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Put a piece of cardboard in front of the radiator. Only if it's cold out though, Usually in really cold weather, the cooling affect of the cold air circulating around the radiator keeps the fluid from getting hot enough to produce heat unless your running in park.
Both of the hoses at the heater core should be hot. If one is cold the heater core is plugged.
You can think of it this way. If the thermostat was stuck closed and you checked the upper and lower radiator hoses with the engine hot the upper hose would be hot and the lower hose would be cool because there is no flow. If you check the hoses with a properly working t-stat both hoses will be hot when the t-stat is open and coolant is flowing.
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You have an air bubble in the coolant system. When the engine is cold, loosen the radiator cap to the first notch. Start the engine and warm to operating temperature. Turn the engine off and top off the radiator and overflow tank. Tighten the radiator cap and allow to completely cool down. After the engine is cold again, top off the overflow tank. The heater should work properly.
Be careful, with engine at normal oper. temp. feel both upper and lower radiator hoses, they should both feel the same, warm to hot, if not the same temp. replace the thermostat.Heat sensor either work's or not, if it reads hot, engine is hot, or will not work at all.
First check if you have any leaks in the cooling system , then check if level is correct(radiator and expansion tank , check level when car is cold , never open radiator when it's hot).
Then verify operation of radiator fan , if it's electrical fan it should comes on when temp. goes over normal,
if it's a clutch fan- it should pick up a speed and be more loud. If fan is not on and engine is starting overheating , check upper and lower radiator hoses.
If upper hose hot and lower hose cold or warm then you might have stuck close or prtially closed thermostat , difference between lower and upper hoses when t/stat works ok is about 10-15 degrees,
not really noticeable for hand.So if you don't feel a lot of difference between hoses temp. , and fan is not on , check fan or fan clutch itself.
If lower hose cold , replace thermostat and purge system.
If everything above is working properly , and it's still overheating , you may have bad water pump or head gasket.
Check your radiator for external and internal obstructions (leaves, bugs etc on the a/c condenser or leaves between the condenser and radiator, rust or calcium deposits inside the radiator core all cause loss of heat transfer) Also make sure that the system is full and that the electric fan and the clutch fan are operating properly. (electric fan should come on just above normal operating temp and clutch fan should not be "freewheeling" above temp either) Note: If you need to have the radiator flushed, do it separately from the engine. especially the inline 6 and 4 engines rust up inside and if the system is flushed with the radiator connected, the debris will pack up inside it and actually can worsen the condition.
Check your coolant level first,then run until normal operating temp.and make sure upper and lower rad. hoses are near the same temp. if that is ok then check the heater hoses going in and out of your firewall to the heater core if they are both hot then you have a hvac case issue.
U didn't say the engine, if it is a 6cylinder then around 195-200 F, if a V8 they run very hot, around 225 F, so you have a problem, is the coolant clean? no rust present? I would suspect that you need the radiator pulled and roded out by a radiator shop.
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!