PO300, That pesky random multiple misfire you can't find.

Random Multiple Misfires

PO300 can cause your head to explode!

I've been there, 96 GMC Sierra pickup truck.

You've changed the plugs, cap rotor, wires and still, A misfire. You check the book and read everything related to OBDII PO300 and the ignition system.
You've changed (Because it tested bad, or not, but it could be....) coil, the EGR, the oxygen sensor, cleaned or replaced the Mass Air Flow Sensor and still you have Random Misfires and the code of OBDII PO300, pulling your hair now?

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You change the crank sensor, the cam sensor, the Vehicle Control Module AND Still you get OBDII PO300 (Beating head on brick wall now?)
You clean, check and maybe even change everything connected to the fuel injectors and what do you get (As the little green Yoda always said) HHHUUuuummmmmm?

Now you have a full mental melt down as you have chased this ghost all around under the hood, up, down, left, right, back to front. You done everything your brother said, brother-in-law said, the local mechanic, your best friend AND Still you have the herky-jerky as you cruse down the road screaming OBDII PO300 WHY?

PO300 Random Misfires Solved!!

Yeah, I've really been there!!!

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I have a simple check you may not have found, it solved my problem and it won't cost you a dime to check.
Pop your distributor cap, yeah I know you've changed the cap and have to remove the wires Again. Trust me I've been there, however try moving your rotor back and forth. If you have a distributor with a mechanical advance (High Energy Ignition HEI ) there should be NO Play before the advance kicks in. If you have a distributor without an advance (Distributor Ignition DI ) there should be NO Play at all.
If you find any play it (AND it was what caused my random multiple misfire) is what is causing your random multiple misfire.
Replace your distributor!
Or at least the gear, now you could also have a worn upper bushing there too. I found that after buying the gear and bushing, with time to replace them, it'd cost about the same as just replacing the distributor.

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PO300 chasing that Random Multiple Misfire!