Coolant overflow

[Paulhowell]

We just bought a 40 foot 1999 Patriot Thunder with a 425hp Cat C-12.  We took it on its first trip and everything worked fine for 800 miles. As I was parking it after we got home, the coolant overflowed out of the coolant surge tank. The previous owner had installed a one gallon jug to the overflow hose from the surge tank and the jug was overflowing.  Today I emptied the coolant from the jug back into the surge tank and I was going to replace the surge tank cap and the heater reservoir cap as well.
The surge tank cap is rated at 7lbs and the heater reservoir cap is rated at 13lbs.
That sounds low for the surge tank cap. Any info would be appreciated. Thanks.
Paul.

[Steve Huber Co-Admin]

Paul,
To my knowledge, the surge tank came with a 13 PSI cap. However many of us replaced them with 7 PSI caps as the 13 PSI cap can cause cracks in the plastic body of the surge tank when the engine gets to operating temperature. The overflow tank does not need a pressure cap as it's only purpose is to catch fluid from the surge tank when the fluid is hot and expands. Once the engine cools, the fluid will be sucked back into the surge tank by vacuum since the connecting hose is on the bottom of the overflow tank. It sounds like the overflow tank had too much coolant in it when cold. When the engine heats up the added coolant will result in an overflow. With a cool engine, you only need enough coolant in the overflow to cover the hose running to the surge tank.
Steve

[Paulhowell]

Thanks Steve!  I found some older posts that addresses this also. I guess the slop can is an approved solution if you put in a 7psi radiator cap.

[LEAH DRAPER]

Guys, pardon my lack of knowledge being female, but are you saying that the lesser psi cap causes more chance of overflow from the surge tank??

Leah Draper

[Steve Huber Co-Admin]

Leah,
Lowering the PSI of the radiator cap lowers the boiling point of the antifreeze. With a 50/50 antifreeze to water mix, a 13 PSI cap results in a boiling point of approximately 261F. A 7PSI cap reduces the boiling point to about 243F. This is normally well above the engine operating point (I think your C9 has a 206F t'stat and C12 t'stats run about 195 or so). The purpose of reducing the PSI is to reduce the amount of stress (repeated expansion/contraction) seen by the plastic surge tank as it will tend to crack and  leak at some point. The reduced pressure cap does not change the engine operating temperature so you should not see any added overflow unless the engine temp reaches the lower boiling point.
Steve

[Jeremy Parrett]

After blowing holes in 3 Ford plastic coolant expansion tanks I purchased and installed the steel  one Beaver sells !! 
Only one issue since...the sight glass cracked. I replaced it with a brass plug.   
Since then no more issues.  I use a 13 lb cap and a Caterpillar overflow bottle.