My 1991 F250 460 has a slight misfire at idle. I tried unplugging the MAP sensor and RPM's will shoot up from 600 RPM to 100 RPM. The steering wheel seems a lot steadier though about once every 2-3 seconds, like it's timed, like it's going through a cycle or something, I will get a very small short period of vibration from the steering wheel. So by unplugging the MAP sensor, I go from erratic intermittent missing to smooth idle with cyclical roughness. Does this mean my MAP sensor could be bad?

 

Why are you trying to diagnose a problem this way? Why not just pull the codes and know if there are any sensor problems.