As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING START CYCLE, PILOT HEARD A BANG. HE ABORTED THE START AND WENT TO LOOK FOR THE SOURCE OF THE NOISE. UPON OPENING ENGINE COWLING HE SAW FUEL POURING OUT OF THE AIR FRAME FUEL FILTER BOWL. PILOT RETURNED TO COCKPIT AND SHUTOFF BOOST PUMPS. HE REMOVED THE BOWL OF THE AIR FRAME FUEL FILTER AND FOUND THE "HEAD" OF THE CAP BROKEN OFF AND SEPARATED FROM THE CAP.