heater

I Have an 88 chevy cheyenne 4x4 that had after market a/c. We took all the a/c out cause it was junk. Now the heater works good when the truck isn't moving, but going down the road blows luke warm. There's a fresh air door we tried tieing shut but get no air flo. It has a vacuum line we hooked up but it's still not right. Any ideas?