last year in 08 i stated getting just warm air coming out of the heater. I set it on feet annd it would only come out warm,then it went cold. the derost stated coming out warm and now its totally cold,actually every setting its coming out cold. And its not set on cold either…lol. Any ideas. I changed out the radiater in the summer when it statred leaking,i thought after changing it it would send the heat back but it didnt.
Start by making sure your bleed all of the air out of the cooling system.
First, is the radiator full of coolant?
Looking at the overflow reservoir won’t necessarily give you the answer. When the engine is cold, open the radiator cap and make sure the radiator is completely full of coolant. There should be no air pockets in the cooling system. Some vehicles have a bleed valve at the highest point to allow trapped air to escape.
Is the engine warming up to normal temperature? A faulty thermostat may prevent the engine from warming up and this will also prevent the heater from working.
Next check the heater hoses going from the engine to the heater core. When the engine is warmed up these hoses should be hot. If they aren’t, coolant is not flowing through the core.
If the heater core is hot but you still have no heat, the problem is with the blend door in the duct work under the dashboard. The blend door controls interior temperature by directing air through the heater core, the AC evaporator, or both, as required.
I am having just about the same problem! Sometimes there is a burst of hot, then it goes cold. Had a cracked coolent bottle replaced. Then a coolent fan. And many hundreds of dollars later, the next diagnosis is the blend door that mcparadise mentions.(the heater core and hoses are hot) I am glad to hear that my mechanic might be on the right track now with this blend door that is under the dash and will take a full day’s of work - I was hesitant to take it back for more time and money since the previous tries had not been successful. But I will go ahead and give them another shot!
The problem is not usually the blend door itself (just a plastic flap) but the actuator that moves the door, or the device that controls the actuator.
Some vehicles have vacuum actuators, some use small electric motors to move the door. I don’t know which system Hyundai uses.
The trouble is these things can be very difficult to get at, which is why fixing them can be expensive. It’s labor intensive, and often requires removal of part or all of the dashboard to reach the malfunctioning part.
The only thing I see missing from the suggestions below is that if the heater core has hot water going in and the outlet isn’t hot, then that means the heater is plugged, unless there is a shut off valve not working. Before you changed your radiator last year, you didn’t happen to put in a can of stop leak or another type of radiator leak stopper did you? If so, those products do a good job of stopping (plugging) up the heater core.