Water pump was the culprit for me. I checked for Head Gasket, Radiator, heater core, flushed the system, purged for air bubbles, etc... nothing worked. I thought that it couldn't be the water pump because it wasn't leaking or anything...
Turns out the water pump was completely shot inside. All the impeller fins were totally corroded away.
see my thread...
zerothread?id=97865
My car drove the same either way. My symptoms were pretty unique.. if you share these symptoms you should guess it's the water pump.
1.) heater worked funny... Turn your heat on as high as you can. The heater core requires a water pump to force fluid in motion through the system. When you car is running hot, when you first turn your heater on to full heat, you will get hot air, but it will gradually cool down and eventually you will get almost not heat. If the water pump is not moving the system, no fluid will get into the heater core, and you will get heat from what's in there, but as you use the heat, it will not cylce more hot fluid in there.
2.) The car overheated only when driven on the highway for extended times, or when boosting. Granted my driving at that time was when it was very cold outside... but, the car didn't seem to have a problem just making short trips in town. But when it ran continuously on the highway it overheated... of course I didn't let it.. I would always pull over to the side of the road.
3.) No other explanation.... like I said.. we pretty much eliminated everything else.. test for bad head gasket, try flushing the coolant system to make sure you don't have some sort of build-up. purge the coolant system so you don't have any bubbles. Try pressure testing the coolant system to see if there's a leak or something somewhere. If none of those things help... it's your water pump.
The only other thing I could think of is perhaps the sensor is bad. But, you should know from popping the hood if the car is really hot.
Get a heat laser. My mechanic said their like $20. Shoot it at the coolant pipe and see what readings you get.
Good luck!!!
oh.. where to get one.. First.. try our sponsors. Call them and ask if they can get you one quickly. If no luck with sponsors, phase2motortrend has them... $129. Email JonPowell... somebody he does business with has them if I remember. RBmotoring?There around... you'll find one.
later