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Is the radiator full or surge tank full of coolant ? Are the heater hoses going into the fire wall hot ? Feel the heater hoses . Are the radiator hoses hot ? Especially the top one going into the radiator , stuck open thermostat if not . Do you feel any air coming out of the vents ? Stuck shut temp. blend door , possibly . Does A/C work , blowing cold ? What make ,model an year vehicle do you have ? Is it manual or automatic HVAC system ?
If you know for sure the thermostat is working, it could be low on coolant and has air pockets in the system. Only one radiator hose should be warm until the engine reaches normal temp, and both heater hoses should be very warm.
I am very doubtful you have air in your hoses causing the problem. Feel the upper and lower heater core hoses at running temp and verify they are both hot, and should be same temp.(make sure heat is on high in car) Some newer cars lose some heat to the cab just because they are low on fluid. More likely though, you may have a bad radiator cap. If your cap is good, watch your hoses and make sure they aren't collapsing( the hoses will look like they are breathing in and out with car idling). You could even have a bad thermostat, or your heater core could be plugged. You can flush your heater core and see if that don't fix it, but since you have heat then it decreases, your core is probably ok.
If you have a viscous fan hub then have it replaced as it is not drawing air through the radiator at idle or slow speed (crawling traffic). Check that are no air locks in the system as they will cause heating problems.
there should be a temperature door, but I don't know if there is a cable or if its electric. turn the temp knob, if it feels like it moves anything? I dont have a diagram here at home, but will monday/
My 2005 impala had the same problem. Here's the answer. The heater core is plugging up. When you sit idling, the fluid movement is slow, therefore the junk settles in the heater core, still allowing enough fluid to get through to heat. When you drive it, the fluid movement is much faster, therefore sending the junk into the heater core, plugging it up. To check for this problem: Get engine temp up to normal, feel both heater hoses. If one is hot and the other one is cooler, fluid is not passing through. Take off both heater hoses at the top of the engine, blow out fluid with low pressure.(5 or 10 lbs.) Fill both hoses with the right radiator flush. (Some engines are aluminum) Let stand for 4 hours. Flush with new antifreeze. (I used a garden sprayer) Stick the sprayer end in the heater hose and tape tight. My vent temp WAS 100 degrees. After fix was 152!!
Do you have enough antifeeze in your cooling system. If not fill & check again for heat. You may have a blockage. If so remove the in & out hoses to the heater core on firewall. Be careful not to damage core. Use a garden hose to flush the core one side to the other untill running clear water. put one hose on tight & clamp, put other on loose. Have someone start engine & slowly remove loose hose to let air escape. when fluid appears put hose on & tighten. Refill rad & resevoir. Heat engine up & check for heat. Also heat cable may be off under dash.
It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks
coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and
then drop, and the heater doesn't work well. This is because the
cooling system of your car is supposed to be a closed system, full of
coolant and no air. When coolant leaks out, the space it used to occupy
is now occupied by air, which does not transfer heat well. When 'air'
is passing through your cooling system, no heat can be transferred from
your engine to the heater and radiator, resulting in a hot engine and
no heat at the heater. Then when a pocket of water passes through the
system, the temperature gauge quickly falls as the water absorbs the
heat from the engine. The hot water that cools the engine is where the
heater gets it's heat from as well, so when water passes through the
heater core, the heater works, but when it's filled with air, it
if the heater hoses are hot on both sides(indicating the engine is at full temp) and you feel the heat controller in dash is working properly, i would suggest disconnecting both heater hoses and backflush heater core with decent amount of pressure. if this improves condition but does not solve completely, heater core replacement is necessary.