91 Sundance heating problems

I just bought a 1991 Plymouth Sundance for use as a work car, 2.2 liter with
a 5 spd. Its got high miles, 161K, but runs real good. I changed the
thermostat right away cause it was aparrently sticking shut. Now its not
holding heat very well, any ideas as to why? Kinda stumped

 

Not holding heat? I guess you mean you have none? What do the heater hoses
feel like going in and back out?

 

You probably didn't bleed the air out of the cooling system after changing
the thermostat. That's what the threaded plug atop the "water box" is for
(the "water box" being the part of the cylinder head where the upper rad
hose connects, where the thermostat is located). On a '91 with 161K miles,
you may find the plug seized in place, in which case loosen the radiator
cap so it holds no pressure, elevate the front of the car as high as
practicable, and run the engine at a fast idle with the heat on until
normal engine temperature is reached, then for another five minutes or so.
This will allow air to travel to the radiator and out to atmosphere.