2004 Grand am 3.4
I am having trouble with the heat blowing hot at higher rpms (120 degrees) and cold at lower rpms (50 degrees)
What I have done... replaced water pump and thermostat took the car in to have old coolant vaccumed out and new coolant vaccum filled no air in the system. Also the system held a vaccum. I have pressure tested it and fixed the only leak I found. It held 15 psi for quite some time. I started it under vaccum and experienced a slow climb I pressure of about 5 psi. It was not a sharp spike. I have block tested and the liquid didnt change color. I believe I ruled out intake manifold gasket or head gasket. There is no coolant in the oil and it doesnt blow white smoke. I can run in 3rd gear on interstate rpm between 3 and 5 and heat works great, car doesn't overheat. Put it back in drive and starts blowing cold again. I had it vacuum fill to eliminate air in the system as a problem. The temp flex is 195 to 220. I dont know what else to do or what else it cold be... think I covered everything. What am I missing?