When you take the hose off, it's the same (to the FPR) as flooring the pedal (vacuum goes to atmospheric). So the FPR does its job and increases fuel pressure, which richens mixture as does for acceleration.

Sounds like it's working OK., but you can't really be sure at this point. Maybe the FP was low to begin with, and you just "forced" it to normal, improving the idle. I'd put money in a FP gauge before arbitrarily replacing the FPR (about the same cost).

A gauge might show that the FP is good. Then you can start looking for an intake leak that you temporarily corrected by raising the pressure, etc., etc.

Forgive the rambling. My main point was to correct what seemed like a misconception about how the FPR works.
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67