How do you flush the coolant system on a 98 STS. Of late my car’s temp has been rising darn near to the red line going down the interstate in fact it got so hot the other day the car went into idle mode until it cooled a little. The gage stays at 12:00 for a long time but then jumps towards the hot side and eventually goes back to 12:00 What would cause this? I can smell antifreeze when the gage goes up so it is defiantly getting hot.
The car has about 140,000 on it and the head gaskets and case half’s were done at 90,000 and water pump/thermostat at 135,000
I have an infrared thermometer and pointed it at the radiator from the front and the temp varied from about 84 to 96 then I pointed at the upper hose housing and got 220 this was done when I got home today after the gage went up on my way home. Any ideas?

 

Needle cycling while car is in motion normally means low on coolant. Thermostat is possible, but I suggest "cooling system pressure test" to start. Heater pipes that run across the rear head will rust out after 5yrs+, normally at the clamps that hold them to engine.

 

Hi when i drive my car at that time in afternoon so much hot & therefore car also getting hot.
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Shawn it's 60 degrees outside. lol