Friday, 3 June 2016

Car Thermostat

How Does  Car Thermostat Work

Car thermostat function:

The automotive Car Thermostat is a temperature controlled on and off valve. Any liquid-cooled car engine has a small device called the thermostat.Car thermostat sits between the engine and the radiator.When the temperature rises to a predetermined temperature, the thermostat opens up to allow the engine coolant to flow through the cylinder block and the radiator. 

This flow is crucial to maintain optimum operating temperature for fuel efficiency, enhanced drive ability, and engine protection.You can say the thermostat reduces engine wear, deposits and emissions.

Working Of Car Thermostat
See  the image shows how thermostat functions when engine is cold and when engine is hot

Car Thermostat controls the engine cooling system.The coolant flow is reduced when the engine is cold, and the flow is increased when the engine is hot. Contrary to popular opinion, operating the vehicle without the thermostat doesn't make the engine run cooler. It will actually run hotter because there isn't a thermostat to slow the coolant flow. This means the coolant doesn't stay in the radiator long enough to dissipate its heat.

Thermostat parts
There is small cylinder located on the engine-side of the thermostat. This cylinder is filled with a wax that begins to melt at thermostats heating temp, different thermostats open at different temperatures.The most common for regular thermostats is 180 F/82 C is a common temperature. In the above picture you can see the copper color metallic rod.This rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve.This is how thermostat functions.

Car thermostat stuck open

Before replacing the thermostat its advisable to check the thermostat.

How To Test Car Thermostat ,before changing thermostat:

There are several ways to test the opening temperature of a thermostat.

Testing Thermostat without removing from engine:

  1. Remove the radiator pressure cap from a cool radiator and insert a thermometer into the coolant.
  2. Start the engine and let it warm up. 
  3. Watch the thermometer and the surface of the coolant.
  4. When the coolant begins to flow, this indicates the thermostat has started to open.
  5. The reading on the thermometer indicates the opening temperature of the thermostat.
  6. If the engine is cold and coolant circulates, this indicates the thermostat is stuck open and must be replaced.

The other way to test a thermostat is to remove it.

  1. Suspend the thermostat completely submerged in a small container of water so it does not touch the bottom.
  2. Place a thermometer in the water so it does not touch the container and only measures water temperature.
  3. Heat the water.
  4. When the thermostat valve barely begins to open, read the thermometer. This is the opening temperature of this particular thermostat.
  5. If the valve stays open after the thermostat is removed from the water, the thermostat is defective and must be replaced.
  6. Several types of commercial testers are available. When using such a tester, be sure to follow the manufacturer's instructions.
  7. Markings on the thermostat normally indicate which end should face toward the radiator. Regardless of the markings, the sensored end must always be installed toward the engine.
  8. When replacing the thermostat, also replace the gasket that seals the thermostat in place and is positioned between the water outlet casting and the engine block.
thermostat stuck open
Positioning a thermostat into an engine.
  • Generally, these thermostat gaskets are made of a composition fiber material and are die-cut to match the thermostat opening and mounting bolt configuration of the water outlet.
  • Thermostat gaskets generally come with or without an adhesive backing. The adhesive backing of gaskets holds the thermostat securely centered in the mounting flange, leaving both hands of the technician free to align and bolt the thermostat securely in place.

How to install a thermostat

Procedure Changing Thermostat For Car OR How To Replace Thermostat

If the radiator is filled to the top with coolant and the engine is run without the radiator cap in place, the coolant will expand and spill over as the engine warms up.

    1. Drain some coolant into a clean container until the coolant level is below the thermostat housing.

    1. Remove the upper radiator hose connection from the thermostat housing.

    1. Loosen the housing bolts and remove the housing.

    1. Remove the gasket and scrape it carefully from the surface of the housing and the mounting surface on the engine. If the gasket remains on either of the surfaces, there will probably bea coolant leak after reassembly. Some engines use a rubber O-ring to seal a thermostat housing.

    1. Compare the size of the thermostat to the old one. They are of different sizes, types, and temperature ratings.

    1. The temperature rating is stamped on the sensing bulb on the bottom of the thermostat. The temperature bulb faces the block.

    1. When replacing a thermostat, be sure that the thermostat fits into the groove in the block or outlet housing. If the thermostat is installed upside down, the engine will overheat.

    1. Install the gasket.

    1. Reinstall the thermostat housing. Refill the system and run the engine or pressure test to check for leaks.

    1. When the engine has reached operating temperature make sure the thermostat opens.

    1. You should be able to see coolant circulating within the radiator.

    1. Another way of checking thermostat operation is to feel the top of the radiator hose or use a thermometer or multimeter with a temperature probe to confirm that the coolant is warming up.

    1. If the engine is overheating, but the top hose is still cool to the touch, the thermostat is stuck closed and must be replaced.    


    When a paper gasket is used and the recess is in the thermostat housing, it is a good practice to position the thermostat into the recess and glue the gasket to hold it in place. If it falls out of its groove during installation, the outlet housing can be cracked or a coolant leak will result. Before tightening the water outlet housing, try to rock it back and forth to be sure it is flush. Housings are often cracked during this step.

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