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87 Renix 4.0, closed system. My engine was rebuilt last year and has only a few thousand mile so far. I believe all small parts were replaced. On Friday, I loaded up the MJ for a weekend camp trip. Nothing heavy, just tent, sleeping bags, cooler, etc. and a passenger. This is probably the most weight it has carried since the rebuild. The outside temp was mild - 70s. I started out and looked down at the cluster. The temp gauge usually stops at the first line. Very seldom does it rise above this. Well, I was watching it continue past that and go above 210. I had only driven 3 miles. Then the temp dropped down to "normal," just above the first line. I stopped at a McD's for coffee. I looked under the hood and noticed antifreeze around the reservoir bottle. The cap was loose and would no longer screw down tight. i turned back and tranferred all my gear into a Suburban. I have ordered a new cap from NAPA. For the big question: Should I replace the T-stat also? What may have caused the sudden, rapid rise, and rapid drop?