Heater problems

[79chevybigred]

this isnt on my chevy, its on a 93 Mazda Navajo. When the truck has been runing for a while and engine temp is good when it is let to idle nothing but cold air comes out. Then when the rpm's go up you can tell the air blowing out is getting warmer (never warm enough)and the temp needle in dash also rises and falls. I'm thinking there may be something clogged in the heater core. Any ideas would greatly be appreciated.

BTW i can drive from anchorage and back approx. 80 miles and the same occurs no change.

[wyldman]

It does sound like a plugged heater core.Have you checked the lines for any kinks ? Feel the lines going in and out of the core.If one is really hot,and the other is cool,then the core is probably plugged.

Also check to make sure you don't have a heater shut off valve.Some trucks with AC do have them.It may be sticking,or plugged,restricting flow to the heater core.