There is a couple of things you are missing here Pat.. a cold heater can tell you a very crucial piece of information.. if your engine is fully warmed up and you only blow cold air.. you either have a blockage of some sort at the heater core, or.. you could be extremely low on coolant.. OR.. your waterpump is NOT circulating your coolant as it should be. There are more than a couple of reasons that can happen with a water pump depending on the engine and pump design. We all saw what happened to franko's impeller.. and it still amazes me that that thing was still circulating coolant. But that was on a 4cyl engine.. smaller water passages & journals.. so the turbulence that that impeller was still creating.. could have been enough to move the coolant around a bit. Now.. in our V6 engines.. the water journals & passages are a fair bit bigger than the 4cyl engines and the cooling system in general holds more coolant does it not? That means if the impeller comes apart or corrodes like Frankos did.. there is even more resistance to coolant flow to be overcome. Another thing that can cause that would be if your fan belt was loose. Not spinning the waterpump will have the same effect that a defective impeller would.The only thing that a cold Heater can tell you is one of two things. The heater core is crapped out or there is not enough radiator fluid. A bad heater core does not cause overheating, just cold heater air, or a leak of radiator fluid in the passanger floor board
Is a cold heater the end all symptom to look for if your having overheating problems? No. But it is one symptom amoung many that can lead you to a solution for the issue you are having. If I was full up on coolant, wasnt losing coolant but was overheating and I was only getting cold air from the heater.. Good bet your waterpump is toast or you have a blockage. And while most waterpumps have a drain hole that will start spewing coolant if the bearing gets to worn.. there is nothing to warn you if the impeller comes apart. For future reference.. a stuck thermostat will NOT prevent hot coolant flow to your heater core.
A cooling system pressure tester kit like this one Can help find leaks in your cooling system when the engine is cool. Hoses, radiators, heater cores, head gaskets.. leaks in any of these things can be located with that kit. Now cracked heads or blocks are a bit trickier.. especially since our engines are completely aluminum. Aluminum expands and contracts with heat.. so when an engines cold.. those itty bitty cracks in the heads wont allow coolant to get past them.. but once the engine gets to operating temp.. those cracks wont seem so little any more and you will start losing coolant into either your oil or into the combustion chamber(s).
I'll stop rambling now.. but in closing.. I will point out that the most overlooked and most common cause of overheating is quite simply.. your radiator cap. When they fail.. they either release their pressure to soon or not at all.. both of which can lead to overheating if the condition persists long enough. Cheers
Tunes67
*EDIT* I forgot to mention that those cooling system pressure testers can also test your radiator cap to make sure its functioning at its proper pressure level.
-Jim
