Not sure how long you've been plagued with temp problems, but make sure the cap is screwed on properly. There are notches along the bottom of the cap and around the reservoir spout...I have at least once, put the cap back on and not screwed it tightly beyond the first set of notches. The result was higher than expected temps and boiling over after driving.

Go ahead and replace the cap first...cheap insurance.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car