Engine runs warmer with heater on. heater works great. when im driving on freeway or slow city traffic the gauge reads hotter with the heater on. It does not make any since to me. I burped the hell out the system like 5 times and put a new thermostat, radiator, and waterpump in it and she still does it. She does not get anywhere near danger zone at all just enough to notice it on the gauge. Oh BTW its a 1990 longbed 2WD 22rec with a w55 trans ALL STOCK. mileage reads 152,401. truck runs like a chapion and heater works fine so please if you have any ideas let me know.