Thermostat

Do not remove the radiator cap, remove the thermostat, or drain the coolant until the engine has completely cooled down.

Inspection

PROCEDURE
1. Before concluding that the thermostat is the main cause of abnormal cooling system operation, check the coolant level (see subsection 2.3) and the coolant temperature sensor (or warning light).
2. If the engine takes a long time to warm up (indicated by a slow rise on the temperature gauge, or slow heating of the air from the heater), the likely cause is a thermostat valve stuck open. In this case, replace the thermostat.
3. If the engine overheats, check the temperature of the upper radiator hose. If the hose is cool and the engine is hot, the thermostat is closed, blocking coolant flow from the engine to the radiator. Replace the thermostat.

* Do not drive the vehicle without a thermostat. Without a thermostat, the feedback circuit of the automatic emission control systems and the fuel system is interrupted, which may cause them to fail.

4. If the radiator hose is hot, the thermostat is open and coolant is circulating through the radiator. For further diagnosis, follow the "Troubleshooting" section at the end of this manual.

Replacement

PROCEDURE
1. Disconnect the battery negative cable.
2. Drain the coolant from the radiator.

3. Disconnect the fan thermoswitch connector (indicated by the arrow) on the thermostat housing, located directly under the distributor on the left side of the cylinder head, and remove the two thermostat housing cover mounting nuts.
4. Remove the thermostat housing cover. Note that coolant will spill when the cover is removed. If the thermostat housing cover is not being replaced, do not disconnect the radiator hose from the cover.

5. Remove the thermostat, noting its position in the cover or housing, and thoroughly clean the sealing surfaces. The thermostat spring must face the cylinder head.

6. The thermostat is sealed around its perimeter by a shaped gasket. Install a new gasket on the thermostat. Make sure the gasket seats evenly around its entire circumference.
7. On 4A-FE and 7A-FE engines, install the thermostat and cover so that the pin faces up. On 4E-FE engines, align the thermostat so that the pin is aligned with the projection on the cover, then install the cover and thermostat.
8. Tighten the mounting nuts to the torque specified in subsection 4.1. Install the remaining components in reverse order.
9. Fill the cooling system with coolant (see subsection 2.8.4), start the engine, and check for leaks and proper engine operation.