Common position of the Engine Coolant Thermostat: At the coolant outlet of upper engine

In the past, most of the engine thermostats were installed at the cooling water outlet of upper engine. It would be convenient for maintenance and also easy to eliminate bubbles in the cooling system when replacing engine coolant. Such installation, however, has some disadvantages. The thermostat may work along with vibration. Some people may install the engine thermostat in the water pipe of the radiator, which can reduce or eliminate the thermostat vibration, and can accurately control the coolant temperature. Due to the complex structure, the cost would be high, and it is mostly used for high-performance cars and cars that often run at high speed in winter.

It's proved that the thermostat installed at the water outlet of the upper engine will lead to unstable performance and high fuel consumption during engine heating, and thus cause poor engine performance engine wear.  Let's go deep into the reason. While the thermostat is controlling the coolant temperature, the temperature of the engine fluctuates greatly. Once the thermostat's main valve is open, the cooling water in the radiator quickly surges into the cylinder, resulting in sudden drop of the water temperature in the cylinder. At this moment, the thermostat's main valve is closed again.  When the temperature in the cylinder rises again, the main valve in the thermostat opens again and the cooling water in the radiator rushes into the cylinder, causing the water temperature in the cylinder to drop again.  Once again the main valve of the thermostat is closed. This will repeat until the temperature of coolant in the radiator and the engine rise to the controlling temeprature of the thermostat.

As the cooling water temperature in the cylinder changes dramatically and repeatedly, the engine gasoline atomization performs sometimes well and sometimes badly, leading to the unstable performance of engine. For the electronically controlled direct injection gasoline engine especially, it is a nightmare.

Installation Position of Engine thermostat to modern models: at the engine coolant inlet

With the rapid development of electronically controlled direct injection gasoline engine, it is no longer suitable to install the thermostat at the upper engine.

The engine thermostat is installed at the water inlet to overcome the fluctuation of the engine coolant temperature.  When the engine cools down, the thermostat main valve closes the main water channel, while the bypass valve opens the bypass water channel. Coolant in the cylinder flows out from the uppper water outlet into water pump via bypass water channel, and later pumped back into the cylinder. When coolant in the cylinder rise at the temperature of thermostat opening, the main valve of the thermostat opens slightly, while the bypass valve closes slightly at the same time. At this moment, cooling water flows in two ways, Some water repeats the above cycle, and other water flows into radiator through water outlet of upper engine and into water pump via thermostat main valve, and finally pumped back into the cylinder.

Now the cooling water flowing out of the radiator makes the thermostat close the main valve immediately. When the cooling water temperature around the thermostat rises to the opening temperature of the thermostat, the main valve of the thermostat is opened again. When the cooling water in the radiator flows through the thermostat again, the main valve is closed again.  This is repeated until the cold water temperature in the radiator reaches the opening temperature of the thermostat.

It sounds like the same working principle as the thermostat installed at the upper outlet of the engine, however, their controlling mechanism is completely different.

If installed at the water outlet of the upper engine, the thermostat controlls the the temperature of water in the whole cylinder, and the open temperature and close temperature are consistent with the cooling water in the whole cylinder. While the thermostat is installed at the engine coolant inlet where the cold water colides with hot water, its open temperature will be consistent with the cold water in the cylinder, but the close temperature will be consistent with the small part of cold water which flows from radiator through thermostat and into cylinder. The temperature accuracy will be better, so that water temperature in the engine won't fluctuate dramatically and thus the egine can work stably.