Code P0300

Hy there , I have 2006 avalanche 2500 8.1 ltr . I recently have code P0300 random / multiplecyl . misfire detected . That happened when i stepped hard on the accellerator , but when i release the accellerator the check engine light disappear .

In minde these message coming after i changed the tires and the new tires are bigger than the old tires .
I checked the fuel pump pressure and it was good and i will change the spark plug today to see .

waht do you think ?

thanks for your help

Can you put your old wheels back? Or just changed the tires?

The only think that I can think of, is the larger tires, less RPMs. I am thinking the original wheel dimensions wound the engine, therefore the exhaust had a higher velocity.

Usually with problem of that nature, the problem seems to be the Catalytic Converter is plugged or just beginning to clog up. Giving back pressure and smothering the combustion, therefore, giving random misfires.

That is the only thing that I can think of, that could give random misfires with a wheel diameter change.

On that truck I'd agree likely a plugged converter!

you either have clogged convertors or a fuel deliverly issue.

That What i think about it .

I changed the spark plugs . For the old sparks they were in white color and i drove it just 6216 Mil (( 10000KM ))
i will try in these days to remove Catalytic Converter and will see .

Thank you very much for your response

Usually white color on the plugs is too much heat, sometimes has deposits embedded. Or the wrong plugs, the one people calls hot plugs or winter type. They are designed to not dissipate heat easy so the engine will start in cold climates. That could give misfires since the fuel mix, once it gets in the chamber, works like a glow plug igniting the mix before the spark arrives. Also, a sign of a lean fuel/air mix. Possible vacuum leak, clogged fuel filter, weak fuel pump.

The ideal color is Tan or Beige.

If they are black is a sign is not happening a complete combustion. Oil is black with some tar like deposits and/or a wetness to it.



Since you gave us another piece of the puzzle....

Plugs do tell a story, that was good that you checked on them.


I would suggest this order...

1) Pour a bottle of Fuel Injector Cleaner and let it run for good while or drive around town.

2) Change the plugs, for the right one according to the Owner's Manual.

3) Change the fuel filter.

4) Check for vacuum leaks.

5) Check Fuel Pressure.

6) Check Catalytic Converter.

I can say, changing those components is not only cheap but is part of the maintenance and is cheaper to do.
When you change one take it for a test drive to see if anything changes.