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Check Gauge - Thermostat shows Hot then Not - Ignore?

My dear ole Suburban w/rebuilt engine just got serviced 2 weeks ago. Since, the temp gauge moves up to the HOT zone about 5 miles into driving 35 mi/hr on hilly road. Turn on heat and temp goes down to normal. Accelerate, temp goes to normal. Big Problems? Or should I just keep the heat blasting…which doesn’t seem to be working exactly right either.

I don’t think the problem is with the temperature indicator. I think there is a real problem with the engine cooling system. Have the service check this very soon.

I have a trip to take (1 hr drive) and thought it’d be ok if it kept cooling off so quickly and not staying hot. Too risky? (Denial is a powerful tool!!!)

Change your thermostat NOW. Quick, simple, cheap and easy. Don’t want that t-stat to stick closed even one time. No coolant to a warm engine turns the engine HOT, and bad things can happen fast. Rocketman

Sounds to me like the cooling fans aren’t turning on. At higher speeds just the normal air movement will keep the engine cool enough. Also turning the heat on the fan for the heater may cool it enough. It needs to be checked out.

It’s up to you, but some of the possible problems could end up standing you or damaging your engine. Of course it could be nothing more than a bad sensor. At the very least make sure the coolant is not low. I suspect the real problem is some air in the system that they did not get out when they worked on it.