While this article was written with the Jaguar XKE in mind, much of the information presented below is relevant to many other vehicles. This is an excellent write up on how thermostats work and affect vehicle operation.
This seems to be the biggest problem facing the XKE driver. Most of the problems go back to either lack of maintenance or improper maintenance. A properly maintained XKE might get a little hot in traffic but should not overheat to the point of boiling over. The thermostat is a critical component for a street car. Race cars usually leave it out and use a restrictor instead. The size of the restrictor is designed along with the other parameters such as ambient temperature, speed of the car, size of the radiator, air flow opening and size of engine. On street cars, most of the items can vary so to regulate the operating temperature of the engine, a thermostat is needed.
One area of great confusion is that if a car is running hot, change to a lower temperature thermostat. The thermostat only controls the minimum operating temperature of the engine. If the car is overheating beyond the rating of the thermostat, changing to a lower temperature will only cause the engine to run cooler when warming up. Once the thermostat is open, it is open. If the running temperature of the engine exceeds the temperature rating of the thermostat, then the cooling system is not adequate to dissipate the heat put out by the engine. This can be caused by a radiator that is partially clogged, debris in the radiator, to high of a antifreeze concentration, inadequate air flow across the radiator and last but not least an engine putting out to much heat because it is not running properly. All of these factors added together will determine the maximum operating temperature of the engine. If you have a car where the temperature goes beyond the thermostat rating, then one of the situations needs to be addressed. Going to a lower temperature thermostat is not the answer.
On the subject of thermostats, there are various designs out there. The Jaguar XKE has never used the “American style” thermostat. The XKE’s all have a thermostat that uses a by-pass blank off system. The Series 1 cars used one that has an external sleeve that moved when the thermostat opened to cover the by-pass opening. The Series 2 and 3 cars used a thermostat that has a washer on the rear that covered the by-pass opening.
The by-pass is used when the engine is warming up to allow coolant to circulate through the engine. When the thermostat opens, this is blanked off and all of the water is forced to flow to the radiator. Water will follow the path of least resistance and without the by-pass blocked off; the water will keep circulating in the engine block getting hotter and hotter. Some of the coolant will go to the radiator and the system will eventually reach an equilibrium temperature if it does not boil over first. Blanking off of the by-pass is not a good idea as there would not be any circulation in the engine block during warm up. In fact, on the picture of the blank off, this actually restricted the thermostat from opening all the way causing even more overheating problems. I have installed a restrictor before to allow some circulation and used a regular thermostat of higher temperature to stabilize the engine temperature. Most of the sleeve thermostats that are being reproduced are the 165 degree rating. The later style is available in 165, 180, and 190 ratings. The 165 rating was used in the early days when engine oils tended to sludge up and coke if gotten to hot. The newer oils can stand the higher temperatures. Contrary to the old tale of cooler is better for power; the higher temperature thermostats will actually help power. The oil will stay cleaner and burn off condensation easier. Friction is also reduced. Formula one cars typically run engine temperatures of 120-130 Celsius. If lower temperatures were better, they would run them. The only drawback is that for every 10 degrees F raise in temperature, there is a 1 point increase in the octane requirement of the engine. As sensitive as the XKE’s are to gas, this makes premium a must.
Another consideration on the thermostats is the air bleed hole. Some of the thermostats have a hole and some have valves to block the hole when the water starts coming through. This is necessary on the Series 1 cars to let the air out of the block when filling coolant. The Series 2 and 3 cars have by pass ports so the hole in the thermostat is redundant. If in doubt as to whether your car needs one, just take a 1/16” drill bit and put a small hole near the outside edge and position this in the car with it in the uppermost position. Not using a vent hole when needed will create a large air pocket which will keep the thermostat from opening until it gets enough steam to open.
As far as making power and temperature, the incoming air temperature has a great effect. The lower the temperature, the denser the air and the greater the oxygen content. The 6 cylinder cars do not suffer from this problem but the V12’s have a lot of heat under the hood and not a lot of air movement so the air inlets are over the top of the radiator. Changing to some of the aftermarket filters can actually hurt power as the air temperature is higher. Another factor to consider is that the greater the difference between the ambient temperature and the coolant temperature, the greater the heat exchange, all else being equal. This is why cars tend to run cooler on cooler days. Raising the running temperature could be seen as similar to lowering the air temperature. Radiators can be a problem as they are where the heat transfer takes place. Aluminum radiators are usually more efficient. This is not because Aluminum transfers heat better but because it is stronger and therefore allows the fins to be larger instead of having multiple fins. Almost all of the newer Jaguars run aluminum radiators as they are lighter and can be smaller for the same heat transfer.
Engine running is one of the other big problems. Distributors have a tendency for the advance weights to seize up causing timing problems. This is a maintenance issue as the manuals recommend a drop of oil under the rotor. This hardly ever gets done especially on the V12’s. The symptoms of this are that the car gets hot on the road but cools down when it comes to stop. The vacuum retard is another device that directly causes the engine to generate more heat. The Series 1 cars used a vacuum advance unit on the distributors. The 1968-69 cars had no unit and the Series 2 cars had a vacuum retard. The V12’s also had a retard unit but also had temperature controls on the right rear of the engine to stop it from functioning if the engine got to hot. The retard unit is designed to decrease the timing to help reduce NOx emissions. A lean or rich mixture can also cause the engine to put out more heat. This is in the carburetion system and to detailed to cover here other than to fix it if there is a problem.
Special thanks to Dick Maury of GeorgiaJag.com for this tip!