by Jim Kerr

Cold weather brings out many problems in a vehicle that would normally not occur or could be overlooked in a warmer climate. One of the more common problems many drivers have is not enough heat in their vehicle.

Checking the thermostat is usually the first step in repairing a vehicle with a lack of heat. Even if the thermostat has just been replaced, I have seen too many new thermostats that only work for a few times to trust one just because it is new. Check an existing thermostat by monitoring the temperature of the engine while touching the upper radiator hose. The upper radiator hose should remain cold until the engine has reached nearly the opening temperature of the thermostat. This is typically about 87 degrees C. If the hose feels warm sooner than this, the thermostat is opening too soon and should be replaced or there is coolant flow around the thermostat. Some universal thermostats do not fit tightly in their mounting location and coolant can bypass them. Try a different manufacturer of thermostat if this is the case.

Another cause for poor or no heat is an air lock in the cooling system. Heater cores are often mounted higher than the top of the radiator. If the cooling system has been refilled, air can be trapped in the highest part of the system. Sometimes one of the heater hoses or a coolant hose mounted high on the engine has to be removed from the heater core with the engine running, to “bleed off” the trapped air. Be cautious of hot coolant. It is safer to bleed the air out of the system when it is cold. If heater hoses don’t come off the heater core fittings easily, it is better to cut them off and install new hoses rather than chance breaking the heater core itself.

Many manufacturers are now installing air bleed ports at strategic places in the coolant passages to allow air to be manually bled out but sometimes a hose still has to be removed to get the air out. At the factory, the coolant is installed after a vacuum has been created in the system. This fills the system quickly without any air locks. Some repair shops are using this method as well but it doesn’t seem to work quite as well as at the factory. It may still need to be bled manually.

Sometimes a corroded heater core causes poor interior heat. The heater core can be flushed with clean water but often this doesn’t work well because there are many corners and small passages in a heater core. A heater core with good flow should have one heater hose very hot and the other only slightly cooler. If the outlet hose for the heater stays cold, then the core is restricted. It will probably need replacing. Some vehicles use a water shut off valve for the heater system. Be sure the shut off valve is fully open before condemning the heater core.

It is possible to have a blockage in the heater hoses or at the engine fittings. Older hoses sometimes collapse inside and need replacing. The heater hose fittings on some engines have a small hole in them to slow the flow of coolant at high engine rpm’s. If they are blocked, they can be cleaned easily with a drill bit. Don’t enlarge the holes though. Coolant flowing too quickly through a heater core doesn’t have time to transfer heat to the air passing through. It can also erode the heater core material, causing it to fail prematurely.

Other causes for poor heater performance are restricted air flow through the heater or a cold air leak past the heater core. Misadjusted temperature doors or binding heater control cables can cause cold air to bypass the heater core. Even a small air leak will decrease heater performance significantly. Many new vehicles use a replaceable pollen filter in the heater system to remove dust and pollen. These filters can become plugged quickly in dirty conditions and restrict air flow.

Systems that don’t have a pollen filter can still become plugged with leaves, dirt and even mouse nests. Sometimes the housing has to be disassembled to clean it out before heater performance is restored.

Getting heat into a vehicle’s passenger compartment isn’t rocket science but there are many things that need to be considered. Fortunately, this is a problem that can usually be fixed fairly quickly.

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