I bought my H2 2005 SUT about 6 weeks ago. CEL came on at the end of the test drive. It was a vacuum leak. Seller fixed it. Anyhow my diagnostic scanner showed the same code with extremely minor misfire on cyclinder 6. Basically I have fixed everything on this Hummer starting with the simplest solution first: oil change and Techron in the gas tank. The P0300 and other codes indicating a lean fuel mix faded away immediately. Mine was well taken care of for most of its life but the last owner is a dealer with a lot of cars and barely used it, so it sat. So most of my issues are from sitting I think. I have yet to add in Seafoam because I won't be able to tell what is working. Just keeping it simple and not pumping money at it and throwing guesses. Right after the oil change I watched the diagnostics in real time and the misfire became intermittent. Engine is practically purring. Anyhow, has your situation improved?

 

Have you resolved your issue yet? Did you try the Seafoam/Marvel additive solutions and air filter/MAF cleaning yet? Those and a plug & w8re change resolved it on my 03.

 

I just noticed you also have an evap issue, fix it first then see if the P0300 is still active. If the evap is a leak then it can cause the P0300 code.

Neal

 

not resolved yet, casual driving is fine, under any sort of load is horrible

 

I might just send it into a shop for one and done . I lead a busy schedule and have exhausted my comfortably and diagnostics.

My fuel gauge works, then reads empty for couple mins then works again, and the evap issue I could probably fix myself too, at the least it will fix that issue. With any luck it may correct the misfire but I have my doubts at this point.

 

UPDATE! It was infact the Evap solenoid down by the canister on frame! No code for misfire now, throttle response is finally there. It has power.

without your comment I wouldn’t have even thought those two codes together could have been because of the other.

thank you

 

mine is repaired! Was evap solenoid on frame. Runs really smooth now. Mine sadly wasn’t taken cAre of, every door panel is broken one way or another, was never undercoated so floors are tough, frame is rough, heater control is dicked, it’s a shame how some people treat their cars. I’ll probably do some seafoam eventually for it, it’s worth saving and fixing up , I bought it for a steal.

 

Glad you got it worked out and thanks for updating the thread, your issue and repair will surely help anyone reading this in the future

Neal

 

I hope it does help someone, to often I see people turn to the forums for help, then never update the outcome.
for the few mins it took me to update, hopefully save someone else some time!