Stant cap (#10233) is a 16 lb and used on the 94-2001 Explorers and other Fords with the 4.0 V6. In 2002-2005 Ford switched to the system with no rad cap and only coolant recovery screw-on cap. In 2006 thru 2010 for some reason they went with the 20 lb. My bypass hose wasn't aged, it burst from pressure and I looked closely in the bends of my upper and lower radiator hoses and there were hair line cracks starting so they got changed also. I've got 300 miles on my rebuilt system running the 16 lb cap and everything is fine with no difference where my needle is on the temp gauge.
Water boils at 212 degrees and when pressurised you raise the boiling point thus saving your engine. The boiling point of water will raise 3 degrees for every pound of pressure your cap is rated:
16 lb PSI = 260 degrees
18 lb PSI = 266 degrees
20 lb PSI = 272 degrees
There is no logical reason for Ford to run 20 lb PSI caps on only the 2006-2010 models; maybe this was a test for some lower emissions they were trying to acheive in which they failed badly. Still trying to get the cap rating for the 2011 models with the new 3.5 V6. From what I've seen the 4.0 is gone. My final take on all of this is the 2006 was where we all saw the big radiator numbers failing, Ford realized the error and designed a stronger radiator and had all other manufacturers do the same but if you have had more than one failure you got one of the first designs. The radiator leak bulletin didn't come out until Aug-Sept 2009 so probably some 08-09 models may hold the first design. My rad went at 56,000 on 10/2010 (I have it boxed in my attic garage) so I feel like I got the newer design models. If there is another weak link in our systems then hopefully I've saved myself some future work on replacement parts but only time will tell now. I did replace a ton of parts due to the bypass hose burst:
-lower t/stat housing ( one of the long bolts had scale/rust on it but the bolt isn't in water; slow leak starting within the housing)
-t/stat (original was stamped 106 C; if this is celsius thats 222 degrees) replaced with 195
-all orings, even new oring on temp sender
-bypass, upper, lower rad hoses
- even cleaned top of intake valley while I could get my hand in there with Permatex rust remover gel; looks brand new