New to the forums here. I have a 2005 Chevy S-10 Blazer. 2-door, 4.3l, 5 speed stick.

Whenever Im using the heater in my car, it works intermittently. It will blow hot about 30-40% of the time, but it will randomly start blowing cold sometimes. The engine remains at stock operating temp, and the thermostat and water pump are brand new and functioning properly.

Ive noticed that typically the heater blows warmest when Im on the freeway, but as I slow down to exit the freeway, or take an interchange, almost immediately the temperature of the air will start to drop.

Anyone else encountered this problem? Is it a blend door issue or is the heater core going bad or something else?

Saw lots of threads about blower motors and stuff, but nothing about the fluctuating temperature issue Im experiencing. Hopefully I can get some help here! Thanks in advance!!

 

Is it the temperature or where the air is directed? Especially if the later (but possibly with the former too) the problem could be with rotted vacuum lines in the engine compartment. These hoses supply engine vacuum to the system that controls the diaphragms that open and close the doors of the HVAC system. The vacuum supply is also used to control the 4wd front axle engagement and the control of the transfer case for the 3-button but not the 4-button 4wd system. (3-button and 4-button refer to the number of buttons on the 4wd selection switch. The 4-button one has the Auto 4wd feature.)

I've been maintaining a thread on information on these hoses. It is in my sig file below.

 

Thanks for the reply, christine_208!

So far I haven't noticed the air ever being directed to the wrong place. When its on vent it always comes out the vent, on on floor it always comes out on floor, etc., but that is an interesting thought. Perhapse the drop in engine RPMs lowers the vacuum output to a point where a broken line doesnt carry enough suction to keep the blend door open? (Assuming the blend door is vacuum powered here, too?) It just seems odd because other than that everything controlled by the vacuum system works fine! When I switch from Floor to vent or the other way around, I can hear it switch, and even when I change from cold to warm and back I can typically hear a similar kinda sound, too.

 

Yes, if there is a leak, the system will be susceptible it changes in the vacuum as the vacuum reservoir cannot compensate for any leaks.

As for the temperature control, the blend door is electrically actuated. However, there are vacuum operated doors that also redirect air through the AC evaporator rather than through the heater so a temperature change could still be due to a vacuum problem.

A hassle with vacuum is that it is one of degree as opposed to electronic problems that tend to (mostly) be either it works or it doesn't.

If you do a search online for "blend door actuator blazer" you'll get lots of hits for videos on how to repair them and replace them. They are different depending on if you have a regular or auto-climate control system.