It could be any of a number of problems. A heater
that's working properly should blow air that's about 75 to 100
degrees F. hotter than the outside air. If it doesn't, any of
the following might be at fault:
A low coolant level (often due to a leak or weak radiator
cap), but may also be the result of not getting the cooling system
completely filled. If you've just recently changed the antifreeze,
check the coolant level in the radiator to see if the radiator
is full. An air pocket in the heater core or a heater hose may
be interfering with the flow of coolant through the heater core.
One way to tell if the coolant is circulating through the heater
core is to feel both heater hoses. Both the inlet and outlet
return hoses should feel hot when the engine is at normal operating
temperature and the heater is on.
Refilling some cooling systems can be tricky. Some front-wheel
drive and rear-engine applications require special filling procedures
to eliminate the air pockets that become trapped in the heater
hoses and heater core. To help vent the trapped air, some vehicles
have a "bleeder" valve (or more than one valve) on the
thermostat housing and/or certain hose connections. Opening the
valve(s) allows air to escape as the system is filled. The valve(s)
should then be closed when coolant starts to dribble out the valve.
On vehicles that lack these special bleeder valves, it may be
necessary to temporarily loosen the heater outlet hose so air
can bleed out as the system is filled.
An open thermostat or one that's too cold for the application
(most vehicles today require a 190 to 195 degree F. thermostat).
One way to tell if the thermostat is stuck open is to start a
cold engine and feel the upper radiator hose. You should feel
no coolant moving inside the hose until the engine starts to get
warm. After several minutes, you should feel a sudden surge of
hot coolant when the thermostat opens.
A defective heater control valve. On most vehicles built
since 1970, vacuum operated heater control valves are normally
open unless vacuum is applied. This allows coolant to circulate
through the heater core even when the heater isn't being used.
To test the control valve, apply vacuum with a hand pump. If
the valve fails to close, replace it.
A plugged heater core. Accumulated crud in the cooling system
may plug the core and block the flow of coolant. The only cure
here is to replace the heater core. To prevent the problem from
reoccurring, the cooling system should be flushed and refilled
with a fresh 50/50 mixture of antifreeze and water. Distilled
water is best since it contains no minerals).
An inoperative airflow control or inlet door in the heater
ducting or plenum. If the defrosters aren't working either, you've
found the problem. Sometimes all that's needed to fix the problem
is to reattach or repair a loose cable or vacuum hose. On vacuum-actuated
systems, however, the vacuum motor or control switch may be defective
and require replacement.
A defective blower motor. If the blower motor doesn't work
(no sound/no air), the motor may be defective. Or, there may
be an electrical problem such as a blown fuse, defective power
relay, heater switch or resistor, or loose wire. A blown fuse
is a symptom not a cause. A fuse blows when a circuit overloads
to protect the wiring and other components against damage. If
the fuse is blown, therefore, find out why it blew before replacing
it. Always use a replacement fuse with the same amp rating as
If a new fuse blows as soon as the blower is turned on, there's
an electrical short in the heater circuit or motor that should be investigated. If the fuse lasts a while and then blows, the fan motor is probably running hot due to worn brushes and/or
bushings and should be replaced.
The motor itself can be checked by using a pair of jumper wires.
Connect one wire to ground and the other to a source of battery
voltage. If the motor fails to spin, it should be replaced.
A pinched or kinked heater hose. In rare instances, you might
even find misrouted hoses if somebody worked on the cooling system
A weak water pump (one with badly eroded impeller blades),
or one that doesn't turn fast enough because of a slipping drive
An electric cooling fan that remains on all the time, or a
clutch fan that's locked up and overcools the radiator (excessive
fan "roar" at highway speeds would probably be noticeable).
CAUTION: Do not block off airflow to the radiator by placing
cardboard in front of it in an attempt to increase heater output.
This common "trick" can cause an engine to overheat
when it is lugging under load or if it is left idling for a long
period of time. There's also the danger that someone might forget
to remove the cardboard when the weather warms up. A better way
to increase heater output is to install a "winter" thermostat
(195 degree F. or higher) -- but only if it is an older vehicle without
emission or computerized engine controls.
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