Have you checked if the aux fan and fan clutch is operating normally? Have you tried changing the radiator cap. Have you pressure tested the system?

Do I have it right that even with heater on when driving, after 15-20 minutes it overheats?

Does the needle peg, go to 3/4 to peg or what?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas