Last month, I found my truck'* battery drained and when recharged, the heater was turned on in spite of the fact that I had turned it off 2 days before when I had last driven it. The battery tested fine, but the heater now only blew out cold air. A mechanic friend told me the ac/heater module probably needed to be re-programmed, but the local chevy dealer said the problem was with a faulty thermostat and wanted to change it and the water pump, pressure flush the system including the heater core and replace the coolant (although I had just flushed the cooling system and replaced the coolant 20,000 miles previously). I asked how the thermostat could be at fault as the engine warmed properly and held a steady 210 degree temperature. His response was that even though the spring valve to the radiator was working, the valves to the heater core were not as the flow of coolant to one of the hoses to the heater core was cool. Sooo, I agreed to having it changed but not the water pump as there was no evidence of bearing or impeller problem. The fix was short lived as now the heat emitted is tepid until I drive around curves at which point the temperature of heated air increases. So me thinks there is air in the cooling system that is blocking the heat transfer at the core but the dealership insists it is the water pump impeller "cavitating" and needs to be replaced. Then, he attempts to justify this claim by showing me that at higher RPM'* the temperature of emitted air actually increases. Let'* see' the faster the water pump impeller rotates, the less the cavitation??? Can I just pull the front of the truck up an incline, remove the top radiator hose and burp the system? Or is this indeed an ac/heater module problem?

If only one hose is hot and the other is cold, then you have a flow problem within the core itself. One thing too watch out for is the plastic engine shield will sometimes pinch the heater hoses, causing low heat output. Very rarely have seen a thermostat or water pump issue with these trucks. It does sound like you have air in the system. Removing the surge tank cap with engine running is enough to bleed the system.

His response was that even though the spring valve to the radiator was working, the valves to the heater core were not as the flow of coolant to one of the hoses to the heater core was cool.

Wow, that'* a good one......the "valves" to the heater core.....separate valves on that thermostat, huh?

It'* either an air pocket, or more likely, a need for a backflush to the heater core(especially if coolant level is ok in radiator and reservoir).........

well i thought he might be talking about the isolation valves some do have, like my yukon has a vacuum actuated valve in line of the heater core lines, they do wear out occasionally. but it doesnt have anything to do with the thermostat. and its a 99 so i dont know if the new body style trucks have them or if its may be electronic, i have not worked on one of the newer trucks with a heat issue.

That'* true, but from his post ALL THEY WANTED TO CHANGE WAS THE THERMOSTAT AND WATER PUMP......there was no mention of replacing a control valve for the heater core, if indeed it even has one.....

well that part makes me think they really dont know what they are doing. some techs are just parts chuckers, they replace stuff till it works at the customers expense. i really disagree with the whole flat rate system, which seems to be the whole reason for the chuckers.

I took my truck to another friend who is a good auto mechanic and his son took a look under the hood and pointed out that the intake hose to the heater core was crimped under the plastic cowling that sits atop the engine. We removed the cowling, repositioned the hose and voila, the heater works. This did not explain why the problem started in the first place, but it still solved the problem. My sincere appreciation to you who invested your time in finding an answer to this dilemma; you gave me avenues to pursue I did not possess. I did take the truck back to the dealership to show the "chuckers" that their diagnostics skills left something to be desired. I also pointed out that the thermostat, in lieu of what they had told me, did not have seperate valves to control the flow to the heater. If I had followed their advice and let them replace the water pump and the heater core, I would have been out $1,600. Kudos to this board!

thanks for updating your fix. i never liked the engine covers, they hold in the heat bake the wires and tubes and you cant see what going on. it serves no good purpose. one of the first things i do is get rid of it.

But it does quiet those noisy injectors...

things make noise, it is what it is. its when they dont that you have a problem.