A defective thermostat is a common cause of engine overheating. The thermostat regulates the flow of coolant between the engine and radiator. The thermostat is usually located in a housing where the UPPER radiator hose connects to the engine. If the thermostat fails to open, heat will build up inside the engine causing it to run hot and overheat. The result can be loss of coolant if the radiator boils over, and possible engine damage including a damaged head gasket. If a thermostat fails to close, the engine may be very slow to warm up following a cold start, and it may never reach normal operating temperature. This can cause reduced heater output during cold weather as well as increased fuel consumption and engine wear. Engine thermostat How the Thermostat Works Inside the thermostat is a small wax-filled noncorrosive brass tube that extends in length as the coolant gets hot. As the wax inside the brass tube expands, it opens the thermostat valve so coolant to flow through the thermostat. The temperature at which the thermostat is calibrated to open is usually stamped on the unit. Most late model thermostats are calibrated top open around 195 to 200 degrees F. The thermostat should be fully open about 20 degrees F higher than its rated temperature for maximum flow, and should maintain engine temperature in the 200 to 230 degree range. As coolant circulates between the engine and radiator, the thermostat usually remains open to maintain a consistent operating temperature (which is important for good fuel economy and emissions). However, when the engine is idling or when engine speeds and loads are very low, less heat is produced in the engine so the thermostat may close temporarily to restrict coolant flow until the engine temperature can come back up to normal range. When the engine is shut off and cools down, the wax element inside the thermostat contracts, allowing spring-pressure on the thermostat valve to close the thermostat so it will be ready for the next drive cycle. Electronically Controlled Thermostats The thermostats in some late model vehicles are also electronically-controlled by the engine computer or a thermostat control module. Electronically-controlled thermostats have a small electric heater element added to the wax-filled brass tube. You will see an electrical connector on the thermostat housing for the heater. Engine temperature is monitored by a coolant sensor. Electronically-controlled thermostats remain closed to speed engine warm-up, and typically minimize flow to maintain optimum engine temperature in the 220 to 230 degree F. range. When additional coolant flow between the engine and radiator is needed to handle higher engine loads or speeds, the heater element is energized forcing the thermostat to fully open. The advantage of this approach is better temperature control under all driving conditions, faster engine warm-up (more heater output sooner during cold weather), improved fuel economy and lower emissions. Some new vehicles do not even use a thermostat to regulate engine temperature. GM's 2020 Silverado and Sierra pickup trucks with a turbocharged 2.7L V6 do not use a thermostat in the cooling system. Instead, the PCM regulates coolant temperature with a dual-valve assembly that splits the coolant flow between the radiator, heater and oil cooler. A second rotary valve inside the assembly then routes coolant to the heater during cold weather as the engine is warming up. The PCM monitors engine temperature with coolant sensors. The PCM also controls coolant flow via an electric water pump. It's a complicated yet sophisticated system for optimizing coolant flow under a wide range of operating conditions. A similar setup is also being used by some other auto makers, and will likely be used on more and more applications as time goes on. Diagnosing a Bad Thermostat Thermostats have a limited service life, so the older the thermostat the greater the chance it might be defective. If your engine has been overheating, carefully feel the UPPER radiator hose after the engine has been running for 5 to 10 minutes. If the upper hose does not feel HOT, the thermostat is not opening and is blocking the flow of coolant from the engine to the radiator. This likely means the thermostat has failed and needs to be replaced. However, the same condition could also be due to a low coolant level, an air bubble trapped under the thermostat or even a bad water pump. Check the coolant level, and if it is okay, the thermostat should be removed for testing (see below). A suspicious thermostat can be tested by removing it from the engine and dropping it into a bucket of boiling water (Be Careful!). The thermostat should be fully closed when cold, then open when it is exposed to the hot water. It should close again when you pull it out of the hot water. If it fails this test, replace the thermostat. If your vehicle has an electronic thermostat, you can check the integrity of the heater element with an ohmmeter. Disconnect