Rough idle 2 mins after cold start
 

Hummer H3 -- 3.7L

Cold start does fine.Goes 1100 rpms. After a couple of minutes as engine tries to idle down severe rough idle occurs. Misfire codes result I can tap the accelerator and it lines out and idles normally. I have replaced the throttle body and MAF sensor with OEM parts.

Any ideas?

Welcome to HF.

Miles on the 3? When and why did you replace the TB? Are you still using the crappy OEM Battery Terminal Connectors? If so, replace them ASAP. All it takes is a simple search here on that topic.

Check the wiring harness to all the coil on plug packs.

Read this thread... click on the link: Acts like it wants to stall/rough idle -No CEL -What To Do:

102,000

I would look for a vacuum leak, PCV and its hoses, brake booster line, Intake manifold gasket.

Thanks for the advice.

It runs perfectly all other times.. What bewilders me is why, if I tap the accelerator, it comes out of the rough idle and idles and runs perfectly?? Could it have anything to do with the accelerator pedal sensor?

Brake booster line?????,

Sticky valves/excess carbon. Fuel system cleaner/induction cleaning/only top tier fuel.

https://cimg2.ibsrv.net/gimg/www.hum...29f92b5088.jpg

Or: The 3.7L engine in the 2006-2008 H3 is a leaky mess. Well, at least its valves are. Somewhere around 87,750 miles the valves and valve seats in this engine are susceptible to wear and tear.

The engine valves open and close to bring air into the cylinders and force exhaust out. If the valves aren’t tightly sealed while closed, the cylinder loses compression, the engine misfires, and you start saying words that your children probably shouldn’t hear.

Or there are some 2006s with over 200,000 miles and still have zero compression issues??? Overhead valves have been around a long time in many types of engines, they have an extremely low failure rate.

Therefore, if some H3s go 200,000 + miles and all overhead valve engines have extremely low valve failure rates, most H3 I5 powered vehicles will have a very low valve failure rate. ;)

Remedy
 

Found fix to idle issue. Replaced EVAP purge control valve and now idles fine during cold start.