Stumbles on cool start up
 

I have a 2008 hummer h3 with a 145,000 miles. It stumbles almost to the point it shuts down. It only happens on cool start. I have no codes. After 10 seconds it smooths out and drives normal. It has a rough idle. I'm stumped. It almost sounds like it backfires, but after 10 sec it runs normal except the rough idle

No Codes?
What scanner did you use?

You have had this issue for over a year?

https://www.hummerforums.com/forum/s...archid=1934035

Btw, if you scanned the computer with a quality obd2 scanner and no codes, then it's a cheap fix.

I've used several scanners even had it to the shop once and no codes please tell me the cheap fix

When was the last time you cleaned your Throttle Body and MAF?

See this: Acts like it wants to stall/rough idle -No CEL -What To Do:

Exactly where I was going Doc. Perform the check list, also make sure the air filter is clean, check for vacuum leaks, and add a bottle of fuel cleaner to the tank( techron or lucas is what I use).

Similar issue here I've been working through but not quite there yet. Same basic symptoms, rough idle and stumble when cold started. P0300 stored typically, but if I took it on the road too soon after cold start I'd also get P0305. 2010 I5, 184k miles.
1. Cleaned throttle body and MAF sensor, replaced air filter.
2. Checked for vacuum leaks, can't find any indication. Recently changed intake manifold gasket, throttle body gasket, and vent hose when I replaced valve cover. (Rough idle and misfires occurred before and after)
3. Replaced plugs (new OEM), swapped coil pack between 5 and 2, still P0305 code occasionally.
4. Two tanks with Techron so far, has smoothed out rough idle but still get misfires, mostly on cylinder 5. Monitoring Mode6 information I see 10-15 misfires after startup, very few afterwards. Some other cylinder random misfires but very low counts.
Question: There are two parameters for each cylinder misfire counter, anyone know what the difference is? I see more on one than the other.
5. Changed oil and filter.
6. Removed/checked/cleaned VVT solenoid, didn't see any missing screens, tested and solenoid moved when powered.
Question: Could VVT solenoid still be bad even without physical damage and although it actuates? I'm thinking maybe some clogs somewhere. I don't want to change a $60 part if not necessary.

I've cleaned the MAF and throttle body regularly I've ran cleaner in the tank nothing helps it's also a new oem MAF

Have you performed an induction service?
I'm currently doing my annual service on mine right now, got the tank from penray rep(pic below). Add a 1/2 can of seafoam, adjust the flow( a slow drip) and run at 1800rpm until the tank empties. Seafoam also carries a spray can with a long hose attached that you can try:


https://cimg9.ibsrv.net/gimg/www.hum...80000dd6ec.jpg

I've done all the checklist this thing couldn't be cleaner it has new plugs, coilpacks, MAF, injectors, new battery terminals, new cayts, new 02 sensor up and downstream. I'm at wits end

have you tried opening the gas tank cap before starting the engine to see if anything changes. could be starving for fuel.