What was found
DURING TAXI OUT, A PASSING AIRCRAFT REPORTED A FIRE IN THE RIGHT ENGINE. NO FIRE INDICATIONS IN THE COCKPIT. THE CREW PULLED THE NR 2 ENGINE FIRE HANDLE, BUT DID NOT ACTIVATE THE FIRE EXTINGUISHERS. RETURNED TO GATE WITHOUT FURTHER INCIDENT. THE AIRCRAFT WAS GROUNDED. MAINTENANCE REINSTATED THE NR 2 ENGINE FIRE HANDLE IAW AMM 26-21-00, & ACCOMPLISHED A NR 2 ENGINE FIRE DETECTION SYS OPS TEST IAW AMM 26-11-00, & FIRE BOTTLE INSPECTION IAW AMM 26-20-00. REPLACED THE NR 2 ENGINE.