After replacing engine coolant in my 99 cherokee 6 cyl., with new stuff, the heater will now only produce tepid warm air at best. A friend said i must have trapped an air bubble in the cooling system since it was working fine before changing both the fluid, thermostat, gakets etc., etc. I know I put everything in correctly, the right amounts and so forth. i'm not a mechanic but is something like a cooling system air bubble that can actually block up the cooling system itself really possible? The heat guage reads normal after warm up. The ol saying about if it ain't broke don't fix it is really applicable here, but the stuff I changed out was almost 12 years old.