Manual - Chapter 4 - Cooling

8. Thermostat: removal and testing

  1. The thermostat is so designed that it remains in the closed position when it is in a normal cold condition. If the thermostat malfunctions, it will remain closed even when the engine reaches normal working temperature. The flow of coolant will be impeded so that it does not pass through the radiator for cooling and consequently the temperature will rise abnormally, causing boiling over.
  2. If the performance of the thermostat is suspect, remove it from the machine as follows and test it for correct operation. Remove the fuel tank and drain the coolant as described in Section 2. Remove its two retaining screws and withdraw the thermostat cover, noting how the sealing 0-ring is fitted. Withdraw the thermostat. On refitting, note that the sealing 0-ring is fitted around the cover inner boss, so that it fits inside the housing when the cover is installed; a smear of grease or similar lubricant will help the 0-ring to slide into the housing.
  3. Examine the thermostat visually before carrying out tests. If it remains in the open position at room temperature, it should be discarded. Suspend the thermostat by a piece of wire in a pan of cold water. Place a thermometer in the water so that the bulb is close to the thermostat. Heat the water, noting when the thermostat opens and the temperature at which the thermostat is fully open. If the performance is different from that specified, the thermostat should be renewed.
  4. If the thermostat is faulty it can be removed and the machine used without it as an emergency measure only. Take care when starting the engine from cold as the warm-up will take much longer than usual, and ensure that a new unit is fitted as soon as possible.

8.2 Thermostat is fitted in housing at side of radiator

8.3 Renew thermostat if stuck or faulty