the heater connector, and connect your ohmmeter to the two pins on the thermostat. A good heater element will show show some resistance (look up the exact specs for your vehicle). If resistance is outside of specs, replace the thermostat. No resistance would indicate a shorted heater element, while infinite resistance would indicate an open or burned out heater element. NOTE: If your engine has overheated, a new thermostat is recommended because excessive heat can damage the wax element inside the brass tube. If your heater is not putting out much warmth during cold weather, or your temperature gauge indicates the engine is very slow to warm up or is not reaching normal operating temperature, the thermostat may be stuck open. The most common cause would be a bad thermostat due to a defective wax element or broken return spring. Either way, you need a new thermostat. Another reason to replace a thermostat is for preventive maintenance on older, high chilometroage vehicles. A new thermostat will reduce the risk of a thermostat failure and overheating. If you are doing other cooling system repairs such as replacing a water pump, radiator or heater hoses or a radiator, or you are changing a head gasket or rebuilding an engine, replace the thermostat too, especially if it is more than 10 years old. Choosing A Replacement Thermostat First, make sure the replacement thermostat has the SAME temperature rating as the original. If someone has previously replaced the original thermostat with one that has a different temperature rating (too cold or too hot for your engine), have the person in the auto parts store look up the recommended thermostat temperature for your vehicle and go with that. We also highly recommend buying a quality NAME BRAND thermostat (original equipment or aftermarket). Some cheaply made thermostats may not open at the correct temperature or may fail after a few thousand chilometri. In some older high performance muscle cars or ones with highly modified engines, a colder thermostat may be needed to prevent the engine from running too hot. But for most street applications and certainly all late model computer-controlled engines, installing a colder thermostat is NOT recommended because you will not gain anything and it will screw up how the engine control and emissions systems function. Your engine will run worse, not better! In very cold climates, a slightly hotter thermostat may be installed to increase heater output. But this trick should only be used during cold weather. A hotter thermostat may prove to be too hot during warm weather, and may cause your engine to run too hot and detonate (engine-damaging spark knock or pinging). WARNING! Do NOT run your engine without a thermostat. Coolant flow needs to be managed so different parts of the engine do not run too hot or too cold. If there is no thermostat in the system to regulate temperature, uneven cooling can increase the risk of cylinder head cracking, head gasket damage/leaks, increased cylinder wear and moisture/sludge buildup inside the crankcase. No thermostat during cold weather also means no heat from the heater because the coolant will never get hot enough to provide significant heat. If your vehicle is running hot or overheating during hot weather, removing an otherwise good thermostat probably won't help. Once the coolant is hot, the thermostat should be wide open and offering no restriction to flow. So if your engine is running hot or overheating, chances are the cause is not a bad thermostat but more likely a dirty, obstructed or clogged radiator, a cooling fan that is not working or pulling enough air through the radiator, or a weak water pump (worn o0r loose impeller inside the pump or a slipping drive belt). If your vehicle has a high horsepower or modified engine and is running hot, you probably need a larger or thicker radiator to manage the extra heat produced by your engine. Replacement thermostats are available with different heat ratings. These include 160 to 165 degree F. thermostats for older vehicles for hot weather driving. This type of thermostat would NOT be suitable for a late model vehicle application because the engine control system is calibrated for engine temperatures in the 195 to 200+ degree F. range. If the coolant does not reach the proper temperature, the engine control system will not go into "closed loop" operation, which adversely affects emissions, fuel economy and performance. For more information on this subject, see Computerized Engine Controls. How to Replace the Thermostat Always replace the gasket when changing a thermostat. The only tools you should need to replace a thermostat are a wrench, pliers, maybe a screwdriver, something to scrape with, a funnel and a bucket. For parts, you will need a new thermostat and a new gasket for the thermostat housing (or RTV silicone sealer to make your own gasket), and antifreeze if you also plan to change the antifreeze at this time. On an older high mi…

Fonte: AA1Car.com