Today the heater started to blow cool air out the vents. I then noticed the temp guage start to rise, but did not beg out. I turned the heater off and maybe 5 minutes later the temp guage comes down and back up to normal. I turned the heater back on and it was fine. It did this twice while I was driving in the city and didn't do it at all when I got on the highway. Any thoughts of what might be going on?

 

[sm=welcomesign.gif]to the Forum. First, tell us which model Blazer you have. Have you checked the coolant level and how long has it been since the system was drained and/or flushed and how old is the thermostat?At this point it could be a thermostat going bad, low coolant level, faulty Engine Coolant Temperature sensor, air in the system, heater core plugged or failing, etc.

 

Its a 1999 4.3 LS 4dr. Looks like I'm going to check some things that you mention.