overheats at freeway speed changed tstat no help. also heater blows ambient temp although car is overheated, could this be a bad water pump even though it shows no signs of leakage. need help

 

Check the obvious first; make sure the belt is tight that runs the water pump-it can't do its job if it's slipping. If you have the serpentine belt that runs everything, often times the belt adjuster goes bad (has to be replaced due to ham-handed mechanics ), so read up on belt deflection for your model engine. Then check the fans. I'm not sure what motor you have, but my 103 has two electric fans and a clutch fan...ALL have to be working in concert to get the correct coolant temp. It could even be that your coolant sensor(s) are faulty. This should be obvious when you open the hood-excess heat more than usual....unless you just got the car and don't know what 'normal' heat coming out is suppose to feel like. If you had a leak in your heater core, most likely you would smell anti-freeze.

In one of the manuals I have, you can take the clutch fan off and install a screw in the bi-metal strip to make the fan engage earlier. But before that trick, check the obvious above.

Kevin