Volvo Coupes/Sedans/Wagons 1970-1989 Repair Guide

Thermostat

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To operate at peak efficiency, an engine must maintain its internal temperatures within certain upper and lower limits. The cooling system circulates fluid around the combustion cylinders and directs this heated fluid to the radiator, where the heat is exchanged into the airflow created by the fan and the motion of the car.

While most people realize that an engine running too hot (overheated) is a sign of trouble, few know that an engine can run too cool as well. If the proper internal temperatures are not achieved, fuel is not burned efficiently, and the lubricating oil does not reach its best working temperature. While a too cold condition is rarely disabling, it can cause a variety of problems which can be mistaken for tune-up or electrical causes.

The thermostat controls the flow of coolant within the system. It reacts to the heat of the coolant and allows more fluid (or less) to circulate. Depending on the amount of fluid being circulated, more or less heat is drawn away from the inside of the engine. While we are beyond the days of having to install different thermostats for summer and winter driving, it is wise to check the function of the thermostat periodically. Special use of the car such as trailer towing or carrying heavy loads may require the installation of a thermostat with different temperature characteristics.

REMOVAL & INSTALLATION



See Figures 1, 2, 3, 4, 5, 6 and 7

  1. Disconnect the lower radiator hose and drain the cooling system.
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Fig. Fig. 1: Remove the two fasteners that hold the thermostat housing to the cylinder head-B21FT engine shown



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Fig. Fig. 2: Pull away the housing from the cylinder head to access the thermostat inside

  1. Remove the two fasteners securing the thermostat housing to the cylinder head on gasoline engines and to the block on the diesel. Carefully lift the housing free. Before removing the thermostat, look at it carefully to know which end is up. It is possible to install one upside down. Pay attention!
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Fig. Fig. 3: Manipulate the housing up in your hand and pull out the thermostat from inside

  1. Remove all old gasket material from the mating surfaces and remove the thermostat. Do not use metal blades to scrape these surfaces; you may gouge the metal and cause a leak. Use a wooden or plastic scraper.
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Fig. Fig. 4: Remove and discard the old gasket, then scrape clean the mating surfaces without gouging the aluminum

  1. Test the operation of the thermostat by immersing it in a container of heated water. Replace any thermostat that does not open at the correct temperature. Most thermostats will begin to open at approximately 190°F (88°C).
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  3. Place the thermostat, with a new gasket, in position. Fit the thermostat housing and hand-tighten the bolts. Tighten the bolts 1 / 4 turn past finger-tight; overtightening can crack the brittle housing.
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Cars with diesel engines require a different bleeding procedure. Please refer to the Cooling System portion of General Information & Maintenance .



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Fig. Fig. 5: Access the thermostat in the location shown. Arrows indicate areas to scrape clean prior to new gasket installation-B23 and B230 engines

  1. Connect the lower radiator hose and replace the coolant. On gasoline engine cars, leave the radiator cap off, start the engine and allow it to reach normal operating temperature. Inside the car place the heater temperature selector to full hot and turn on the blower. This allows the coolant to circulate fully throughout the system and purge any air bubbles which may be trapped. After a few minutes (during which you may observe air bubbles in the radiator neck) shut the engine off, top up the coolant and secure the radiator cap.
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Fig. Fig. 6: Exploded view of thermostat mounting-make sure you install it in the correct direction



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Fig. Fig. 7: Exploded view of the diesel engine cooling system components and thermostat mounting

 
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