(Home) Thermostat

Thermostat mounting and gasket - John Davis

Unusually, it's where the bottom hose goes into the left (passenger) side of the engine block "behind" the aircon compressor. When the thermostat is closed it has to sense the rising temperature before there is any flow through the radiator. To do this the wax capsule (in the middle of the spring) faces the block and it seems that the thermostat frame may even make contact with, and conduct heat from, a specially positioned protrusion inside the block. It senses the rising temperature from the circulation around the block via the rear heater matrix. Coolant always flows through the rear heater matrix (there is no valve) provided the water pump is working and that matrix and the hoses to/from it are not blocked.

Most Toyotas use the same thermostat unit 90916 03046 which requires rubber seal 16325 63011 ordered separately - it's a bit like an O-ring but with a groove around the inside which fits around the outer circumference of the thermostat body. Together they cost about £15. Only fit the Toyota parts, substitutes are said not to open so far, or at the right temperature, and the frame doesn't touch the block as it should (see above). In summer some owners use a Toyota thermostat in which they have drilled two or three 3mm holes near the rim to get earlier flow through the main radiator for better cooling - see discussions elsewhere. You may find that your car still has a very old Toyota thermostat which has a threaded adjustment on the end which gives some control over the temperature at which the cooling system will stabilise - you have to assess the risk of using that.

Removal: You can get to it underneath the vehicle but most people seem to prefer to do it from above, removing the coolant expansion tank. The coolant inlet casting has two 10mm mounting bolts. One is directly visible. You need a mirror to see the other, and suitable socket/extensions to get to the bolt heads. Drain coolant. Take off hose. Remove the two 10mm mounting bolts and remove the water inlet from the cylinder block.

Replacement: Take care that the seal is seated properly and that you fit the thermostat the right way round. The side with the wax capsule and spring goes inside the block.

Dave Mason, editor, Jun 2004