What was found
AT FL360 WITH LESS THAN 22000 POUNDS FUEL ONBOARD, THE NR 2 MAIN TANK FUEL DECREASE FROM 8.55 TO 7.33. THE IMBALANCE LIGHT ILLUMINATED WITH CENTER TANK PUMPS STILL IN USE THE NR 2 MAIN TANK FUEL GAUGE FLUCTUATED BETWEEN 8.3 & 7.3, MOSTLY STAYED AT 7.3 AFTER CENTER QTY WAS EMPTY, BOTH MAIN TANK GAUGES OPERATED NORMALLY. ACFT GROUNDED. COMPLETELY SUMPED NR 2 MAIN FUEL TANK EMPTY AND ACCOMPLISHED FUEL QTY INDICATION SYS TEST, IAW AMM 28-41-00-730-801 AND FOUND TANK UNIT CAP, 399.1 AND COMPENSATOR CAP, 57.99 IN LIMITS. REMOVED A HAND SIZE PIECE OF ICE FROM AREA UNDER THE COMPENSATOR PROBE. FUEL QTY COMPARISON CHECK GOOD, REF AMM 28-41-00-720-801.