heater is all or nothing????

Hey there....my '91 heat works great....but gets damm hot, and doesnt get cooler on the temp selector until I go full cold....and then it is cold! My brothers 96' is the same way....what is up???

Check for vacuum leak.

Dont have my manual handy, could you be a little more specific? Which lines to check, etc...? Thanks

WhatI did was take a can of starting fluid and spray around all the vacuum lines under the hood till I noticed a change in rpm (upward). You can also pull the controller out of the dash and do the same thing. I found many rotted vacuum lines under the hood and replaced them. All was good in the dash though. Fixed my problems.

Does'nt the 91 use a plastic mechanical mixing valve on the heater lines that is controlled by a cable. It the cable works then I would think the valve is messed up.