I've seen weird stuff happen like a chipped edge on a piston preignite fuel. So the answer to your problem may not arrive soon or be eaay to find. If modifying the fuel does not "cure" the pinging I sure would hesitate to take it on a long trip. Might as well hammer the fuel with Chevron Premium plus add Techron. Sometimes a chipped center electrode insulator of a spark plug will cause pinging. In any event yank the plugs and read the clues on the center insulator. Be especially on the lookout for a mottled or speckled insulator.
