Truck is an '04 Hummer H2. Ive only had a short time. When I test drove it, everything worked great, ran great, etc.
Being a new to me vehicle I changed all the plugs, wires. I went with AC Delco wires and had NGK TR5GP plugs. The truck ran like crap and I was getting the code above. I swapped in AC Delco cheap copper plugs and it runs a lot better. No more code if I stay out of it. But if I floor it, or add any real throttle the check engine light blinks and the motor stumbles. I have Torque, the program for the Android, so I can read the codes and see some parameters. Ill try to log as much as I can and figure out how to post them.

But driving it today I had an idea. I parked the truck, left it in Park, and floored it. No code, no CEL flashing, no stumble, nothing. Revd great all the way up to 5500-6000 before I let off.

I do have some mechanical knowledge, but I am mainly an LT1 guy from my old Camaro.

Reading around online, it seems the P0300 is a misfire that could be cats, coils, lifters, plugs, etc. But I am wondering why wouldnt it break up while in Park?
Any ideas? Thanks.

 

Put some iridiums back in it and be done with it.

BTW what did you gap the plugs too???

 

Copper plugs were gapped to .050.
If its the plugs why would it rev fine in park? Not being argumentative, just not wanting to spend $56 on plugs and it not fix it.
If you really think thats the problem should I go with the 41-110's? Autozone is the only place around me that carries them, and they say the plugs wont fit!

 

The plug gap Spec is .040 on the factory Spec plugs. Originally was .060 before they swapped from Platinum to iridium.

Being coper you probably want to go .035 or so the guys on here say worked best for them.

They Factory spark system and spark settings was based on a long life platinum or iridium type plug.

Spark gap and a change in plug type can affect the way it runs. These newer automotive electrical systems are very sensitive to that. Hence the P0300 code...

Try changing the gap and clearing the code and see if that does it..

 

Swapped in some factory 41-110 and still no change.
Ive checked every vacuum line, hose, etc.
I did notice today that it was running hot. About 240ish and its only 85* here today.
I did try a data log. Ill have to look online to see if it was captured.
Im willing to PM the link to anyone if they can look at it.

 

If it was 240 you have a serious problem going on, probably a stuck thermostat.

Misfire is ignition related, most likely cracked plugs or burnt wires.

 

It usually runs in the 210 range.

And I changed the plugs but not the wires.
The wires I had from the loaner car were to short.

 

My buddy hooked it up to his laptop and was able to tell that #6 was missing, then #8, #4 and gradually #2. Only that side had a miss. Got under the truck and felt the cat and it was extremely hot.
His best guess is clogged cat.
So Imma replace the manifolds with LTs. The kit I want comes with hi-flow cats.
And I figure while Im in there, Imma swap on some 799 heads, small cam, and NNBS intake. Will do lifters, trunnion upgrade etc.
All this for stupid cats.

 

Sell the cats to gain back some cash.

Recylce centers might give yeah 50-100 a pop.

 

check the power wires and grounds to the coils and packs ,then check the coil pack junction plug and see what happens.