BuNo 157986 was the 7th prototype built by Grumman. It was used as an engine testbed to find a replacement for the underpowered Pratt & Whittney TF30s which were never meant to be the final engines used in the F-14. In 1973 it was fitted with the Pratt & Whittney F401-P400 after which it was placed in storage until 1981 when it was fitted with the General Electric GE F101DFE. The Navy decided against it and the aircraft was placed back in storage. In 1984 Grumman took the aircraft out of storage again and fitted it with the General Electric GE F110-400. The tests were very successful and the F110 went on to power the F-14A+ later redesignated F-14B as well as the F-14D.