PO300, chasing that Random Multiple Misfire!

PO300, chasing that Random Multiple Misfire!

A random misfire OBD2 code PO300, what the books don’t tell you to look for.

I have a 1996 GMC Sierra with a 4.3 Liter engine. When it was given to us by my father-in-law it had a slight misfire but had not set the PO300 code yet.
I talked to my brother who was a mechanic with his own shop at the time and we checked everything it could be to no avail. After a few months the code PO300 came up in the OBD2 (On Board Diagnostic 2)

PO300 misfire

#4 All engines have some sort of ignition system.

So the mad chase to find PO300 The Random Multiple Misfire

In the books you’re told it could be:
Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked/leaking EGR valve / passages
Faulty camshaft position sensor
Defective computer
SOURCE:
Read more at: Copyright © OBD-Codes.com

If you have an OBD2 system, you need a scanner tool, once a code is set, this tool makes it easy to find what code was set.
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This video explains more of what mechanics are told to look for:

If your vehicle does not have OBD2 then a timing light can be used to find a misfire the timing mark should remain steady. Check several plugs, if they all are unsteady then it’s the Distributor, either the gear or the bushing up top OR Both. You can also use this on an OBD2 system first, too tell you if you need to pop the cap and check for slop.
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Random Multiple Misfires

Now days the newer engines don’t have points.

How I found my random multiple misfire, Pesky PO300 Ghost

None of the above checks worked. I spent at least $1000 over several years to replace parts that either tested good but could be bad OR tested bad but could be good.
I even went so far as to replace the fuel injectors because someone I trusted said it could be them.
My oldest daughter, awhile back, had a 96 Suburban, 350 engine. It up and quit one day. I when over there and found it was getting fuel and spark but had a back fire and would not start.

My GoTo Tool Set, anytime I work on most anything this is the first tools I pull out.
Stanley 92-839 Black Chrome and Laser Etched Socket Set, 99 Pieces

Random Misfire PO300

Fine old flat-head Ford.

So I brought #1 cylinder to top dead center, and checked the rotor. Sure enough it was off, Timing Chain Right?
We changed or went to change the timing chain but when we got the cover off, the chain was tight and lined up, it looked brand new!
Well I did a bump over and checked again to see if the rotor would line up and it still did not, I was stumped I’d not ever seen this before.
They wound up taking to a local mechanic in their town and told him what we had done. Of course he, because of what we had done, knew it was a worn distributor gear or upper bushing.
Ya see those gears are not steel but some sort of composite that wears and causes the slop to create your PO300 random multiple misfire.
I checked my distributor for play and it had some slop so I replaced it….

Too fix this PO300 you’ll need a Distributor.You can buy one from Amazon.

How to fix a random multiple misfire PO300…

Boy I wish I’d learned this sooner, I’d never heard or seen such a thing. So before you go hunting a Ghost PO300, pop your Distributor cap and check the rotor for any play, back and forth.
If you have a High Energy Ignition (HEI) it’ll have the coil on top of the distributor cap and a spring loaded mechanical advance there should be NO play before the advance starts moving.
If you have a Distributor Ignition (DI) you’ll have an external coil and no advance (It’s handled by the Vehicle Control Module (VCM).
If you don’t have a vehicle with OBD2 but you can feel the Herky-Jerky (random multiple misfire) as you cruse down the road check for play in your distributor, that could be your problem.

Featured Image created at Wordie build #1439 by Doug’s Projects.

Image #4 Source:
By Rich Niewiroski Jr.
[CC BY 2.5 Attribution], via Wikimedia Commons
via Wikimedia Commons
Image #2 Source:

By Sonett72 at en.wikipedia [Public domain], from Wikimedia Commons

Image #3 Source:
By granada_turnier (Ford-T engine) [CC BY 2.0 Attribution], via Wikimedia Commons
via Wikimedia Commons

Author

Husband to Teresa, father to four of the best children in the world (Don’t tell them that) Grandpa to the four BEST Grandkids in the world. Grandkids are great I get all the fun my daughter gets to clean the fun. Fishing is one of my pass times (When I can that is.) Learning not to beat my computer to death when it is only doing what I stupidly told it to is my other pass time when I am not Remodeling someones home or building decks.
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