I have had very weak heat and getting worse. I changed the thermostat and gasket. Almost the same. Its been so cold I just ran with weak heat.
A week later I get mad and look at the heater hoses to attempt to maybe flush the heater core and spy the real problem. The philips screw on top of the heater control had come loose and I basically was on cold all the time. Move the lever to cold, align the notch in the control box top with the cable end and tighten the screw. HEAT !!!!! Freaking hot heat too.
I hope this helps someone.
