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.

Keep throwing parts at it... :rolleyes:

Here's some good reading for you:
(Page 1 & 2):

https://static.oemdtc.com/TechLink/TechLink2011Sep.pdf

pip3146d:

www.askatech.com/AskATechLive/forums/ResourceHandler.ashx?a=3311



What fuel to buy:



https://www.toptiergas.com/

My fuel pressure is 55 psi it does lose 1lb every min so I lost 10lbs in 10 min on leak down

Do you smell fuel anytime parked/not running?

It has no fuel smell but I'll try cranking without the cap but it's also new ...i left the old one at the gas station

I tried the seafoam cleaner so far 2 cranks it hasn't stumbled thanks so much and fingers crossed

As I stated above; Cheap Fix
Now you know what caused your issue. Only use top tier fuel, and if there is a performance issue: Decarbonize.

The stumble came back just as bad I cleaned it several times with the top engine cleaner they work for few days

Where are you fueling?
https://www.toptiergas.com/licensed-brands/

have you noticed if it is happening at a certain fuel level maybe something is off with the sending unit in the pump or the fuel gauge is miss reporting the amount of fuel or the fuel pump pickup is disconnected or damaged.

Good questions.
I always ask many questions to the customer before servicing their concern.
A problem here: post an issue and give limited information.
From the limited information I gathered, fuel contamination is the cause?

It will do it at anytime with a full tank of gas or a qrt tank I did a fuel pressure test I had 55lbs of pressure and my leak down test I lost 10lbs in 10min

Well, this seemed to work. Used the spray can and followed the video procedures. Still got quite a few misfires right afterwards, but drove around a bit and they stopped. Have cold started a couple of times since then and no misfires reported.I think that indicates it was a sticking valve issue, just need to see if it comes back. Since the Hummer isn't a daily driver, sitting up seems to cause it to misfire more.

Thanks for the help!

I only average around 3k miles per year, however I only use top tier fuel, I also keep the oil clean with a quality full synthetic oil, and keep the air filter clean. If you ever experience a performance issue in the future, you now know where to start.