Hey guys I was wondering how can you tell if your thermostat is bad and not working I have a problem with my heat and don't know what it is it gets hot then cold then hot and it sounds like something is opening and closeing when it changes from hot to cold its a 98 blazer any info would really help I checked all the hoses and they were hot heater core hoses were hot so i have no idea.

 

If your temp goes from hot to cold, you are probably low on coolant in the rad (due to a leak somewhere).
Have you checked to see if the temperature feels the same on your upper and lower rad hoses and both heater hoses?
With your engine cold, you can take off the radiator cap, check the fluid level, top up if nessessary, run the engine and watch the coolant. When the thermostat opens, you should see the coolant circulating in the rad.
Another way to check the t/stat is to remove it, put it in a cup and add hot water frfom a kettle and watch to see if it opens, but for the price of a thermostat, I'd just replace it if I was to take it out.

 

sounds like maybe a vacuum leak somewhere. the opening and closing sound your hearing is probably the heater door. its controlled by vacuum. check for leaks. the vacuum line that goes to the vacuum ball on the drivers side usually goes bad.

 

if the thermostat is stuck closed then you should see the coolant back up out the overflow resevoir cap after it has run for a few minutes. if its stuck open then, if you have the rad cap off and the engine cold you we will see it circulate right away when you start it up. the change in temp is most likly a leak. i had a slight hole that i could not find because the coolant was hitting the fan, thrown around and left no signs of a leak. i changed the entire rad with a brand new(150.00) and found the small very small crease that punctured one of its pipes. just beware if you have a internal oil cooler or not(the internal cooler will work in a standard application as well, but not vice versa). anyway no more coolant problems for me.