[c.w. fields]

Changed out the antifreese on my 75 Grand Am and while I was at it I changed out the thermostat with a fail safe thermostat that I bought a while ago and had in the glove compartment. I was having what I thought was problems with the thermostat prior to antifreese change out. The car would heat up and watching the temp gauge it would go over the half mark and then it would go down as I drove the car so I thought it might be sticking. This new fail safe has the temperature going all over the place from all the way to the left to 1/4 mark where it normally runs. Yesterday I found the top radiator hose partially collapsed and went to NAPA and got a new one. As the overflow bottle was almost full I noticed it went down to add after a run to a local car show. On the way back home from that the temp gauge went over to hot and then back to 1/4. Any thoughts on perhaps this fail safe being defective?

[fiedlerh]

In a normal engine temperatures change slow and steady so, in no particular order...
Sending unit has a problem,
Sending unit wiring has a problem like a loose wire or loose connection at the sending unit.
Sending unit is seeing air + antifreeze = erratic temperatures, which would want me to pressure test the cooling system or look for bubbles in the radiator.
Radiator hose collapsing means the radiator cap is not be allowing the coolant to suck back in once everything cools down.

[Lightfoot]

Any chance the fan belt is 'stretched' or clutch fan is too loose?

It's not clear if the temp is going to extreme while the car is sitting still, or in motion.

I once had a fan belt that behaved like a rubber band. I'd draw it up tight and a week later it would be all flexed. I was too cheap to buy a new one. Finally, the engine pitched it off while I was driving down the interstate.
No problem until I got to my exit. Then I had to take back street shortcuts to get to work before it overheated.

[camerjeff]

All I can tell you is that I used to change the thermostat every other year as normal PM, but about 10 years ago I had a lot of issues with "NEW" cheap thermostats that were made in China, they would either not open or no close, or act as you are describing.

I now only change thermostats when they have operating issues, and have only used Robert Shaw High Flow models. I have not had an issue sense then.

[rexs73gto]

When you changed the coolant did you let it run with the cap off so it could completely fill & get ALL the air out of the system before you put the cap on. Also did you replace the hose when you found it collapsed. It use to be when we had cars in the shop for coolant replace we would jack up the right front side of the car so it would get the right side higher then the rest of the car so the air would all be out be getting the right side up higher then the rest of the car. That way all the air / bubbles would come out because the radiator fill side would be higher then the rest of the car. You need to get ALL the air out because the things your talking about is what we had problems with to.