With a COLD engine, open up the radiator cap to see if you have enough coolant. If you are low, it may still be good enough to cool the motor, but will only be able to circulate coolant through the heater circuit with high revs.
Do you have a temperature gauge or just a light? If the motor is overheating, you must be just about out of coolant as well.
If you share the cold with me in Canada, you MAY have a frozen radiator/heater if the coolant antifreeze wasn't strong enough for the winter. In that case you if you get in and drive the car it WILL likely overheat since the liquid coolant is solid or slush and can't move through the radiator to cool.
OR... maybe a cable for the heat/cool control is disconnected or you have lost vacuum control for fancier models... did you ever hear a "shhhh' when you moved the heater controls.?
Let's hope it's just low antifreeze... if you add some, make sure you maintain about a 50/50 mixture of antifreeze/water.
Hope this helps...
/denis from Canada
SOURCE: air condition/heater
sounds like the air-conditioning expansion valve is probably not working correctly here is a way you can fix your air conditioning:
SOURCE: heater blows only cooled air
my heater only blows cold air please could you give me some advice on this thanks . everything else is ok temp ,engine etc
SOURCE: 93 gmc sierra heater not working
THERMOSTAT REPLACE IF NOT SEE IF HOSES ARE CLOGGED SHOT OF AIR COULD HELP OR DISSCONNECT BATTERY CABLE 10-15 MIN CONNECT BACK TRY HEATER
SOURCE: heater only blows cold air but hoses are hot
Hi!
It
appears we have an Air lock scenario and you will need to perform a
system Bleed.
Park
the vehicle on level ground, when cold remove coolant filler cap,
start engine and leave to idle, turn heater on full and blower to
max. When engine reaches operating temperature watch and listen near
coolant filler, keep clear as gurgling and hopefully a boil over
should occur. Top up with very warm coolant and wait as it may do it
again.
Check
for heat inside vehicle if warm replace coolant cap but keep an eye
on temperature gauge as the ~Air lock may have moved on from heater
matrix/core so proceedure needs to be carried out again from COLD.
If
persistent boil ups/over attention must made in the cylinder head
or
gasket area, or possibly water pump?
Please
press the Blue button to appraise my FREE Efforts, Thank You!
Paul
'W' U.K
SOURCE: Why doesn't the heater blow hot air?
If your "Check Engine" or "Service Engine Soon" light is on, you need to have that checked out first.
If not, the next thing you should check is your engine coolant level. (Antifreeze) If the coolant level is low, the engine coolant does not circulate through the heater core and the the heater doesn't work. The coolant level could be low for a number of reasons including a leaky hose, water pump, or head gasket. In any case, if it is found to be low, you need to find out why.
If the coolant level is OK then the thermostat function needs to be checked to make sure that the engine is reaching operating temperature.
If all of the above checks out OK, then the heater control valve and/or the blend air door in the HVAC housing needs to be checked for proper operation. (NOTE: some cars are not equipped with a heater control valve)
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