FUEL IMBAL IN A B767 CAUSED BY AN OPEN XFEED VALVE THAT WAS NOT DISCOVERED UNTIL LOW FUEL WARNING WAS DISPLAYED.

Narrative

FLYING FIRST FLT OF THE DAY FROM LGA TO ATL ON ACFT THAT WE HAD BROUGHT IN THE NIGHT BEFORE. SOMETIME DURING THE NIGHT; SOMEONE HAD OPENED THE FUEL XFEED AND DIDN'T RECLOSE IT. DURING DAYTIME OPS; THE FUEL PANEL CAN BE DIFFICULT TO READ FROM EITHER PLT POS DUE TO GLARE OFF THE PLASTIC COVERS. AT NIGHT; EVERYTHING IS BACKLIT AND IS EASIER TO READ. I MISSED THE FACT THAT THE XFEED WAS OPEN DURING THE BEFORE START CHKLIST. WE HAD ABOUT 2600 LBS OF FUEL IN THE CTR TANK WHICH IS CONSIDERED UNUSABLE; SO THE CTR TANK BOOST PUMPS WERE OFF. IN THIS CONFIGN; A 'FUEL CONFIGN' EICAS MESSAGE IS DISPLAYED CONTINUOUSLY; MASKING ANY FUEL IMBAL. AFTER A LONG; MUCH DELAYED TAXI OUT FROM LGA AND CLB TO CRUISE; I CHKED THE BAL ON THE FUEL INDICATORS. HOWEVER; BY THIS TIME; WE WERE 10000 LBS OUT OF BAL AND DUE TO THE AFOREMENTIONED; DAYTIME GLARE ON THE FUEL PANEL; I READ IT AS BALANCED IE; 16.5/16.7 VERSUS 16.5/6.7. OUR FIRST CLUE THAT SOMETHING WAS WRONG WAS WHEN THE 'LOW FUEL' EICAS MESSAGE WAS DISPLAYED WHEN THE R TANK REACHED 2500 LBS. THIS WAS AT TOP OF DSCNT; SO WE ACCOMPLISHED THE 'LOW FUEL' CHKLIST AND LANDED NORMALLY AT ATL.