Was driving from Flagstaff, Arizona to Phoenix, Arizona and while driving my 2006 F250 SD [5.4L engine], the Check Gauges light and chime came on. The engine coolant gauge was pegged at the top.

Immediate action was to turn off the A/C, pull to the road shoulder, and then turn the heater on full. After about two seconds, the temperature gauge immediately dropped to mid level. No loss of power or check engine light. The engine coolant was system was visually inspected for leaks or damage and none was seen. I was then able to drive to the outskirts of Phoenix without incident.

I stopped into the store and left the engine running. According to the wife, she got hot and turned on the A/C again just before I got back into the truck. As I re-entered the freeway, the chime again sounded, the Check Gauges message and chimes came back again. The engine immediately suffered a loss of power and Check Engine warning light lit up. I pulled to the side of the road and shut off the A/C [while yelling at the wife for turning it on. I repeated the immediate action drill and got no response. I let the engine idle for about two minutes and then shut it off. Another visual inspection showed nothing and when the engine was restarted, it ran without a loss of power, although the Check Engine light stayed on. I was able to limp home without furhter problem. I don't have an OBD2 reader presently.

Last week, the engine cooling system was flushed at 89,000 miles, the oil filter and synthetic oil were changed, a new fuel filter installed, and all new spark plugs were installed [one plug was found 'burned' - according to the service advisor]. The serpentine belt and coolant hoses are 1 year old.

A search showed no similar problems and I am stumped. It is clear that the A/C system is a trigger to the problem.

Any ideas, hypotheses, or areas to check?

 

A failing thermostat or radiator that's starting to plug are my first inclinations. The cooling system is "taxed" and the added heat from the AC condenser puts it over the edge.

However,
"Immediate action was to turn off the A/C, pull to the road shoulder, and then turn the heater on full. After about two seconds, the temperature gauge immediately dropped to mid level."
It takes some time to dissipate that much heat. Much more than 2 seconds.

I'm thinking the gauge temperature sender is getting flaky and giving false gauge readings. Your lack of other signs of overheating seem to bear that out.
On older models (through 2001, that I'm familiar with) the gauge sender and Engine Control sensor were separate, so a flaky gauge sender would indicate overheating, but wouldn't cause the driveability issues or a CEL that you mention.
By 2006, they might have been combined somehow. Thus the high gauge readings and "fail safe" driveability issues, but no external signs of overheating.

 

I'd be suspicious that there's an air bubble in the system and it just happens to sit under the CHT sensor once in a while.

 

Good point.
I originaly read that as the flush was done to try and solve the problem. If theproblem occured After the flush, that's a real possibility.

 

Thanks for the answers.

The truck was flawless in the first 89,000 miles, needing only normal maintenance. The radiator flush was done as an item of normal PM prior to any issue and not as a response to a repair requirement.

Yesterday, the truck was driven at <25 mph for thirty minutes, then driven at 55 mph for 15 minutes, and then driven home and idled for 10 minutes with the Max A/C setting on. No repeat of the previous gauge temperature issue or any problem.

The driving test will be redone today with a longer duration at highway speed to see if that has any effect.

 

You may have a failing fan clutch too.