A FORTUITOUS DELAY AFTER DEPARTING THE GATE ALLOWS B717 FLT CREW TO DISCOVER FUEL IMBALANCE DETERMINED TO HAVE BEEN THE RESULT OF THE FUELING CREW'S FAILURE TO PROPERLY SECURE THE FUELING SERVICE DOOR.

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AT FIRST THE 'LAT FUEL IMBAL' MESSAGE MY CAPT AND I NOTICED WHILE WAITING ON THE TXWY FOR OUR DEP SLOT SEEMED INNOCUOUS. ANTICIPATING A DELAY FOR TKOF AND THE PROSPECT OF WX DEVS THE CAPT HAD REQUESTED AN ADDITIONAL 1000 LBS OF FUEL JUST BEFORE PUSHBACK. WE HAD BURNED ABOUT 300 LBS DURING OUR WAIT AND FIGURED THE FUELER MUST HAVE ACCIDENTALLY LOADED THAT EXTRA FUEL INTO ONE SIDE. AFTER NOTING 8700 LBS IN THE L TANK AND 7300 LBS IN THE R; WE SET UP OUR FUEL SYS TO FEED EXCLUSIVELY FROM THE L TANK. SINCE WE STILL HAD A SIGNIFICANT WAIT AHEAD OF US AND PLENTY OF FUEL TO BURN WE SAT BACK AND CONTINUED WONDERING IF WE WERE EVER GOING TO BE RELEASED. OUR BOREDOM GAVE WAY TO BEFUDDLEMENT AS WE SAW THE R TANK QUANTITY CONTINUE TO DECREASE; 7250; 7200. IT HAD BEEN A LONG DAY; BUT THIS WASN'T BRAIN SURGERY. THE SWITCHES WERE CORRECTLY SET; THE FUEL PAGE SHOWED THE L PUMPS ON; R PUMPS OFF AND THE GREEN LINES TOLD US FUEL WAS FLOWING FROM THE L TANK INTO THE L ENG AND THROUGH THE XFEED VALVE TO THE APU. A CALL TO MAINT CONFIRMED SOMETHING WAS GOING ON THAT SHOULDN'T BE. WE HEADED BACK TO THE GATE SO A MECH COULD COME OUT AND HOPEFULLY SOLVE OUR PROB. DURING THE CAPT'S CONFERENCE WITH MAINT I HEARD AN OUTBOUND REGIONAL JET CALL GND AND REQUEST A 180 DEG BACK TO THE GATE BECAUSE 'THE FUELER LEFT OUR FUEL CAP OFF.' I WONDERED IF THE FUELER MAY HAVE HAD A HAND IN OUR PROB AS WELL. AFTER PARKING WE DISCOVERED VIA OUR GND CREW THAT THE FUELING PANEL ACCESS DOORS WERE OPEN. AN INSPECTION SHOWED THE FUELING PANEL TO BE PWRED; THE L FUELING VALVE CLOSED AND THE CTR AND R FUELING VALVES OPEN. THE CONTRACT MECH WAS ABLE TO CONFIRM THAT OUR FUEL SYS WAS WORKING PROPERLY. AFTER HIS SIGNOFF WE HAD THE R TANK FILLED TO MATCH THE L TANK. I CONFIRMED THIS TIME THAT THE FUELING PANEL WAS INDEED SHUTDOWN AND PROPERLY SECURED BEFORE WE DEPARTED WITHOUT FURTHER PROBS. THE REASON THIS FLT CREW DISCOVERED THIS OCCURRENCE WAS BECAUSE OF THE 'LAT FUEL IMBAL' MESSAGE. HAD WE NOT WAITED SO LONG FOR TKOF; I BELIEVE THIS PROB MAY HAVE BEEN DISCOVERED DURING THE BEFORE TKOF CHKLIST. IT HAS BEEN MY PRACTICE TO USE THE FUEL PAGE WHEN VERIFYING 'FUEL XXX LBS; XXX LBS REQUIRED' DURING THE TAXI CHKLIST. THIS ALLOWS A LOOK AT THE SYS TO ENSURE PROPER BALANCING PRIOR TO TKOF. IT IS NOW APPARENT THAT THIS SHOULD BE DONE DURING THE BEFORE START CHKLIST AS WELL. THE PRIMARY CAUSE OF THIS INCIDENT WAS THE FUELING PERSONNEL'S FAILURE TO SECURE THE FUELING PANEL AFTER SVCING OUR ACFT.