AN MD80 HAD #2 ENG REPLACED 'OFF LINE' BY ENG TECHNICIANS. WHEN TRIMMING ENGS FOUND IT NECESSARY TO REPLACE #1 ENG FUEL CTL UNIT. WHEN #1 FUEL CTL WAS REPLACED THE ACFT WAS FLOWN WITHOUT USE OF THE ENG TEST BOX.

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ZZZ1 ENG SHOP CHANGED THE #2 ENG IN ZZZ2 ON DEC/SAT/05. BOTH ENGS WERE TRIMMED AT THAT TIME. FORWARD THRUST WAS IN LIMITS WITH BOTH THROTTLE KNOBS ALIGNED. THE LAST STEP OF THE ENG TRIM WORK CARD IS TO VERIFY REVERSE THRUST KNOB ALIGNMENT. WE WERE UNABLE TO RIG OUT THE KNOB SPLIT WITHOUT AFFECTING THE FORWARD THRUST KNOB ALIGNMENT. WE CONTACTED TECHNICAL SVCS FOR ASSISTANCE. THEY INSTRUCTED US TO RE-CTR BOTH FUEL CTLS AND REDO THE ENG TRIM. WHILE RE-CTRING THE #1 ENG FUEL CTL; THE IDLE ADJUSTMENT SCREW SEIZED. THE ACFT RETURNED TO THE HANGAR TO AWAIT THE #1 ENG FUEL CTL UNIT CHANGE. THE NEXT MORNING (SUNDAY) THE #1 ENG FUEL CTL UNIT WAS CHANGED AND BOTH ENGS RETRIMMED. PER THE MD80 MAINT MANUAL 71-00-00 CONFIGN 8 PAGE 501 ITEM 1B; IF 2 ENGS ARE INSTALLED OR 2 FUEL CTLS ARE INSTALLED AT THE SAME TIME; YOU ARE REQUIRED TO INSTALL THE TEST BOX. NEITHER EVENT TOOK PLACE. BOTH ENGS WERE ORIGINALLY TRIMMED. THEN AFTER BEING UNABLE TO GET THE REVERSE KNOB SPLIT RIGGED OUT; WE RE-CTRED THE #1 FUEL CTL UNIT AND SUBSEQUENTLY SEIZED THE IDLE ADJUSTMENT KNOB DURING TROUBLESHOOTING. THE NEXT DAY WE CHANGED THE #1 FUEL CTL UNIT AND TRIMMED THE ENGS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ENG CHANGE WAS COMPLETED AND ALL THAT WAS REQUIRED WAS TRIMMING THE ENG FUEL CTLS IN FORWARD AND REVERSE THRUST. FOUND UNABLE TO TRIM #1 ENG DUE TO TRIM ADJUST SCREW SEIZED. AFTER REPLACEMENT OF THE #1 FUEL CTL UNIT; THE ENG WAS CORRECTLY TRIMMED BUT ON READING THE MAINT MANUAL IT WAS DISCOVERED THE INST TEST BOX WAS REQUIRED AS 2 FUEL CTLS HAD BEEN INSTALLED. THE ENG CHANGE PLUS THE FUEL CTL CHANGE. THE TEST BOX CHKS THE ACCURACY OF THE ENG INSTS ON BOTH ENGS AND IS REQUIRED. THE BOX WAS ORDERED AND WAS HUNG UP IN CUSTOMS WITH NO ESTIMATE OF WHEN IT WOULD BECOME AVAILABLE. THE DECISION WAS MADE TO FLY THE AIRPLANE TO A COMPANY MAINT FACILITY AND PERFORM THE TEST.