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Heat hot at high rpm, cold at low

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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?

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Sounds like you've done most of the standard stuff. Have you checked your electric fan(s) to make sure they are working properly and perhaps checking the fan temperature sensors to make sure they are working / calibrated correctly? I am wondering if the fan is coming on and off at the wrong temperatures. For that matter, you might check your fuses for the radiator fan. Also check the electrical connections on the fan for corrosion or dirt contamination.

Can I assume for your note above that you are running 50/50 mix water to antifreeze? 

Edited by Frosty
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Is thermostat bad or maybe it didn't seat correctly when installed. It's happened to me before. Like to much cold water going through. Maybe try a hotter thermostat????

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This may sound like a stupid question... Am not trying to be condescending...

Are you 100% sure that the system is full including the heater core..??

Something that might be worth looking at... Check the hot water  valve that regulates the water flow into the heater core (controlled by the temp switch on the HVAC panel on the dash) for proper operation or for  some sort of blockage or maybe sticking or something.....Are the heater hoses routed correctly (inlet usually the top hose )  going into the heater core from the engine and or radiator..and not squeezed off or kinked somewhere...Or possibly installed backwards...



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