I have a 1997 Blazer with heating problems. When the truck isn't moving it blows almost totally cold air. Then when it gets moving the air will get a little warmer but not as warm as it should be. The temp gauge goes up and down too. The temp never gets high, but it fluctuates between about 190 and 160 instead of sticking at one temp. I replaced the thermostat but I still have the same problem. When I feel the hoses to the heater core one of the hoses is hot and the other isn't.

 

Flush the core to make sure that it isn't restricted. Once you are done with that, make absolutely sure that all of the air is out of the system.

 

How do I make sure the air is out of the system? I can't see any kind of valve that looks like it would be opened to let the air out of the system.

 

The first thing you have to do is flush out the heater core. There are multiple posts here on how to do it but basically it involves connecting a water hose to the heater core and hope it doesn't blow up. Once that's done you need to elevate the front of the car and run it for about 1/2 hour after it's warmed up with the radiator cap off which should get the air out. Once you see it "Burp" check the heater and if it's OK then fill the radiator and put the cap back on.

If for some reason you can't flush the heater core one of the other users here said he used a 50/50 mix of vinegar and water and let it sit in the core overnight and it worked. Not being a real mechanic I can't comment on what vinegar might do to the core but I did sleep at a Holiday Inn once.

 

Check out the Tech Article section for the heater core flush how-to article.

As far as bleeding the air out of the system, you need to get the front of the truck elevated as much as reasonably possible with the passenger side of the truck slightly higher than the driver. This will get the radiator cap location at the highest point in the system which will more easily allow the air to escape.

Once you are all setup, you need to run the engine up to operating temperature with the cap off, adding coolant as necessary. Then let it cool down for 15-20 minutes and repeat until the level doesn't really change all that much. That should get the air out of the system sufficiently.

I have used the vinegar and water trick a number of times in the past with stubbornly clogged heater cores. If a core has been clogged for quite some time, you may end up with a leaking core after you clear the clog just as a warning. The act of clearing the clog didn't cause the leak, the clog was just keeping it sealed and allowing corrosion to make things worse. Sooner or later, it would have leaked anyway. Just FYI.

 

Flushing the heater core worked like a charm. Even easier than changing the thermostat. Thank you very much for the help.

 

Glad it worked out for you!