THE AUTOMOTIVE THERMOSTAT
by Jim Miller

The basic purpose of an thermostat is to trap the coolant & water around the engine block in order to help the engine reach operating temperature more quickly. As soon as the temperature gets hot enough, the thermostat must open, allowing the coolant to flow throughout the entire cooling system (hoses, radiator, etc). When taking your vehicle to an auto repair shop, beware of any mechanic who immediately suggests a new raditor; the problem may be as simple as a stuck thermostat. Automotive engine operating temperature must be reached as quickly as possible and it must be kept at a specific level. The proper operating temperature of a vehicle affects fuel economy, performance, emissions, and longevity. If an engines operating temperature is too low, fuel delivery is affected, carbon builds up in the engine, wear is increased and performance can suffer. In addition, heater outlet temperatures can be too low. Because of this, a thermostat is used to maintain the engine at a proper temperature.

Thermostats used in most vehicles are of the pellet type, poppet valve design. A pellet, containing a material that expands with temperature, moves a rod that opens a poppet valve, to allow coolant flow at a required temperature. Thermostats also contain a small hole that allows air and steam to escape from the engine into the radiator when the thermostat is closed. Most cooling system designs provide a means for coolant to flow to the water pump when the thermostat is closed. This is called bypass flow. Coolant is circulated by the water pump through the engine until it reaches the closed thermostat. A small amount of coolant flows back to the water pump via a passage or a hose, to be recirculated through the engine. This helps to provide faster warm up for the engine and to prevent water pump damage. Once the thermostat opens, normal coolant flow through the radiator is resumed.

Almost all vehicles today use a thermostat that opens at 195 Farenheit. In order for a vehicle to operate properly, the correct thermostat should be used. An automotive engine that overheats should not be repaired by using a lower rated thermostat. The cause of overheating should be diagnosed and corrected.

A thermostat that sticks in the closed position will cause severe engine overheating conditions, since coolant will not flow through the radiator. Consequently, a thermostat that sticks in the open position will not maintain engine operating temperatures at a required level. A thermostat that is stuck open may not be noticed by the driver until ambient air temperatures are colder and only then when heater temperatures may be too low. However, a thermostat that is stuck open should be replaced as soon as possible due to its affect on fuel economy and engine wear.

For people who live in very hot communities (eg. the desert), it may be OK to remove the thermostat entirely. There is no advantage in doing this unless you have had consistent problems with thermostats getting stuck in the closed position, in which case it may be acceptable to remove the thermostat. Again, this only applies to very hot communities and in particular, during summer months.

(Jim is a lifelong fan of Dodger Baseball and used to race sprint cars in the 1980s.)