Hummer H3 For the Hummer driver who wants the rugged look and off road capabilities of the Hummer, but in a smaller size and with a more fuel economy friendly engine.

Rough idle

Old 07-25-2018, 01:39 PM
Doc Olds's Avatar
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Join Date: Jul 2007
Location: Boat Town USA MI
Posts: 8,662

All of what you are describing could be normal for an I5, it is all a matter of degree.

I5s are inherently unbalanced and do vibrate at idle.

When you turn on the AC, the AC Clutch kicks on, the PCM increases idle to compensate, and the vibration is reduced.

The H3 has the GM smart Regulator Voltage Control system with fuel management. When you stop, the PCM and BCM compare input regarding fuel, speed, and voltage needed to operate whatever is on/being used, and the engine idle speed is reduced to the lowest RPM capable of delivering on reduced fuel consumption and just enough for the alternator to generate sufficient power to operate everything needed -nothing more- all while the alternator itself adjusts its output changing the load on the belt. While the comparisons of input, signals from the PCM to TB etc take hundredths of a second, some lag can occur when the system adjusts itself, and it can create the sensation of a rough idle or even a false stall for a second or so. The classic situation where this happens is braking to a stop, brake lights on, speed going to zero, and RPM reducing with speed. Many customers would see headlights dim a moment and think something was wrong, just Smart RVC and fuel management doing their things.

It's all a matter of degree and we can't see/feel exactly what you are describing.
Old 07-25-2018, 04:38 PM
Junior Member
Thread Starter
Join Date: Jul 2018
Posts: 12

I understand what your saying but when i bought it in never did this. The lights would dim when i hit the brakes but that never bothered me. It had a vibration but not this bad, i will try to get a video of it if i can. Just started within the last 6 months or so. Could this be the altenator going bad or somthing along them lines. Ive changed the battery terminals to since the stock ones are junk thinking it might have been that. One thing that did happen was after i canged the intake gaskets the altenator was making a squeal noise is the best i can describe it. But that has went away so i figured it was just sand/dirt in it.
Old 02-06-2019, 05:29 PM
Junior Member
Join Date: Feb 2013
Posts: 5

wow ihope you reply. ihave identicle problem. yes, the TPS and IAC motor are sealed to the TB.
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