Heat comes back temporarily after opening the bleed screw, but numerous mechanics have checked it out and said there's no air in the system. Replaced the T-stat 2 years ago, and still looks to be good, as the coolant temp guage is almost center. I have a suspicion that the heater core is clogged/blocked pretty good, because there doesn't seem to be very good flow of hot engine coolant through the heater core unless I rev the car to 4000+ RPM for a couple minutes straight. Ussualy blows cold air when I turn up the Climate control to full. Very little air coming out when the heat is on, and the dial on the vent is set to Three red dots. I want to isolate the problem before I rip out my dash, and replace the heater core.