My 85 GMC Jimmy boils?

[gerrythibodeau]

HI. I'm having a problem with my 1985 GMC Full Size Jimmy. It has the 305 engine. My main concern with this vehicle is the coolant system. today i was idling for about 15 minutes and the overflow started boiling so i shut the truck off. After that i watched it for a good 20 min and the whole time antifreeze was boiling out of the overflow comming out of my radiator. i drained and flushed the whole coolant system and put in fresh antifreeze. could any1 please help me figure out why this is happening? my only other thought is that maybe the water pump isnt pumping. because the fan is spinning fine and the radiator is new. thanks

 

[Sterling Auto Works]

thermostat is a good Idea way to check it out is after it has been running and starts over heating check the gage to see what it says and if you are brave feel the bottom radiator hose to see if it is hot if it is cold thermostat is not working! if you replace the thremostat and the problem happens again then go for the water pump!