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Faulty thermostats can manifest themselvs in several ways. Some stick open, some stick closed and some stick in one position for a while then suddenly open or close. The ones that stick open will never allow the engine to completely warm up and you will get cold or cool air coming from the heater. The ones that stick closed will cause the engine to warm normally but then the temprature continues to climb and you can loose coolant and possibly harm the engine. The ones that stick for a while are interesting because the vehicle will run cold then turn warm quite rapidly or more likely the thermostat will stick closed and not open until the vehicle is quite warm and possibly you have started to loose coolant. In any case, the easiest way to detect a bad thermostat is to replace it. If the problem goes away, it was the thermostat. If you REALLY need to know if the thermostat is bad, remove it from the vehicle, put it in a pan of water and bring the water to a boil on the stove. Watch the thermostat. If it opens smoothly when the water gets to the designated temprature then the thermostat is ok. If the thermostat opens suddenly, fails to open or is open when you remove it from the vehicle, the thermostat has failed.

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โˆ™ 2011-09-13 04:59:17
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Q: How do you know if a thermostat is bad in a 1994 astro van?
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