3.1 - Thermostat Replacement ( ,  )

X300 thermostats appear to fail in the open mode after 4 - 5 years. This shows itself by the temperature gauge being slow to reach, or in some cases not achieving, the Normal mark. It is not unduly damaging to drive the car like this for a few days, but fuel economy will suffer and the throttle body is likely to get gummed up after a while, potentially causing it to stick partially open.

Thermostats can also fail in closed mode, causing rapid overheating. The car should not be driven whilst overheated otherwise severe damage will result.

To replace the thermostat, undo the two small bolts attaching the thermostat housing to the thermostat block. This can be found on the right hand side of the engine, attached to the large pipe leading from the top left of the radiator.

It may be necessary to pull quite firmly on the thermostat housing to break the seal. Note that a small amount of coolant will be spilt, though this can be avoided if you prefer by first draining some liquid by disconnecting the lower radiator pipe for a short time.

Remove the thermostat, noting which way it is fitted. Some vehicles do not have a gasket fitted, but if one is used then remove it. Carefully clean the faces of the thermostat housing and block.

It appears optional whether to use a gasket on replacement. It is not strictly necessary, but many feel more comfortable using a gasket or gasket compound.

Fit the thermostat into the block following the orientation of the old one; the actuating spring is normally on the engine side. Refit the housing, and tighten the fixing bolts. Whilst these need to be tight, excessive force will damage the threads.

Refill the cooling system to the maximum mark and run the engine up to operating temperature whilst checking for any leaks.