Car overheated on highway. Have replaced radiator, water pump, thermostat, etc. Passes headgasket test, but mechanic used the pellets just in case. He is stumped. Does fine for short trips around town, but still overheats after 25 miles or so on highway. Even after short trips, pressure stays high in the overflow tank hours later. (May be unrelated but recently heater stopped working and air blows hot.) I can't keep this car, but I can't sell it to anyone either. HELP!

 

Check if the thermostat is the right kind. some cars have dual port thermostat that redirects water to transmission separately. If you use a non ported one it may not work correctly. Also check the timing to see if it is advanced. Also dilute the antifreeze a bit unless the manual says not to do that. If all looks good then the fluid cooling passages may be blocked. That's expensive to clear out.

zahid