Engine coolant expansion tank

[Gerald Farris]

Tom,
Jeremy is right about the Ford designed tank failures. Plastic welding will not work because the tank design is not strong enough to withstand the 13 to 15 PSI cooling system pressure that they are operating under. The last one that I had fail had at least 6 cracks in it before it split wide open and started dumping coolant. The only way that I have found to make the Ford coolant surge tanks last is to lower the radiator pressure cap to a 7 PSI cap. I did this 3 or 4 years ago and I have not had a tank failure since, and before that I was buying a tank about every 2 years.

Gerald  

[Jeremy Parrett]

Gerald,
  there is a relationship between pressure and temperature.  Something to do with raising the boiling point maybe?
   I wonder if lowering the cap pressure could affect the way the system is supposed to work?  
   Maybe another member here can add his 10c worth  !!    Jeremy

[David T. Richelderfer]

Yes... increasing the pressure on a liquid will raise its boiling point.  Conversely, that is why cooking times are longer at altitudes... because the liquids will boil at lower temperatures - that is, not get as hot before they boil - and cool themselves through evaporation.  Liquids have to absorb heat to evaporate.  That's why we perspire... to cool ourselves through the evaporation of the perspiration.  The evaporating perspiration absorbs and carries away heat.

Lowering the cap pressure will effectively lower the boiling point of the coolant.

[Edward Buker]

A 50/50 mix of coolant will boil at 223 degrees F at normal atmospheric pressure. At 15lbs of pressure the same mixture will boil at 257 degrees F. The relationship is about 2.25 degree rise of the boiling point per pound of pressure. A 7lb cap would provide a boiling point of approx. 239 degrees F.

I also run a 7lb cap and have not had an issue with the expansion tank (yet) and mine looks like the original which would make it 12 years old. I think the highest coolant temp I have seen when driving was 204 degrees so I am not worried about a boiling point of 239 degrees. Given the parts involved, the hoses, the overflow tank, the radiator, the transmission cooler, and the gaskets I think the lower pressure would stress these parts less as Gerald's observation regarding tank life seems to indicate. When you run the system full there is no room for expansion without forcing the 15lb or 7lb cap relief valve up, in my case that goes to a second overflow tank and gets drawn back in when cooling takes place. If your coach runs hotter on climbs then the extra margin may be worthwhile but these coaches seem to be quite well cooled.

 I also run a 7lb cap on my Aqua Hot to stress that system less and hopefully not induce so many leak points.

Later Ed

[Larry Dedrick]

Ed:

          Excellent information. Although I have ordered the new expansion tank, I will use your approach on my AquaHot and go to a 7lb cap.

                Thanks:                       Larry D.

[Jeremy Parrett]

Interesting thing happened ...the new expansion tank sight glass cracked......it is not glass but plastic. I removed it and installed a brass blank,as well as a Caterpillar overflow tank.  
Pics to follow.

[Jeremy Parrett]

The  new Caterpillar overflow tank catches the coolant that might  otherwise escape after lowering the expansion tank cap pressure to 7 lbs.
Since this picture was taken I have removed the cracked plastic sight glass and installed a brass blank in the opening.   The  Caterpillar overflow tank makes checking coolant level easy and topping up a breeze.
I had to relocate the washer bottle for the backup cam.

[Edward Buker]

Nice job and you probably have saved some future headaches with the 7lb cap. I also added the extra overflow tank after Gerald passed that tip along.

Later Ed

[Jeremy Parrett]

Thanks Ed.  It seemed a logical final step to solve this problem.  Now I have to fix the wandering tachometer !!
Jeremy