another p0300 question

[Xotik H3]

The evap is more than likely not causing your issue. I would look at changing the fuel filter first, then you can check you intake gasket with some "starting fluid". Spray the starting fluid around the intake ports, if it leaks the RPM will increase with starting fluid... always the simple things first.

Your O2 sensors could cause such an issue - which is what I would lean towards, they can be pricey though.

Curious, what color was the smoke and was it a one time thing or every time you start it - cold or warm?

Have you pulled any spark plugs to check for color?

How much fuel is in the tank? Have you filled it up with the same symptoms?

[HUMTECH]

A scanner with live data would be very helpful. It can tell youwhat cylinders are misfiring. A restricted converter can cause random misfires across one bank. Leaking intake will set a lean code when cold. If you do dcide to buy a better scanner might be worth it to spend more rather than less to get one that will read body and chassis codes as well. I bought a mac tools task master cheap it was a demo on the mac truck. Comes in real handy when im not at work wth access to the tech 2. Has great live data capabilities

[Hummeron37s]

I agree a better scanner will help but why not buy a programmer instead of a scan tool. My Diablo Predator help me to diagnosis alot of codes,, besides checking and clearing codes it has live feed. It also a programmer and corrects tire sizes, changes shifts point, etc...It was about $350 great all around investment.
My P0300 code was a clogged cat which I replaced both with Magnaflow high flow cats that I got on amazon.com for $75 a piece and just had to cut the old ones out and weld the new ones in.
What year and how many miles on your Hummer.

[HMH2SUT]

xotik
i havent done anything yet except clean the maf sensor road side and check to make sure plugs wires tight.The truck was cold it had been sitting for a week. The smoke was white, only did it once, never before or since... Im not gonna get a chance to do anything till im off this weekend. im gonna make a list see what i can get done in two days . Im really over due for some maintenance, I change my fluids more then needed oil, trans, diffs, coolant etc... but i havent touched the motor other then changing a thermostat. KNock on wood till now I havent had any major issues.

Tech, on37s.
Im gonna pick up something better for sure. i think i pad $50 for my scanner... I can get the magnaflow cats at autozone near me for $80 thats not bad so im gonna throw 2 of them on if I get time this weekend.
My SUT is an 05 with 120k

[HMH2SUT]

small update... took my sut to the chevy dealer near bye to get it scanned. they said it is misfiring on numbers 2,7. and that 7 seemed to be the worst.They recommened a leak down test to check my valves... i understand a valve could cause some of my issues. But what are the odds of valves going in both heads same time? I left while the scan was still free...

so has anyone had a similar problem?

[HUMTECH]

I would pull the driver side fuel rail and check for water. It will pool towards the back and cause a number 7 misfire. Number 2 may be showing misfire counts due to 7 misfiring and not actually misfiring. If no water is present remove the injector and inspect the inlet screen for rust or contamination plugging the injector.

[HUMTECH]

If you had a scanner with live data you could swap ign coil, plug and wire with other cylinders to see if the misfire follows any of those parts.