wanted:lower PSI coolant cap..how to find?

[wesalexleft]

I have a 2003 Mercury Moutaineer with the 4.6L V8 engine. It's getting coolant in the oil. In trying to trace that down, I found that coolant pressure leaks down until about 13 PSI, then the leak stops. I'm looking for a lower pressure coolant cap to replace the stock 16 PSI cap until I can find the leak so that coolant will stop being forced into the oil. It's my son's college car and he is away at school. From what I understand, the coolant caps are actually in ranges and a 16 PSI cap is from 16-20 PSI. A 13 PSI cap would be from like 12-16, etc. Does anyone know of a cap that will fit with a 7-10 PSI rating? This is the plastic cap on the degas tank with internal threads on the cap. This year does not have the cap with access directly to the radiator, but on the overflow only, and is an odd size. I had to get the adapter to do a coolant pressure test. I can't find online, but I'm hoping that someone "in the know" will be able to ring in and say that a Mazda XX will fit, or a Ford Focus cap will fit, and has a lower PSI rating.
I understand that this will lower the boiling point of the coolant, but this truck isnt doing any towing, etc, and we'd watch it closely to be sure there is no boiling over.

Thanks!

 

[D Hook]

Not sure this will help but Presco makes quite an array of caps.

http://www.presco-radiator-caps.com/radiator_caps_new.htm

Might have to buy/make some kind of adapter neck to fit that tank.

I googled "7-10 PSI cap". There's more entries to check.