Chevy 6.0 P0300 error. Ugh

Streetwiseguy wrote: In reply to Knurled: I bet I haven't seen a misfire caused by an "inside the engine" spark plug problem in 20 years. On the other hand, its pretty rare to have a week that doesn't need a coil or boot or wire. And, I don't consider an oil fouled plug to be the fault of the plug. Fuel trims tell me more than looking at a plug any day. And Ford just builds crap coils.

Ford may build crap coils but I had a misfiring 5.4 last week with plugs worn to a gap larger than my largest plug gapping wire, which is .080".

I agree that most ignition related misfire issues are outside the combustion chamber, but a lot of times those issues are exacerbated by the added stress of reactive maintenance like waiting until the truck kills a coil or two before having someone change the plugs. It's rare to see a carbon-tracked outer porcelain that didn't also come with a way out of spec plug gap, so the electricity had an easier time jumping the outside of the spark plug rather than going through the gap.

I've decided to throw a set of intake manifold gaskets at it and ordered a couple ignition coils to help me trouble shoot. That should remove a couple question marks at least.

I have two gm products in my driveway and both need that done. You get good at it with your truck and I will pay you to do both of mine.

It wasn't that hard of a job to do, there is no oil or coolant running through the manifold. The hardest part is having to stand on a stool and lean way over into the engine bay to work on it, in used to small cars.

Any chance you have a leak between the MAF and throttle body letting in unmetered air under higher load/rpm?

Possibly a small split in the intake tube that opens as the engine twists on the mounts or leaks more as the throttle plate opens?

dean1484 wrote: I have two gm products in my driveway and both need that done. You get good at it with your truck and I will pay you to do both of mine.

It's actually pretty easy. I already had the injectors and fuel rails out and there's not a whole lot to remove beyond that to get the manifold off.

In reply to jstand: I gave the intake a once-over and didn't see anything concerning. The engine does miss while idling (ever so slightly) it just doesn't happen enough to wake the ECU up I guess.

Changed the intake gaskets out for a nice new set of FelPros. Made no difference.

Scratching my head now for next steps.

To recap: slight miss at idle, triggers CEL under load.

Changed:

Spark plugs

Wires

Intake manifold gaskets

Ohm'd injectors (all good)

Compression 165psi+ on all cylinders

Cleaned MAF

Cleaned throttle body

Bunch of seafoam in the gas

One new coil to replace one that just looked crusty.

I've had my 99 GMC 4.8 LS for a long time. I've run into 2 simular issues over driving from 140K to 286K. I had a coil going out that acted simular, but bad enough on the scanner to trigger a specific cylinder miss-fire. 2nd simular occurence, as mentioned by another board member JStand, was a crack in the intake tube close to the MAF sensor - it was hard to see. The MAF is very sensitive. I took the tube off to inspect it - because the crack was at the bottom where you couldn't see it at first. I then installed a Spectre cold air intake. Problem was then solved.

Well I gave up on this and handed it off to a professional. Unofficial diagnosis (pending road testing) is that it needed an ECM relearn.

That's something I couldn't do on my own, so I feel a little better about handing it off.

While he's in there have him turn off the rear O2 sensors and the VATS function for when you decide the hillclimber needs less Wankel and more torks

ECM relearn definitely fixed it. $90 later I'm back on the road with a short list of things on the to-do list like a new powersteering pump and reservoir.