Thanks. So I guess the picture I shot was the wrong one. it is the bigger diameter cap just next to the radiator.

I also found there is another cap close to the back passenger side just below the big wind shield washer fluid container.

so is the smaller cap the low pressure value? the one next to the radiator is bigger cap.

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2004 X5 4.4 Sport --- for her
1998 GS400 --- for me
1992 Legend -- totaled
1995 Civic EX -- stolen