Wintertime heater operation presents numerous questions and problems.
Here are some guidelines for professional technicians to ensure the
heating efficiency of their customers’ vehicles when the weather turns
Verify coolant level and condition. If the system is low, the heater
core is the first to lose coolant flow, causing a lack of heat in the
passenger compartment. Old coolant can cause corrosion and scale
buildup that can restrict coolant passages and reduce flow to or
through the heater core.
Check the temperature of the engine coolant to see if it has reached
normal operating temperature. If the engine thermostat is opening
prematurely, the engine temperature may not be sufficient to heat the
vehicle cabin. A scan tool can read the coolant temperature sensor, or
for older vehicles, a digital pyrometer can be used for this test.
Check the temperature of the thermostat housing to determine the
approximate opening temperature of the thermostat. Remember, normal
operating temperature will be 20º to 25ºF higher than the temperature
stamped on the thermostat.
The heater core should be checked for proper coolant flow. Simply touch
both of the heater hoses at the firewall to see if they are hot to the
touch. If one hose is hot and the other hose is cool to the touch, then
the coolant supply to the heater core is insufficient to heat the
cabin. The problem could be a closed heater control valve, a defective
water pump, air in the system, coolant system restriction or a clogged
If both heater hoses feel equally hot, then the most likely problem is
the temperature blend door not allowing air to pass over the heater
core. Follow the manufacturer’s procedure to test and adjust the blend
door. Another problem that could occur is a bypassing heater core. Some
core designs have a separator plate in the tank between the two pipe
fittings. The separator plate can erode away, allowing coolant to flow
from the inlet pipe to the outlet pipe without passing through the core.
If an air pocket develops in the heater core, coolant will not
circulate through the core. Check the manufacturer’s recommended air
bleeding procedure. The technicians may also be able to bleed the air
from the system by using a cooling system pressure tester to lightly
pressurize the cooling system. Loosen one of the clamps and hose at the
heater core but do not remove the hose. Lightly pressurize the coolant
system, allowing the trapped air to leave the system via the loosened
hose. When coolant starts to leak from the heater core fitting,
re-tighten the clamp.
At times the heater core may have a buildup of sediment, which
interferes with its ability to be a heat exchange unit. Flush the core
Remember, an 85º to 100ºF increase from ambient temperature is the
normal operating range of the heater. If the ambient temperature is
zero degrees, then the expected duct temperature should be about 90ºF.
Four Seasons Inc.