Special20

I have a 2003 GT Convertible. Can someone please explain why the coolant cap has a PSI of 16? Would it not make sense to have a lower PSI(7) so there is not as much pressure on the hoses, radiator, and heater core. I have a 1978 F-150 that I use a 7 PSI cap on and have no issues. Would there be any harm to do the same to the Mustang? Thanks.

Z28KLR

Because when in a closed system and otherwise under pressure, water has a higher boiling point.

Given just purely atmospheric pressure, water in an open container begins to boil at 212°F when at sea level. At approx. 6,000' elevation (lower atmospheric pressure) it reaches boiling point at about 200°F. With your car's cooling system, the cap essentially eliminates atmospheric pressure from the equation and while allowing up to 16 psi of pressure (think, higher atmospheric pressure) the boiling point is then effectively raised to 250°+.

Ever notice how when removing the radiator cap on a hot engine you can hear the water begin boiling and bubbling as you open the cap? The water temperature, whatever it is, hasn't changed. But the decrease in pressure as you are removing the cap is what causes it to begin boiling.

imp

"Would there be any harm to do the same to the Mustang?"

Probably yes. Z28 is exactly right. One part I always wondered about regarding pressure, and you didn't mention it, is the water pump shaft seal.


I've always figgered a far better solution to radiator, heat, gradual coolant loss, possible boiling over problems, etc., would be to upgrade the cooling system's heat transfer capacity. This means larger radiator, if possible, adequate cooling fan size, possibly a high-capacity water pump. But using a radiator cap having a lower pressure rating only lowers pressure long-term IF it doesn't allow loss of coolant or boil-over. Using a 16 lb. cap does NOT mean the system "sees" higher pressure than it does with a 7 lb. cap. The pressure determines it's own level based on the operating temperature, which varies, but never exceeds 16 lbs., so long as the radiator type is one having an "air space" designed in, to allow the liquid to expand as it heats up.


Now, very few of today's vehicles employ an expansion space within the radiator itself. Instead, the pressure ALWAYS rises to the cap's rated pressure, at which time the excess liquid represented by expansion leaves the radiator, sealed within the cap's upper interior by a second rubber seal which seats on the top rim of the radiator neck, and exits at ATMOSPHERIC pressure through a hose, usually, and is caught within a "catch bottle".


Upon cooling down, the coolant in the radiator "shrinks", creating a vacuum which "draws" liquid out of the catch bottle and back into the radiator. Thus, the radiator is always full. (whew!) imp

Special20

Wow. OK. Hmm... I just had flashbacks of high school. Thanks for the quick replies. I think I will leave the 16 PSI coolant cap in place.

imp

FWIW, I have seen the "BEST" solution to radiator coolant fullness without nasty catch bottles, drawback into radiator, etc. twice now in all my years. 1st: Early Ford big blocks, 352, 390, etc., used a separate copper expansion tank mounted to the intake manifold replacing the water outlet housing. The radiator had NO CAP. The cap was on the expansion tank, it being a standard radiator cap. Gave the expanding hot coolant a place to sit. Drained automatically back into engine as the juice cooled back down. Expensive though.


2nd: My 2004 Explorer V-6 uses exactly the same technique as above, except the expansion tank is now strong heavy plastic, connected to the radiator by a thin hose, radiator itself has no cap. The cap is on the expansion tank, and is of that new variety that "clicks" upon being properly tightened. Thus, the tank, hose, and radiator are pressurized all to the same level. Works very well. imp