Hello all i bought my dads pickup 86 f250 4.9L and its been great. After a few years of daily driving i lost my job and the pick up sat. After 3 years i put flushed the engine/rad. then put on a new water pump. started it up and drove it a little to notice i was overheating and heater wasnt working. so i went and got some flush for my radiator and a new thermostat. followed the directions for my flush then installed the new thermostat. all was great heater core worked and didnt over heat at all stayed between the o and r on gauge. then a week later as i was driving to school the temp went just passed the R but didnt pass. On the way home from school driving down the freeway it shot up all the way to the L so i shut it down and pulled over. It spit all the coolent out the overflow just like it did before. I have no idea why it runs good and has never had the problem before. any clue?

 

 

Are you sure you put the thermostat in the right way? Also, had there ever been any stop leak in the system? I would do a full flush then pressure text the system to make sure all air has escaped.... Could also be a faulty aftermarket thermostat..

 

Prestone flush is acidic and known to eat old gaskets. I would ensure the t-stat is installed correctly, there is no air in the system, and I would use GM coolant system tablets. The coolant system tablets will clean out your coolant system, without eating gaskets.