It was probably a one time thing then....
I would not bother with replacing the gas cap, when they go bad they usually throw off a P0456 code.
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With 60k your WJ might well benefit from a bit-o-cleaning in the engine starting with the throttle body. That requires an old toothbrush and a can of good quality cleaner. I used GumOut Jet Spray ($3.19).
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And in case your intake valves, cylinder walls, piston rings, and injectors have deposits you might want to clean them also. There are several good brands, I used SeaFoam.($5.99)
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Will you WJ run just fine without these cleanings? Probably but then the cleaning cost will be offset by using regular 87 octane plus you get the benefit of doing something nice for your Jeep....