Engine overheating/Radiator cap

A new thermostat did not cure the overheating on my 1976 300D. A new radiator cap did. Would someone please explain how the radiator cap effects the cooling temperature.

Thanks

[JimSmith]

Jaycorn,

The cooling system is designed as a pressurized system and without the seal on the radiator cap there is no way to maintain the system pressure. This leads to two problems.

The first is that the car will tend to be low on coolant most of the time, which can increase the coolant temperature, and will speed up the loss of coolant mechanism enabled by the leaking cap. This ends up in a "do-loop" until the car overheats.

Simulataneously, the next issue is the coolant is supposed to be heated to a temperature internal to the engine that exceeds the boiling point of water at atmospheric pressure. When the water reaches the radiator there is a higher temperature difference between the air and the water, which improves the efficiency of the heat transfer mechanism of the radiator. The engineers then make the radiator as small as is practical to lower weight and cost.

When the cap fails to maintain the pressure required to prevent boiling inside the engine coolant passages, the water boils. Steam bubbles do not pick up heat anywhere near as well as liquid water and the ability of the system to remove heat from the engine is compromised. The engine metal surfaces will start to get warmer. Which boils more water and so on. Finally, the coolant temperature and pressure are lower when it hits the radiator than was intended, so the amount of heat rejected to air is less (little bubbles form in the passages in the radiator too, as the velocity of the water changes follow the passages and leads to cavitation). As a result the whole system temperature increases (radiator is less efficient and heat removal from hot engine surfaces is less efficient due to bubble formation) and you get a high temperature indication on the gage. If the water level is lowered too much the system really fails and you cook the engine.

I hope that helps. Jim

Jim:

Many thanks.

Jaycorn