Posted 01 November 2008 - 08:43 AM
Okay Dan, here's how it works:
Your heating system is like the heating system in most houses, a boiler heats the water and it runs over to the wall radiator, and there it passes thru a series of tubes, and air moves over the tubes, and takes the heat from the radiator and puts it in the air.
In your car's case the engine is the boiler and you have 2 radiators: one at the front of the vehicle called the radiator, the other insider the heater box called the heater core. Engine coolant circulates in the water jackets in the engine and takes some heat away from the combustion process, then it's pumped by the water pump to the radiator (and the heater core sometimes if the dial is set on warm) and the heat is taken out of the water and put into the air by the action of the fans blowing air over the radiator/heater core.
The radiator is capable of radiating more heat than the engine is capable of producing. If the coolant is cooled down too much, the engine won't operate efficiently. For this reason, there is a thermostat in the engine (at the engine end of the upper rad hose) that opens and closes. If the coolant is cold in the engine, i.e., under about 180 degrees, the thermostat stays closed and does not allow coolant to escape the engine and flow to the radiator. Once the coolant heats up over 180, the thermostat opens and lets the coolant flow into the radiator. The water pump operates constantly, as it is belt driven, but just cavitates if the thermostat is closed.
Coolant flows into the rad thru the upper hose and back to the engine thru the lower hose. Coolant flows to the heater hose thru a diversion hose drawing off the top of the engine, and back to the water pump. The heater hoses are much longer than the rad hoses, and thinner, they flow less coolant (obviously).
Temperature in the heater core is controlled by the amount of coolant which is allowed to pass thru the heater core. This is controlled by a gate valve, which is actuated either by a cable (manual temperature control) or by a solenoid (climate control). Find the gate valve and control cable on the heater hose, and actuate it by moving the temperature control and watch if the cable moves the gate valve.
Your cooling system uses coolant. The level can go down merely due to evaporation. This should be a very, very slow process but you could lose a liter or two over a number of years due to evaporation. Anything more than that and you have a leak.
The heater draws coolant from the top of the engine, to a skinny-hosed loop far from the engine, so if that circuit fills with air, it can create a "vapor lock" and coolant won't flow to it. More often than not if the coolant level in the engine is low there is enuf for the coolant to flow thru the engine, thermostat, and radiator, but there isn't enuf for it to make it all the way to the heater core.
You may have had two problems: first, the coolant level may have been low because your water pump was leaking. So, you changed the water pump and filled up the coolant. But, in a system as large and loopy as a car's cooling system, there are bound to be air pockets, and you can expect to have to let the system "burp" or "settle" for a week or so. Keep an eye on your coolant level.
If your heater core is still cold, there are four possible additional problems: first, the thermostat is stuck open, allowing coolant circulation at all times, thus never allowing the engine to warm up. You will be able to tell this from your temperature gauge, it shud be somewhere in the middle. If it's too low, change out your thermostat (and don't put it in upside down).
Second, the cable could have slipped off the gate valve or it may be seized. This is not likely but it's worth checking.
Third, there are leaves/crud on the outside of your heater core, which are keeping air from flowing over the heater core and taking heat away from it. You will have to visually inspect this and vacuum out the heater core. In addition, your truck may have a cabin air filter (search the forum for the thread) and if that air filter is blocked you won't get any air moving over the heater core either. If it's dirty/blocked, replace it.
Fourth, the inside of your heater core could be blocked with corrosion/crud, keeping coolant from circulating thru it. If the rest of your cooling system is okay this shud not be a problem, but you never know - fix this problem last if all the other ones check out.
Sounds like you'll be spending some more time in your engine compartment......
Marko
2005 Sierra 1500Z71 SLT Extended Cab
