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Low coolant, if there is an air pocket in the engine it will make it to the heater core. Once there , the air coming out is cold/cool. Once the coolant fills the heater core, it gets hot. If the coolant is fine hot or cold, then have the HVAC system scanned.
After warmup it still blows cold, or only when cold? Normally the fan will operate independently of engine temp except in some climate controlled vehicles. As long as it blows warm air after engine is at temp that's a good thing. the heater uses hot water from the engine. Obviously the heater can't be warm before the engine is. If it's cold after the engine is hot, there are vacuum controlled "motors" on top of the heater box under the dash that move doors that control heat. Either one of the control motors or one of the doors have stuck or failed. Many also have a control valve on one of the heater hoses under the hood as well. If the engine comes up to temp then the thermostat is good (should be at about 210f) Only remaining item is the core itself which may be internally or externally obstructed. All this being said, if it's good when the engine is up to temp, leave it alone...it's fine.
First you have to identify what causes the problem.
To do this you have to: 1)When engine is cold check the level of coolant. If it is low fill it up. 2)Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced. 3)If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core. 4)If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator. 5)If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.
if the air is definatly being blown through the heater core , then the core is blocked.if there is no hot coolant passing through the core then ------no warm air ---give it a good look see if you can see the deflector guiding the blown air is going far enough to send majority of air through heater good luck
you should have hot water on all rad. hoses when the motor is running at a norm. temp. the water is not going through the heater core like it should. you have cutoff valve bad low water are a stopped heater.try a hoter thermostat if none of the above are true.
Notorious for plugged heater cores. Also updated plastic restrictioncouplers mounted at heater core available from dealer. Try back flushing heater core. Run engine till its at operating temperature then carefully hold the inlet and outlet hose breifly with heater fan on high if hoses are nearly the same temp then flow is good , if not then core is plugged.
My 05 sentra with 79k miles heat will warm up and then get cold after a mile of driving, we have tried to get the air pocket out, the mechanic stated he thought it could be a blown head gasket, another mechanic said it was a bad heater core. Any advice would be appreciated.
um...strange ...engine temp reaches 210 before thermostat opens and lets water circulate ..fans go on before that normally..180 or so..hot air from heater going to cold air? strange...could you elaborate for me /us?