Check your coolant expansion tank located on the driver side closer to the front. They are notorious for cracking and leaking (check the bottom of it). Also, check the all hose connections. Report back?

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Current:
2017 640Xi Convertible
2011 Acura MDX
2018 Santa Fe

Past:
2013 Honda Accord
2008 Acura TL Tech.------2004 BMW X5 4.8is----2004 Acura MDX Tech
1994 BMW 525i.----------1994 Lexus 300 GS.----1994 Acura Legend GS
1991 Acura Legend LS.----1991 Honda Accord.----1991 300ZX twin turbo
1996 Buick Regal GS.-----1984 300Z 50th anniv.--2002 BMW X5
1982 280Z