Originally Posted By: OVERK1LL
That is not necessarily correct. There are a number of different types of cooling systems in use. Almost all of them are pressurized, but WHERE that pressure is restricted is a function of the type of system.
Three examples:
1. Old-school 8psi-style systems found on 60's and 70's cars. Many of these did NOT have overflow tanks. There was always some sort of air in the cooling system. Yet they functioned fine. They typically had larger rads, and copious quantities of under-hood space that allowed for much better heat dissipation than the modern cramped engine compartments which rely VERY heavily on an efficient cooling system.
2. Newer systems with a non-pressurized overflow. The system found on all older Ford automobiles for example. My Town Car, Mustang, Explorer (1997)...etc all had this system. The pressure cap (15psi for the Lincoln for example) is on the radiator. The overflow tank simply has a clip on cap. There is a level of coolant in this tank and two lines indicating full cold and full hot. As the coolant expands, it eventually overcomes the 15psi limit of the cap on the radiator, causing the expanded coolant to exit the radiator into the overflow container and increasing the level to the "full hot" mark. When the car is shut off, and the coolant cools (and contracts), the negative pressure inside the radiator draws back in the coolant from the overflow and brings it back down to the "full cold" mark.
3. Pressurized overflow. My dad's 03 Town car has this, my mom's 2000 Expedition has this system. There is no rad cap. The cap is instead a screw-on cap on the actual overflow container. This container is opaque plastic and has the same "full cold" and "full hot" marks as the older system, except the pressure in the system is controlled by the cap on the actual overflow.
And a couple of points:
- Ethylene Glycol has a higher boiling point than water (198C). A 50/50 mix of traditional green antifreeze (Ethylene Glycol) and water NOT pressurized has a boiling point of 107.2C (225F).
- Pressurized coolant has a higher boiling point than non-pressurized coolant. So while a 50/50 mix of EG and water will boil at 225F, under 15psi, water boils at 250F, and with a 50/50 of EG, boils at 265F.
OVERK1LL,
That's an excellent well written post with lots of good information. Nice work.
