How to replace a car’s thermostat

How to replace a car's thermostat

Thermostats in cars are often neglected, if not totally forgotten, units. Yet they play an important role in the general performance of cars by restricting water flow until the engine has warmed up.

Thermostats are heat sensitive valves that open and close. On cars, they prevent the circulation of coolant to the radiator until the engine is warm enough. When the thermostat is closed water only flows through the water pump and water jacket to let the engine warm quickly. When the thermostat opens, water can then pass through the radiator for general cooling.

Having a car that warms quickly is particularly useful in cold weather when you want to use your car heater as soon as possible.

The thermostat is in a small housing positioned where the top radiator hose connects to the engine.

The best time to check or change a thermostat is when you are servicing your radiator because you need to drain the cooling system first. Hence, a thermostat change is a good time to flush and clean your cooling system, check all radiator hoses, and the radiator pressure cap. Be sure you have the correct cap on your radiator and it is not rusted.

Thermostats are small, inexpensive items. You can test them by heating in water and checking their operating temperature with a thermometer. However, it really is quicker and ultimately more reliable to simply fit a new one.

Removal and replacement of the thermostat involves the following:

  • Put the car on level ground. Drain the cooling system and unscrew the thermostat housing and connecting the water hose. (If you are flushing and cleaning the radiator and cooling system, do this before you tackle the thermostat).
  • Look at how the thermostat is positioned and remember to put it back the same way.
  • Inspect the accessible part of the water jacket for corrosion. Carefully remove any thick-crusted corrosion but do not let it drop inside the water jacket.
  • Also, clean the rims of the thermostat housing cover from old gasket or glue remains.
  • Insert your new thermostat.
  • Put on a new gasket and replace and tighten housing.
  • When everything is back in place, run the engine to full operating temperature, and check for leaks.
  • Take the car for a run and check that the thermometer needle stays at normal with the car at full operating temperature. If the needle did not do this before you changed the thermostat, it should do so now.
  • After your run, make a final inspection for coolant leaks.

There are two types of thermostats, one with a bellows and the other with a wax element.

The bellows type includes a metal container of fluid that expands the bellows when heated, making the thermostat valve open.

The wax type has a rubber diaphragm surrounded by wax and with a rod. The wax is sealed in a brass container that is in contact with the coolant. The valve is closed when the wax is cold and water cannot flow between the radiator and engine. When the wax is hot it melts and opens the valve, allowing full flow.

The wax type

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