If your car has air conditioning, it will have a vacuum-controlled water valve. You can watch the actuator lever move when someone in the car switches it from AC to heat. If it doesn't move, suspect a vacuum hose problem.
You can also move that lever by hand, then wait to see if the heater hoses get hot. Dandy observation, by the way. Information like that helps a lot in the diagnosis. If they still don't get hot, suspect a plugged heater core. To prevent the possibility of breaking the solder joints on the core, I like to remove the hoses from the engine, then back flush the core both ways with a garden hose. Limit the water pressure to under 15 pounds. Be aware too that due to its age, it's always possible the sediment is blocking a leak caused by corrosion. There's a small chance you'll end up replacing the heater core. Only heard of this happening a few times, but it's worth mentioning.
Another good observation is the hot radiator hoses. A lot of people run into an overheating problem after replacing the thermostat because they don't know you have to burp the cooling system. Hot air will not cause a thermostat to open; it must be hit with hot liquid. There will be a big air bubble behind the thermostat until you remove one of the hex plugs or temperature sensors on the thermostat housing, and refill the radiator. Sometimes people get lucky and just run out and drive the car, oblivious to the overheating. Going up and down hills and over bumpy roads can cause the coolant to splash up to the thermostat causing it to open. That will allow the air bubble to move to the radiator. Problem averted, and they never even knew it. Sometimes though, the engine overheats and warps the cylinder head. Simple maintenance turned into costly repair.
Saturday, December 12th, 2009 AT 3:15 AM