You either have a bad water pump or there's air in your cooling system.
To test for a bad water pump, start your engine and let it reach full operating temp. Then have someone rev the motor up and down while you feel the upper radiator hose. You should feel pressure building up as the motor revs. If it doesn't, then you pump is bad.
If you do feel pressure, then you have air in the system. This air gets trapped inside of your heater core, and sometimes your radiator and water pump. This prevents your coolant from circulating correctly, and thus, your overheating condition.
Now how that air is being introduced into the system is where your worries lie.
If you've recently had your system flushed, or added coolant, then you may have failed to bleed the system of air.
You have a blown head gasket. As you place your engine under load (driving at highway speeds), exhaust gasses are getting into the coolant system and are getting trapped in the heater core.
The quickest (but not surefire way) to tell is to pop the cap on the overflow tank and start your engine. While someone revs the motor to at least 1500 rpms. Look at the coolant in the tank. If you see large bubbles, then that's a blown head gasket.
If you don't see air bubbles, then try bleeding the system.
Start by removing the radiator cap or coolant surge tank cap. Then let your engine reach full operating temp. And place your heater controls on high/full heat.
Have someone keep the motor reved at 1500 and squeeze the upper radiator hose several times until you don't see anymore air bubbles in the coolant.
Tuesday, April 27th, 2010 AT 6:41 PM