Hi,
I have a 2002 Dodge, I am the only owner, and 2 weeks ago for the first time the check gage light came on, while I was driving on the interstate. I pulled off and let it cool down and it was fine for about 4 days, then on my way to work on the interstate, the check gage light came on again, this time when I stopped, there was steam coming out from the hood. My husband came and got the truck, took it in and they replaced the thermostat, measured the heat, visually checked the radiator and re filled it. After scratching their heads as to "why". My husband took off towards home, the gage went up about 3/4. Today we took it for a ride going about 55 -70 miles an hour with the A/C on it hit the black line, no light and it didn't overflow, but when we slowed down the temp dropped quickly.? We then went down a bad dirt road, put it in 4 wheel drive and had the A/C cranked, and the temp gage stayed normal.

What is wrong? All suggestions will be greatly appreciated as we and the mechanics are baffled.:confused:

I would immediately flush the cooling system, including heater core. If you have not been using a 50/50 mixture of manufacturer recommended antifreeze (likely G-05) and distilled water, there may be mineral buildup. Minerals found in tap water tend to combine with coolant additives. When this happens, they form a chemical complex called “phosphate scale,” which coats the heat-transfer surfaces of radiator and heads, resulting in decreased heat-transfer efficiency. When this occurs, your truck will overheat and run much hotter than it was designed to. This scale can also lead to the destruction of water pump bearings. It wouldn't surprise me if you needed to replace the radiator. The links below may help:

https://www.askmehelpdesk.com/cars-t...tml#post235973

https://www.askmehelpdesk.com/cars-t...ml#post1365453

Overheating problems are very serious. It can result in warped heads, blown head gaskets, and damaged valves. Repairs can easily run $2,500 or more.