P0300 Random/Multiple Cylinder Misfire Detected

A a common code I know, however it only occurs at revs go above 4000 and not every time. My H2 runs on LPG or gas but the code occurs on either.

The plugs look pretty good (gaps at 1.02mm/0.040") a clogged fuel filter would only affect gas (I suspect).

Could be air-filter or air-leak (I'll check later)

Is there anything else like the cat that could be the cause?

Cheers

 

Air filter looks a little bit clogged so will change this 1st and see if it makes a difference.

 

I was having this code come up on cold starts. I could hear the truck missing on start up but it took a month to throw a PO 300. I reset it. It still misfired and took another month to throw the code. My plugs were 3 months old. Denso platinum.

Took truck to my mechanic where he cleaned the MAF sensor. From research and gut feeling I was thinking intake gaskets. Brought the truck back and he cleaned the MAF again. Still no fix.

Brought it to mechanic and asked him to smoke test intake. He couldn't find any leaks. But he also hosed the motor down which I found out on the way home flooded my knock sensors and caused the truck to run like crap. I was not happy.

So to change the knock sensors the intake gaskets have to be replaced. Replaces the gaskets, knock sensors and harness. Truck ran great again.

The random misfire code is a tricky one. You need to get lucky as there are so many things that can trigger that code.

 

Have you ever replaced the plug wires?

 

Thanks for the suggestion as it re-assures my plan to change both the ignition wires and plugs and possibly coils after that.

To be honest I've struggled to find the new ACDelco part numbers (nobody in the forum seems to know them) anyway I believe I have them now and new SP's+Wires are on their way (UK delivery takes a couple of weeks)

I'll post the results which will hopefully support other members with the same issues.

Thanks again