I just replaced my therostst and flushed my radiator and put a new water pump on to fix overheating. It pressures up as soon as I start the pick up,...why

 

may be getting combustion pressure into cooling system through a leaking head gasket or cracked head, most shops can check for exhaust gas getting into the cooling system by using an exhaust annilizer, they also have a kit you can get at your local parts store called block check that contains a blue fluid that will change color if combustion gases is detected.

 

The man who flushed my radiator mentioned this. Although I was still hoping for the best.

Thanks for such a quick response!

 

Type of vehicle? Year? Engine size?