the starter installed.
Starter hanging
I missed getting a picture of the sendor in place. Here is the hole where it goes. (To the right of the inner starter bolt hole, with the blue coloring in it from the sensor seal o-ring) It is held in with a single bolt, and will require lots of wiggling under slight pressure to remove it from the block. Be patient and keep moving it back and forth and it WILL eventually come out without breaking it. You really don't want to break it as there is a supposedly "dealer only" calibration procedure that needs to be done when a new one is installed. That is NOT necessary when removing/reinstalling the old one though, so take your time.
See the " lip " to the outside of the sensor hole? That is where the hard scale has built up. It won't just brush away, you will have to scrape hard at it with a screwdriver or something similar. You know how the scale on an old brake rotor can be removed with several decent hammer taps? That is what you are doing here.
Get that crap outta there!
Here is the sensor. There was no crud or goo on my sensor, a simple wipe off with a shop towel was all I did. I did need to put some vaseline on the o-ring in order to get it to reinstall. Its a close tolerance fit so be sure to keep it aligned straight with the sensor hole. I installed the bolt and gently drew down on the sensor while giving it gentle taps and pressure.
Pop the starter back in and give it a shot. My 2006 Chevy Express 6.0 LQ4 has not given a P0300 since I did this. Its cheap to do and only took me an hour.
Hopefully your luck is similar!
......
Fast forward a couple years. The frequency of this has become once every 50 miles or so. Okay, some work with the diagnostic tool and still no definitive direction to go.
And this year. It has gotten to the point that it does it on every short trip. Along with some bad stumbling, flashing Check Engine Light, and lack of acceleration for many seconds.
Have done new plugs and wires at 130,000 miles. The plugs were due but all were still serviceable. NO difference at all.
Finally came to the opinion that there was actually nothing wrong with either fuel OR ignition. In fact, my opinion is that there is in fact NO MISFIRE AT ALL being detected.
Rather there is something going on that the poor ECM computer is confused on, and is simply lost and INTERPRETS AS A MISFIRE.
Enter the crankshaft sensor. There is actually nothing wrong with the sensor, but my theory is that rust scale builds up on the engine block - under the sensor lip - ever so slightly forcing the sensor out of calibration with its reluctor wheel on the crank. Ever had a wheel sensor act up from scale under it on the spindle hub surface? Same thing.
So with that in mind, here is my cure.
Remove the starter. There are only two bolts and you can leave the wires connected if you are careful. Slide it forwards and then down between the block and the transmission cooler lines. The Crankshaft position sensor is hidden behind the starter, no way you can access it with the starter installed.
Starter hanging
I missed getting a picture of the sendor in place. Here is the hole where it goes. (To the right of the inner starter bolt hole, with the blue coloring in it from the sensor seal o-ring) It is held in with a single bolt, and will require lots of wiggling under slight pressure to remove it from the block. Be patient and keep moving it back and forth and it WILL eventually come out without breaking it. You really don't want to break it as there is a supposedly "dealer only" calibration procedure that needs to be done when a new one is installed. That is NOT necessary when removing/reinstalling the old one though, so take your time.
See the " lip " to the outside of the sensor hole? That is where the hard scale has built up. It won't just brush away, you will have to scrape hard at it with a screwdriver or something similar. You know how the scale on an old brake rotor can be removed with several decent hammer taps? That is what you are doing here.
Get that crap outta there!
Here is the sensor. There was no crud or goo on my sensor, a simple wipe off with a shop towel was all I did. I did need to put some vaseline on the o-ring in order to get it to reinstall. Its a close tolerance fit so be sure to keep it aligned straight with the sensor hole. I installed the bolt and gently drew down on the sensor while giving it gentle taps and pressure.
Pop the starter back in and give it a shot. My 2006 Chevy Express 6.0 LQ4 has not given a P0300 since I did this. Its cheap to do and only took me an hour.
Hopefully your luck is similar!
......
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