Not turning the fan off is very hard on the ignition switch contacts and is a common cause of melted wires in the switch connector.
The heater core is a small brass or aluminum radiator in the dash that has hot engine coolant running through it. There are two problems that develop. Most commonly, sediment settles in it that blocks the flow of coolant. When hot coolant can't flow in, the air blowing through it doesn't get warmed up. Cold air out of the heater is the result. Ususally the blockage can be cleaned out by using a garden hose to force water backwards through the heater hose connections.
The second, less common problem is a leak caused by corrosion. Antifreeze is alcohol and will retain its freeze preventing properties indefinitely, but it has additives that wear out. Most notably, water pump lubricant and corrosion inhibitors last about two years. Conbustion gases from the engine tend to find their way into the coolant and forms acid over time. Flushing the cooling system and installing new antifreeze is done every two years to remove the acid and replenish the additives that fight acid buildup. Flushing the system also helps to prevent blockage of the heater core.
If acids are allowed to build up in the cooling system, corrosion results. The antifreeze comes in contact with aluminum, steel, lead and tin in solder, and possibly brass. Two different kinds of metal and an acid forms a chemical reaction identical to a battery. The chemical reaction eats away metal. Since the radiator and heater core are made from very thin metal to promote efficient heat transfer, they are the first items to corrode through. If your car does not have air conditioning, coolant leaking from a corroded heater core will drip onto the passenger front floor. Cars with air conditioning have a drip pan to collect condensation from the evaporator. Dripping coolant will also usually fall into that drip pan and will drain onto the ground between the two front tires. In addition, you might smell a sweet smell, and coolant blowing up onto the windshield will leave an oily residue.
It is also not unheard of to have a combination of both problems. A hole corroded through the heater core can be blocked by sediment. The symptom will be cold air when heat is desired. Flushing the heater core will remove the sediment but that exposes the hole so now the heater core leaks. Naturally, uninformed owners are convinced the mechanic is to blame when in reality they just exposed a problem that developed a long time ago but was hidden.
Replacing a leaking heater core is a big job that requires removing the entire dash board to gain access to the heater box. Usually the repair also necessitates discharging the air conditioning system so the heater box can be removed and disassembled. Flushing and replacing the antifreeze every two years is cheap insurance to prevent this problem.
Thursday, January 28th, 2010 AT 8:22 PM