FMEM is Failure Mode Effects Management.
This means that since there is something wrong with your mass air meter the computer is going to just use look up tables based on other information. It's basically a limp home mode.
Try resetting your computer. If it is still a problem afterwards, I would suspect that your MAF sensor was the cause of the problem for which you changed the throttle body.
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351W 89 Mustang GT Convertible