What was found
BOS- AFTER TAXIING TO GATE THE LT ENGINE WOULD NOT SHUT DOWN. FIRE T-HANDLE WAS PULLED, NO EFFECT. THE ENGINE CONTINUED TO RUN. MAINTENANCE VERIFIED THE FIREWALL SHUTOFF VALVE WAS CLOSED. THE MANUAL SHUTOFF VALVE WAS CLOSED AS WELL WITH NO ENGINE SHUT DOWN. TRANSFERRED FUEL OUT OF THE LT TANK AND THE ENGINE FLAMED OUT. MAINTENANCE FOUND THE MOTIVE FLOW CHECK VALVE WAS STUCK OPEN ALLOWING FUEL TO BE DRAWN FROM THE COLLECTOR TANK BYPASSING THE SHUTOFF VALVES. THE MOTIVE FLOW CHECK VALVE WAS REPLACED AND OPS CHECK WAS PERFORMED. DURING THE OPERATIONAL CHECK THE ENGINE WAS STARTED AND THE FIRE T-HANDLE WAS PULLED AND THE ENGINE SHUT DOWN WITHIN 10 SECONDS. THE AIRCRAFT WAS RETURNED TO SERVICE.