AN ACR FLC IN AN LGT HAD A FUEL IMBAL DUE TO MISMGMNT AND WHILE DISTR BY THIS PROB THE FO OVERSHOT THEIR ASSIGNED ALT.

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I WAS THE FO AND PF ON AN IFR SCHEDULED FLT FROM DFW TO ORD. OUR FUEL LOAD FROM DFW WAS SUFFICIENT TO BURN ALL ENGS FROM #2 FUEL TANK. ALL FUEL TANKS WERE APPROX EQUAL (ABOUT 45 MINS INTO THE FLT) WHEN THE SO SAID; 'I'M GOING TANK TO ENG.' THIS WOULD NORMALLY BE THE CONFIGN FOR THE DURATION OF THE FLT. I THINK; MIDWAY THROUGH THE FUEL PANEL CONFIGN CHANGE THE FLT ATTENDANT KNOCKED AND ENTERED THE COCKPIT WITH OUR CREW MEALS. THIS DISTR WAS ENOUGH FOR THIS NEW SO TO INADVERTENTLY LEAVE A XFEED VALVE OPEN. THIS WENT UNNOTICED FOR ABOUT 25 MINS. THIS CONFIGN LEFT US WITH A FUEL IMBAL AS TANK #2 WAS FEEDING ENGS #1 AND #2 AND TANK #3 WAS FEEDING ENG #3. ZAU GAVE US A LATE XING RESTR WITH A SPD REDUCTION; SO I WAS DSNDING RAPIDLY WITH THE AUTOPLT ENGAGED. I DISCONNECTED THE AUTOPLT TO MAKE A SMOOTH LEVELOFF AND THE ACFT FELT WING HVY AND WAS VERY DIFFICULT TO TRIM. I MENTIONED THIS TO THE CREW AND WE DISCOVERED THE FUEL IMBAL. AS I WAS OCCUPIED WITH TRIMMING AND CONFUSED ABOUT THE CONSTANT ROLL; I FLEW BELOW THE ASSIGNED ALT OF 12000 FT BY ABOUT 400 FT. I IMMEDIATELY RETURNED TO THE ASSIGNED ALT AND THERE WAS NO MENTION OF THIS TO OR FROM ATC. WE DISCUSSED THE FUEL PROB AND THE CAPT INSTRUCTED ME TO FLY THE AIRPLANE AND HANDLE THE RADIOS; WHILE HE AND THE SO COMMUNICATE WITH OUR DISPATCH AND MAINT TO WORK OUT OUR PROB. THE SOLUTION WAS TO BURN DOWN THE FUEL LOAD TO WITHIN LIMITS FOR LNDG. I ASKED ATC FOR DELAYING VECTORS AND AFTER ABOUT 15 MINS WE WERE READY TO START OUR APCH TO ORD. THE APCH AND LNDG WERE UNEVENTFUL AND WE TAXIED TO THE GATE.