Caution: Do not open the expansion tank cap or remove the thermostat until the engine has cooled down completely! See also Section 2.
General information
1. The thermostat is designed to speed up the engine warm-up process and divides the cooling system into two circuits: large and small. Until the coolant temperature reaches a certain value (as a rule, thermostats are adjusted to a temperature of 80-90°C), the thermostat valve is closed and the liquid circulates along the small circuit without entering the radiator. As a result, the engine quickly warms up to normal operating temperature. When a certain threshold value of the coolant temperature is exceeded, the thermostat filler begins to expand and opens the valve - the liquid begins to circulate along the large circuit through the radiator.
2. The thermostat housing is attached to the engine. The location of the thermostat may vary depending on the layout of the power unit, but it is easy to find - it is always connected to the upper radiator hose.
Removal
3. On petrol models, the thermostat housing is located openly and does not require any special preparatory work before removal (except for the Z22YH engine). It is necessary to drain the cooling system (see Section 3), disconnect all hoses and electrical wiring from the thermostat housing.
Note: On the Z18XE engine, the engine cover must first be removed (see Chapter 2).
Then unscrew the mounting bolts (see accompanying illustrations), remove the thermostat housing and extract the working element. On the 222YH engine, the thermostat housing is installed under the exhaust manifold - in addition to the above work, it is necessary to remove the manifold protection. Unscrew the mounting bolts and stud (see illustration 19.3e) and remove the thermostat housing together with the cooling system pipe.
4. The layout of diesel power units is somewhat more complex and to remove the thermostat housing it may be necessary to dismantle components that block access to it, such as the battery with the tray, air ducts of the intake tract, and in some cases, fuel lines. Before performing work, locate the thermostat housing and determine which components need to be removed first.
Examination
5. The main reason for thermostat failure is the aging of the filler and its loss of ability to significantly change its volume depending on the temperature. Mechanical damage to the thermostat valve may also occur during operation.
6. You can check the thermostat's functionality beforehand without removing it from the engine. With the engine running, open the hood and touch the upper radiator hose.
Caution: Be careful - high coolant temperatures can burn your hand!
If the engine has just started and has not yet reached operating temperature, and the liquid in the upper hose is already warm, then the thermostat valve is not closed. If the control lamp has been activated (see Chapter "Controls and operating techniques", Section 16) coolant temperature on the instrument panel, and the upper pipe is cold - it is guaranteed to say that the valve does not open the large circuit.
Note: This is only a rough check, and you must be sure that all other components of the cooling system and temperature control are functioning properly and that the coolant level is normal.
Follow the recommendations given in the Section on diagnostics of cooling system faults.
7. The most accurate results are obtained by checking the thermostat on a workbench.
8. Remove the thermostat from the engine (see above).
9. Attach a piece of wire to the thermostat assembly and lower it into a container filled with water (see accompanying illustration). Put the thermometer there too.
Attention: Make sure that the thermometer and thermostat do not come into contact with the walls and bottom of the container!
10. Start heating the water slowly, monitoring the changes in the thermometer readings. The valve should not start opening until the water temperature reaches 80°C. The thermostat should also be replaced if the valve does not start opening when the water heats up to 92°C or more.
11. Estimate the value of the full opening of the valve, - the nominal value is b - a = 8 mm (see accompanying illustration). If any deviations are detected, replace the thermostat.
Installation
5. Installation is carried out in reverse order. Please note the following points:
- a) The mating surfaces must be thoroughly cleaned and all sealing elements/sealant replaced;
- b) When installing in the casing, orient the thermostat correctly;
- c) The cover mounting bolts must be tightened to the required torque;
- d) After connection, the position of all hoses and pipes must correspond to the original, without kinks or twists.
6. Fill the system with coolant (see Section 3).
7. Start the engine, warm it up to operating temperature and check for leaks.
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