Overheating

Have you checked if the fan is turning on? If that is the problem, the overheating will not occur at highway speed.

Removing the thermostat can create all kinds of new problems and will not cure the cause. In some cases, the hot coolant will circulate so fast it won't stay in the radiator long enough to give up its heat. That will make the overheating worse. More commonly, the engine won't get up to normal operating temperature. Parts will not expand to fit right resulting in increased wear. Most engine wear takes place when the engine is still cold. Blowby will condense in the oil and not burn off creating sludge and reduced lubrication. Engine oil also helps remove heat and gives it up in the oil pan. Once the coolant temperature sensor reaches a predetermined temperature, it signals the Engine Computer to go to "closed loop" which means it uses the oxygen sensor signals to fine tune fuel metering. By remaining in open loop, fuel mileage will drop, emissions will increase, and blowby will increase.

If you suspect a leaking head gasket, have your mechanic perform the "sniffer" test. That involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.