Cold air comes from heater ducts after car reaches operating temp. Coolant level and thermostat are fine. Operating temps are fine. Does these cars have a water flow valve to heater core like older models and if so where is it located?
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Check coolant level first for being low. IF level ok, possible plugged/restricted heater core. With engine at operating temperature, check hoses under hood going into heater core. They should be close to the same temperature. If one is hot and the other warm/cold, suspect a restriction in the heater core and you will require a back flush to remove the restriction. If both hoses warm/cool, suspect a thermostat issue and the engine will not reach proper operating temp to supply adequate heat.
Check to see if the coolant is reaching the heater core and that the heater core isn't leaking. Make sure vehicle is reaching operating temp and the thermostat isn't stuck open. Also check for proper coolant levels. Coolant systems should be flushed every 3 yrs to maintain optimal service.
It could be a plugged heater core or heater control problem. I assume the blower is working, but the air is cold. The first thing to check is the coolant level, make sure there is coolant in the resivoir. Next see if the temerature gauge reaches the normal operating temperature range. If the gauge reads cold most of the time, it would tell you that the thermsotat is faulty and not allowing the engine to reach full operating temperature. If your car doesn't have a temp gauge, the blue cold engine light might stay on for an extended period of time. The thermostat is supposed to not allow coolant flow in the radiator until the engine reaches operating temperature. If you drive the car for a couple of minutes, from cold start, the radiator or upper hose should be cool until the engine reaches operating temp.
Check your coolant level and make sure its full, next i would look into replacing your thermostat, if your thermostat is stuck open your engine isnt able to reach operating temperature therefore heater output suffers, your coolant temp gauge should read around halfway when the engine is hot if its not look into replacing the thermostat, if your gauge is reading halfway check the cable going to your heater box when you adjust your temperature on your heater controls its possible that the cable broke or the blend door inside the heater box is stuck into the cold position allowing only cold air to come through the vents.
If I understand you correctly, your temp gauge shows correct operating temperature, your blower fan blows air that you can feel with your hand from the right floor or dash outlet, but that air doesn't feel warm until you are moving?
If so, several possibilities come to mind. One is that your coolant level is low. Even though your temp gauge reads in the right range, the coolant level could be low enough that it doesn't reach the heater core until there is enough engine RPM to create more pressure from the water pump and force what coolant is there toward the heater core. Another possibility is that your heater controls are not able to do their job. A stuck flapper in the duct or a malfunction with the temperature control system could be causing the problem, and at 15-20 MPH there is enough air coming through the outside air vents to provide some heat.
I'd check the coolant level first, though. The other possibilities are more far-fetched.
if your vehicle has the electronic climate control, it is probably the blend door motor/actuator that has failed and is stuck at the temp that was last entered in the climate control unit when it failed. this can be a very time consuming repair to do as it is under the dash. the other possibility is your coolant level is low and not allowing the heater core to bring the outside air up to the temp you want. if that is the case,your low coolant temp sensor should have come on. if not, then the thermostat could be either stuck open or closed. if stuck open the engine would not always show the temp gauge at normal operating temp of 195-200 degrees. if stuck closed the engine should be overheating according to the temp gauge. you did not mention what the operating coolant temp was!
you have air trapped in the system.
Park facing uphill (on car ramps if you have them).
with cold engine, remove radaitor cap and fill radiator.
leave cap on loose.
Turn on heater.
start engine and run until upper radiator hose gets hot (meaning thermostat has opened).
Check coolant level and top off. Repeat until temp gauage reads normal and heat is coming from heater ducts.
Hi! Looking at your description I feel a Air lock scenario? Have you tried a system bleed? Park vehicle on level ground, when cold remove coolant filler cap, start engine leave to idle, turn heater to max and blower full. Wait for engine to reach operating temp, keep clear of coolant filler, listen as hopefully gurgling then a boil over will occur, top up with very warm coolant wait as it may boil over again. Check for heat inside vehicle, if hot replace cap but keep eye on temp as air lock may have just moved out of heater matrix/core so boil up may happen again. If the bottom hose continues to constrict and heat inside car then possible head gasket issues. Good luck! hope I sorted it for you? if so press the Blue button please! for my FREE effort's I Thank You! Paul 'W' U.K.
First, check to make sure you are not low on coolant. Check hoses going to heater and see if they are getting hot.(one should be hot, the other should be slightly less hot but not cold or just warm) Check heater control valve to make sure that it is operating, Check that plenum door is opening properly inside the cabin, in the heater box. If you have done any work at all on cooling system, run the engine 'till it reaches operating temp with radiator cap off...sometimes htr core can become air-bound. Also...goes without saying, but make sure that engine reaches normal operating temp while driving. If not check thermostat!