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You make sure reservoir is full if it's full after running 10 minutes or so check top and bottom radiator hoses and see if both are hot if only top one is hot may have a bad thermostat if both are hot it may have a heater control valve in heater line Goin into heater core
It sounds like your thermostat is stuck closed. Your thermostat also acts as a heater valve to let hot coolant go to your heater core to blow hot air if your thermstat is stuck close it prevents your coolant to run through your radiator to cool down and to blow hot air from your heater core. A way to check to see if your thermostat is working while its at operating temp feel the bottom and top radiator hose if your lower hose is cold and top is hot your thermo is stuck closed but be careful hoses might be very hot
Check the radiator-it may have blockage. Take it off and turn upside down and flush from the bottom.
If the top hose gets hot, that means the thermostat is opening. The bottom hose should be at least warm, and quite warm if radiator flow is good.
While it doesn't seem likely, if you've changed, or had the thermostat changed.... I'd say you got a faulty thermostat. I am curious as to what your temperature gauge says the temperature of the engine and coolant is.. If your gauge is showing hot, I can think of two more possibilities... the water pump isn't pumping right... and, if the water pump isn't working right, the problem can be a cracked head or block, which can cause an airlock in the coolant system. Check the coolant or radiator cap.
Make sure that the radiator is full and if you still dont have hot air then check to see if the top rad hose and the bottom rad hose are hot after driving for 30 minutes. Now if the bottom rad hose is not as hot as the top hose then replace the thermostat. If both the hoses are hot then check the two hoses going into heater core at the fire wall, both of the hoses should be hot, if not then problem is in the heater core.
The key is the heater hoses. The top hose is the inlet for the heater core and the bottom is the return line. If the bottom hose is not hot, there is little flow through the heater core. It does not have to be completely clogged to reduce the heat output. The core needs to be flushed multiple times IN BOTH DIRECTIONS through the heater hoses. If this does not restore the heat, try a commercial/radiator flush. Many people have had alot of success by filling the core, through the heater hoses, with a CLR type product and letting it sit for 10-15 minutes to remove scale followed by a thorough flush.
Make sure the reservoir coolant tank is at 'full'. Start the car from cold and observe. After a couple of minutes when the engine has begun to drop in revs ( the coolant temperature sensor should tell the ECU to begin to shorten the injection cycle as the engine warms) the top of the engine and the coolant passage to the thermostat housing should become warm to the touch. When hot the top rubber hose to the radiator should become warm and then hot as coolant begins to circulate. If the top hose fails to get hot at any time then the thermostat is at fault. If you have electrically driven radiator fans these should now come on. If they fail to come an at all you need to check the coolant temperature sensor, the electrical motor fuses and the motor relay. Lastly check the motor itself (just run a 12v supply to it directly). As the ar continues to idle, with the fans running, the bottom return hose from the radiator to engine should also become warm. If the bottom hose fails to become warm this points to circulation problems. Either the radiator had become clogged with rust sludge and needs an extensive flushing out with a high pressure hose or the pump is not functioning as it should. Quickly check that the interior heater works by asking for 'full heat with fan on full'. If the interior heater works this indicates that the main coolant pump is probably OK.
To check the thermostat more thoroughly, remove it from the car and drop it in a pan of near boiling water; the 'stat should pop open. As the pan cools, at 88 degree centigrade or so, the 'stat should pop closed. Failure to observe this tells you the 'stat is a dead and needs to be renewed.
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