P0300 Code - bad stumble - only for a minute at cold startup, then runs perfect for rest of day

[Mojo88]

I have a 2003 Silverado 1500, with 4.8L 130k miles, very well maintained. It usually runs like new.

 

But then the temps dropped into 30's, and I started getting a bad stumble, but the misfiring only lasts for maybe a minute. Then the engine smooths out and it will run perfect the rest of the day. First few days, no CEL. Then finally it set the CEL and I put my code reader on it and got P0300 (multiple cyl misfire), which is about the worst code to get because it means so many things, LOL.

 

I'm kinda thinking the clue here is that it only acts up for the first minute when cold. Can anyone suggest a likely problem area to look at? Thanks

[CamGTP]

When was the last time you looked at the spark plugs and wires?

 

I would start there. As well as cleaning the MAF sensor and throttle body. Also putting some seafoam or injector cleaner in the fuel tank just as a service thing.

[richard wysong]

I agree with Cam, also before you start the truck hook your scan tool up and see what the temp sensors are reading. air intake and coolant should be close to ambient after sitting over night. 

[Mojo88]

17 hours ago, CamGTP said:

When was the last time you looked at the spark plugs and wires?

 

I would start there. As well as cleaning the MAF sensor and throttle body. Also putting some seafoam or injector cleaner in the fuel tank just as a service thing.

 

Thanks........... I run Marvel Mystery Oil  in the gas almost every tank, so should be good that way. Plus, the very first thing I did when it acted up, was to thoroughly clean the MAF (using correct spray cleaner) and I looked inside the throttle body, it was clean.

 

I am gonna change the fuel filter next. Then remove/replace all the plugs (70k miles on them).

 

I was kinda hoping that a big clue was the stumbling only for a minute or so. It really stinks that the P0300 code seems to cover EVERYTHING that could possibly go wrong. Jeesh

[Mojo88]

11 hours ago, richard wysong said:

I agree with Cam, also before you start the truck hook your scan tool up and see what the temp sensors are reading. air intake and coolant should be close to ambient after sitting over night. 

 

Ahh, yes, this is great idea about checking the sensors prior to startup. Thanks