p0300?

DareDog · January 3, 2017

2005 1500 5.3L with 121k miles on it. noticed past few weeks the truck had a small skip but no CEL. well yesterday the CEL finally came on and it ran on a few cylinders then it got better and ran on all cylinders. then CEL went of and has not come back on but still got a got code of p0300 engine misfire detected. What could it be? plugs were changed at 100k with iridium spark plugs. plug wires are original .

DareDog · January 13, 2017

Now CEL went off and has not come back on.

carnau · January 13, 2017

Could have been trash in an injector. Cracked plug. Ect... What are you checking the code with? If you know someone with a good scan tool you can go into the data list under misfire data and see exactly what cylinder was misfiring. That would lead you on a better track of figuring out exactly which cylinder was missing.

txab · January 13, 2017

Fuel pressure issue, plug wires, intake gaskets.......

Strong25 · January 20, 2017

P0300

carnau · January 20, 2017

P0300

?

DareDog · April 23, 2017

Got CEL back. It seems to skip only when motor is cold and once warmed up it goes away.

Bad Bowtie2 · April 25, 2017

Start with new AC Delco or Taylor spark plug wires. Use lots of dielectric grease here.

Then check your fuel pressure.

Report back to us....

carkhz316 · April 25, 2017

Start with new ~~AC Delco or Taylor spark plug wires. Use lots of dielectric grease here.~~

Then check your fuel pressure.

Report back to us....

Start with properly diagnosing it, as mentioned above and in your post. Checking fuel pressure is a good start. I just don't want to see people throwing parts at their vehicles.

Podor · April 26, 2017

It could be an intake gasket on one cylinder causing a lean misfire.

There are a lot of scan tools on the market that will show misfire data. GM gives current and history misfires. You may find one or more cylinders with a significant amount of history misfires compared to others, which is a huge help pinpointing the problem. If you can pinpoint the cylinders, use carburetor cleaner and spray it around the intake runner for that cylinder and see if the miss goes away, or if the Rpm changes. Verify the plug condition on those cylinders also. I've heard that cirtain brands don't work well in gm products. I stick with A/C Delco.

It is highly unlikely it is fuel pressure related. You would have some significant performance issues if it was. It's possible that it could be an injector, but not likely. The injectors I've seen fail had issues shutting off the fuel flow, causing the pressure to bleed off with the engine off and cause a hard start/flood condition.

Long story short, pinpoint the cylinder(s), check for vacuum leaks, check plug condition/gap/torque.

Blackty · April 26, 2017

0300 is a random misfire. Used to get that one in my old 03 2500. I never felt a hesitation at all. Ran like a top. Never resolved it. O2 sensors, checked cats, plugs, wires, maf, checked grounds, verified fuel pressure, sprayed around intake to check for leaks...gave up. Good luck...Jim

Bad Bowtie2 · April 26, 2017

Start with properly diagnosing it, as mentioned above and in your post. Checking fuel pressure is a good start. I just don't want to see people throwing parts at their vehicles.

Symptoms & 121K, it's time for new wires either way....

DareDog · April 26, 2017

It could be an intake gasket on one cylinder causing a lean misfire.

There are a lot of scan tools on the market that will show misfire data. GM gives current and history misfires. You may find one or more cylinders with a significant amount of history misfires compared to others, which is a huge help pinpointing the problem. If you can pinpoint the cylinders, use carburetor cleaner and spray it around the intake runner for that cylinder and see if the miss goes away, or if the Rpm changes. Verify the plug condition on those cylinders also. I've heard that cirtain brands don't work well in gm products. I stick with A/C Delco.

It is highly unlikely it is fuel pressure related. You would have some significant performance issues if it was. It's possible that it could be an injector, but not likely. The injectors I've seen fail had issues shutting off the fuel flow, causing the pressure to bleed off with the engine off and cause a hard start/flood condition.

Long story short, pinpoint the cylinder(s), check for vacuum leaks, check plug condition/gap/torque.

When I replaced the knock sensors 2 years ago also replaced the intake gaskets to. I dont think they would go bad that fast?

DareDog · April 26, 2017

Wouldn't it throw a code for plug wires? Took 3 of them off and they were not really in all the way. Maybe that's it? Put 3 wires on it to test it and skip went away. Ordered new wires for it.