91 tracer cooling system wont pressurize

i have a 91 mercury tracer, and the heat is almost non-existant. i canged the thermostat, and that didnt help so i figured id actually hafta troubleshoot it. i pressure tested the cooling system and it does not leak at all. i checked the cooling system pressure while running, and there was none, after letting it run for a while it went as high as 4-5 PSI but nothing more. when i give it gas and watch the gauge, it does drop under acceleration and rise under deceleration. it was time for a timing belt so i did the waterpump while i was in there, that didnt help either. i threw a new radiator cap on cause the other one tested a little low, but that didnt do it either. each part was done seperately, and the coolant was drained, flushed with a garden hose, and refilled with new coolant each time, so thats been done 4 times within the year, and has come out very clean every time. so basically, 91 mercury tracer, cooling system has no leaks, has new waterpump, thermostat, heater core, and radiator cap. only reaches 4 PSI while running. please help me. i miss my heat I don't think the system pressure has anything to do with your heater.

With it being so chilly out (if it is in your area) it's not alarming for the system to operate with very little positive pressure. That pressure is more important during the summer, when you want to increase the boiling temp of the coolant.

Have you felt the heater hoses running to and from your heater core? Are they both hot?

I think about the only thing you haven't mentioned yet is the heater core. Did you flush through that? Did it seem to flow freely?

Other than that, is there a heater control valve on one of the heater hoses? Perhaps it's staying closed for some reason. Are you sure the air control flaps are operating in the ductwork? If they are cable operated usually you can hear them slap closed if you move the temp control lever quickly.

Good luck..Car Repair Talk's forum.