Hummer Forums - Enthusiast Forum for Hummer Owners - Acts like it wants to stall/rough idle -No CEL -What To Do:

H3 & H3T Hummer - False stall (acts like it wants to stall), Rough Idle with no check engine light 3.5L L52 or 3.7L LLR, what to do:

1) Clean the TB with TB cleaner only.... there are multiple threads on this, the best way to do it is remove it, clean it, re-install it;

2) Clean the MAF sensor using MAF cleaner only... there are multiple threads on this (might as well while you have the intake apart to clean the TB);

3) inspect the coil on plug packs and their connectors to the wire harness and any obvious signs of corrosion from water on top of the head;

3.5) Check to see if ALL of your intake manifold fasteners are tight (they do vibrate loose on occasion) to 10 N.m/89 lb in.

If the problem still persists, and you have more than 70K miles (plugs should go damn near every bit of 100K in these trucks). If you do not have more than 70K miles, skip to #6 or feel free to change plugs for the sport of it:
4) change the spark plugs and inspect them for evidence of any fouling;

5) while the plugs are out, look closely at the plug on coil packs to see if there is internal corrosion, if so clean them with electrical cleaner spray and allow to dry before placing them back on the plugs.

If the problem still persists:
6) Pull the camshaft actuator solenoid and look for crud on the sensor, on the screen in the motor, or a missing screen... replace if necessary, tighten mounting bolt to 10 N.m/89 lb in.

If the problem still persists:

7) Then do a compression test, a normal read would be appx 1482 kPa/215 PSI. You can follow that with a leak down test. According to Hummerz when a leak down test shows a valve not seating properly, he performs an induction cleaning first, worked on his rig, and many of his customer's vehicles.

Watch the temp gauge, better yet get a scanner an observe actual operation temp. If that is lower than normal, or coolant is low (like at the 1/2 level) consider a T Stat replacement.