Cooling System and Pressure

by Larry Barickman
Cooling systems seem simple, right? Well, for the most part they are, but here's something that may not be so obvious.
The pressure cap serves as a relief valve so the system won't explode as temperatures rise. The low slanted radiator in our cars presents a slightly different approach. Some models that have a remote pressure cap located on a surge tank (NOT the plastic coolant recovery), some have the cap on the radiator, but this applies to every system. Replacement hoses, especially the flex type may not route the same as the original. They clear everything o.k., no big deal right. WRONG!! Let's say you have an upper flex hose that goes up and over the alternator. You now have the highest point of the system at the hose instead of at the cap. Air (in gaseous form) seeks the highest point and will not escape when the cap relieves the pressure because it is not at the cap but in the hose. Something has to escape! 
COOLANT escapes every time pressure exceeds the cap's designed setting! When the engine cools there is less coolant and more air. Remember your high school physics? Air expands a whole lot more than water. Now MORE water escapes. A never-ending cycle has begun! Also the air trapped in the upper hose acts like a vapor lock, impeding coolant flow. So if your corvette is always venting coolant and you keep adding water for no apparent reason, you may have the high point at a location other than the cap. If you look at the late model LT-1's(C4) you will notice that there are bleed screws at the thermostat housing and the throttle body. These are to bleed the air from the high spots. My big block upper hose actually has a molded in trap much like the ones in household plumbing to keep the hose lower than the top of the surge tank. Clever, those GM engineers!!! 
The guys at the parts store don't have a clue about this and a "cut on this line to fit" type hose may actually cause a problem that will defy all logic. After all, "a hose is a hose" Right!