engine temp gauge fluctuation

[markm]

Any thoughts on what would cause the engine temp gauge to drop from 190 to 160 while idling in a parking lot with the A/C on? Outside air temp about 80. No error message from the message center, no service engine soon alert. Also noticed that the RPMs automatically sped up from idle to about 800-900. Opened the hood, no coolant leaks, belt is fine. Truck has 34K miles on it. Serviced at regular intervals at Chevy dealer. Shut the engine off for about 5 minutes and then restarted. The engine temp gauge stayed at 160 for about 10-15 seconds and then came up to normal temp at 190-200. Drove 6 miles to home, everything seemed OK.

[More Power]

I do know that a Duramax 6600 idling without a fan will take 40 minutes just to reach 160 degrees.

I suspect with the AC on, the fan-clutch could be engaged - even at idle, and might cool the engine temperature.... I dunno....

I'd just watch the temperature gauge to see if you notice any other odd behavior. What you saw could have been normal for your engine on that day.

MP