why does car idle get worse when we replace MAS? on 1998 Chevrolet Lumina

Code showed MAS was bad, replaced with used part. Light went out, but car would barely run - it would rev high upon startup, then rev dropped and it would sputter and die/almost die. It just kept doing that in a cycle. When we unplugged sensor, the car runs much better, however check engine light is then on.

Asked by for the 1998 Chevrolet Lumina
Check the big rubber intake tube to be sure it is not damaged and installed/tightened correctly. - Try a new MAF sensor!
I agree with pushrod. Sounds like you have a HUGE unmetered air leak.