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Defective thermostat / defective coolant regulator? Our information!

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Recently updated on January 27, 2022 at 10:19 am

Defective thermostat / defective coolant regulator? Our information!

For cars there is two types of thermostats (also called coolant regulator). With the mechanical thermostat, an expansion material expands due to the temperature so that the thermostat opens or closes. The electronically map-controlled thermostat with electric actuator will, however, from a control unit regulated and can regulate the temperature faster and more effectively. There are two cooling water circuits in the car. A small and one great cooling water circuit. While the engine is warming up, the cooling water circulates in the small circuit. The thermostat opens when the engine has reached the correct operating temperature and the cooling water can circulate in the large cooling water circuit. From this point on, the cooling water also reaches the cooler, which releases the temperature to the environment. The thermostat also regulates the temperature while driving so that the engine keeps its operating temperature within the perfect range and cannot overheat. Indirectly, the thermostat also has an influence on the fuel consumption and the exhaust emission. The main components of the small cooling circuit include the water pump, the heat exchanger for heating in the interior and the engine block itself.

The function briefly explained:

  1. both types of thermostats have one Spring plate
  2. the plates open/close the path between the small/large cooling circuit
  3. cold condition -> Spring plate closes the cooling water's way to the cooling water cooler
  4. warm condition -> Spring plate opens and releases cooling water
  5. Closing / opening always happens when the opening temperature is below / reached

identify a faulty thermostat

Defective thermostat / defective coolant regulator? Our information!

Will the Motor, the Heating or both not warm, the thermostat is permanently open or does not close properly. If the engine does overheated, i.e. more than 90 °C is displayed on the temperature display in the area of ​​the fittings, is the thermostat permanently closed or does not open properly. It may also be that the Diesel particulate filter either not properly regenerated or not regenerated at all. Increased fuel consumption can also indicate a defective thermostat. And, these symptoms do not always appear together. The following problems can occur as symptoms with the classic thermostat & the map-controlled thermostat:

  • The engine warms up / hot too quickly
  • Engine does not come up to temperature
  • red coolant lamp in the instrument cluster lights up or flashes (The instrument cluster shows the cooling water temperature if installed)
  • Water vapor is released (Every cooling system has an overpressure valve, the overpressure is released in the form of water vapor, the overpressure valve is usually located in the cover of the expansion tank or in the radiator cap)
  • electric radiator fans from the cooling water radiator (when installed) a permanently closed thermostat can partially compensate (this can be seen from the fact that the electric cooling fan is almost permanently active)
  • The engine control lamp lights up (possibly)
  • Heater does not get warm
  • Info: the yellow coolant light only indicates that the coolant level is too low

Repair or change thermostat

If the thermostat turns out to be faulty, it will be against a new changed. While should no used part, since it is not clear how old and worn it is. The repair process is not always identical, since the thermostat is installed differently in every vehicle. However, the underbody protection is almost always removed and the cooling water drained from the system. In most cases, some lines and the cooling water hoses must also be removed so that work on the thermostat housing is feasible. Now the old thermostat can be exchanged for a new thermostat with all the seals. After that, the coolant circuit is new again filled and then still vented.

Causes – Defective thermostat

The main reason for a permanently open thermostat is one jammed piston or of corrosion. Sometimes, however, it is also jammed foreign body the open thermostat. The holders on the thermostat housing also tend to break off, which means that it only hangs loosely in the flow of cooling water. And at some point it will poetry leaking inside the expansion element. Then the wax, which acts as an expansion material, can escape. This means that the expansion element can no longer work correctly. The map thermostats often have one defective electric actuator as the cause. At some point, the movement of the working piston causes cooling water to enter the actuator and destroy it.

Continue with a defective thermostat?

When the thermostat "no longer opens“ can one Engine damage from overheating develop. If the cooling water temperature is too high and the resulting pressure is too high, additional parts (bursting cooling water hose, breaking away plastic cooling water flange etc.) are affected. So you should not continue more.

When the thermostat "no longer closes"It's not that dramatic for a short moment. You can go on. Nevertheless, the defect must be rectified as soon as possible. Because if the engine does not get warm, then all mechanical components are exposed to a high level of wear.

The following procedure can help to reach the workshop with a permanently closed thermostat:

  • Switch off the engine from time to time while driving and let it cool down (at best with the hood open)
  • Set interior fan to "ECO" (Air conditioning off)
  • Temperature & fan at maximum
  • drive at night if possible (because of low outside temperatures)
  • Driving at full load / avoid high revs

Cost of changing the thermostat in a workshop!

A mechanical thermostat including all parts costs about between 15 euros and 75 euros. Up to 300 Euros on the other hand, the more modern ones can map-controlled thermostats costs. Of course, the costs for the spare parts depend on the spare parts manufacturer and the vehicle. Well-known manufacturers of thermostats are Behr, Febi Bilstein or Gates. And as already mentioned, the cooling water also has to be changed, although the costs vary here 20 euros and 70 euros move. This depends on the engine and the associated amount of coolant that is required. A liter of cooling water costs ca. 5 to 10 euros. Must be the Intake manifold or Air filter housing be disassembled and assembled, then changing the thermostat becomes more complex. However, assuming an average hourly rate of 100 euros, the labor costs add up to a total of 100 to 300 Euroas it takes between one and three hours to change the thermostat. If you add the labor and material costs together, the change costs between 100 euros and 750 euros.

Defective thermostat / defective coolant regulator? Our information!

Of course, that wasn't the end of it!

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About Thomas Wachsmuth

Thomas Wachsmuth - He has been an integral part of tuningblog.eu since 2013. His passion for cars is so intense that he invests every available penny in them. While he dreams of a BMW E31 850CSI and a Hennessey 6x6 Ford F-150, he currently drives a rather inconspicuous BMW 540i (G31/LCI). His collection of books, magazines and brochures on the subject of car tuning has now reached such proportions that he himself has become a walking reference work for the tuning scene.  More about Thomas

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