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You will know of your thermostat is going out when your car overheats, then turn on your heater really hot on full blast, then the temp will instantly drop. This is a sign of a thermostat gone bad.
roll down the windows and turn the heater on full blast. It pulls hot air from the engine bay. Also check to see if you have a coolant leak or if your thermostat isn't opening all the way. you can test your thermostat in a pot of boiling water, boil the water to what ever degree thermostat you have. You could also burp the cooling system.
turn the heater on full blast, that will make use of the heater core to help cool things down.
It sounds like your heater coil is low on coolant and drains your radiator every time you turn it on. Try letting your van warm up completely, then turn your heater on full blast and then check your radiator coolant level, if its low, that's your problem. Just keep adding until full.
The most likely cause is a bad blower motor resistor.
Then you will have a bad a/c-heater door actuator located behind the glovebox mounted on the top of the a/c-heater box...............
This era gm vehicle was famous for having clogged heater core problems. Part of the problem is the the 'dexcool' coolant used. It gels up in the heater core, and needs to be forced cleaned with a strong water spray. This is easily done by a mechanic. They will disconnect the inlet and outlet of the heater core, hook up a hose to the inlet or outlet, and blast away. The symptom you describe could also be a faulty Thermostat. Good luck!!
there's probably a big snot wad of dex-cool in the heater core! mine was! disconnect the two hoses from the heater core and blast it out of there with a garden hose.......you're gonna lose some coolant, so you'll have to re-fill
bad fan clutch, clogged radiator.....................when you turn the defrost on, the heater core is acting like the radiator...............good luck, Mike
Usually if the resistor is bad the heater will only run on full blast.
The amount of propane a 400,000 BTU pool heater will use depends on factors like the heater's efficiency and the temperature you desire. On average, a 400,000 BTU pool heater can consume around 4-5 gallons of propane per hour when running at full capacity.
The problem is either the blower motor resistor or the blower speed selector switch.