I have a 1996 chevy blazer. When I start out in the mornings the heater works fine. The temp. gauge eventually goes all the way over to the overheating mark and the check engine gauges light comes on. At the same time the heater starts blowing only cold air. The motor is not overheating and in a few minutes the temp. gauge goes back to normal and the heater begins blowing hot air. The process repeats over and over. A couple of weeks ago I did smell a weird smell coming out of the vents...almost like a burning smell but it has gone away. I am baffled. Anyone have any ideas?
Thanks,

 

Is there water in the radiator and overflow bottle....?

 

I had a very similar issue... Though I could very easily be wrong, I would check three things first. One, do a pressure test to see if you have any leaks. Two, replace your thermostat and thermostat sensor (the latter is a PITA). Three, (not really a check) pray your intake manifold gasket isn't shot.

I recently rebuilt my entire cooling system. My gauge would do the same exact thing, but I also was loosing coolant (which you didn't mention).

Good luck

 

Thanks Dark Rider. Unfortunately I am losing some coolant so it may be the gasket. I will check for leaks tomorrow. I replaced the thermostat 2 years ago so do you thing it could have gone bad so quickly? Thanks again.

 

parts can be bad - new right out of the box - could be a air lock in the system , bad thermostat, bad radiator cap, bad intake gasgets or even a bad water pump - I had a pump sheared inside it would be fine as long as shaft was makeing contact with it's self as soon as it didnt no circulation then no heat and rise in temp, make contact again then everything would go back to normal