Coolant overflows with cap off.

[jimbo231]

My temp guage on my 1998 explorer was going up and down for a while. Now I have no heat. I though I had an air leak so I ran the truck with the heat on high and the rad cap off. Coolant was slowly overflowing from the rad. Is that normal or is that air making it do that?

 

[4x4xbrian.com]

It is normal, the fluid actually expands quite a bit with heat this is normally held in by the 16 lb cap and any overflow is pushed past the cap into the resivoir. temp fluctuation is usually the water pump.
(bad w.p. can yield air in the system)

 

[jimbo231]

I was wondering, can thermostats fail shut these days? Some one said no heat could be caused by a failed thermostat that won't open?

 

[CDW6212R]

Yes, but your temperature gauge should show that quickly.

 

[kzgrey]

I have had a bad thermostat which appeared to get stuck closed. After running the car for a bit on the highway, I would notice that I could not get any hot air from the blower. I'd suddenly see the temp gauage jump, then regulate and the air from the blower would suddenly become super hot (as if the coolant was suddenly pumping through the engine block and eating the blower air properly).