I found an O ring in my collection that worked. Now the cap fits snugly enough that I am comfortable just stabbing it onto the fill tube. There was no O ring in the fill tube. Either it was never there or was lost when I removed the cap. I am still very interested in why my cap won't latch onto the fill tube with a new lock tab. Any thoughts??

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'99 E300
'01 F250 PowerStroke