Your problem could be caused by an air bubble in the heater coil. Take off the thermostat, turn the heater control valve off and on. If it starts working, replace the thermostat. If not, it is probably the heater control valve which will need replaced.
SOURCE: smoke coming out of vents need to change my heater core but how?
Iv'e done just about everything anyone can do on a car (a few times more than once)..and, heater cores are not my favorite job! Good luck!!
SOURCE: 2001 Lincoln LS heater blows
When the engine is cold,check the coolant level.
Make sure it's on max.
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A coolant system should be flush every two years.
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If you did a coolant flush recently then air pocket can be the cause of the un-even heating problem.
The DIY method is to drive you car onto a ramp or a steep hill.
Iet the engine cool off and top off the coolant tank.
Set MAX heat + MAX fan
Let the engine ran for 20 minutes wih the cap off and the trap air will escape from the coolant reserve tank,
Check coolant level and top off coolant when the engine cools down.
A shop will charge $75+ for this but using an air operated excavate tool!
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A car with over 90K miles may experience linkage problem to the heater control valve.
The heater control valve (near the firewall) may be stick over time.
Open the hood and move the heater to max and min. Back and forth.
Clean and lube the linkage with WD-40 to free the heater control valve.
If it's a vacuum type heater control valve then cash for leaky vac. hoses.
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Move the fan switch back and forth to re-new the electrical conductors.
Pass a vacuum over the control panel to prevent dust build up behind the console.
SOURCE: heater blows only cols air-
sounds like a restricted heater core. if you remove the heater hoses from the engine, pour some cooling system cleaner in. let it sit for 10 or 20 minutes, then you can flush with a garden hose.
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