I've been in the field for over 30 years and I've never seen the opposite effect on a running system with a PSC motor. Were you checking the amps both ways or were you reading the amps off the tag and measuring them against your measured amp draw? There might be some other scenarios but that's the only case I can think of where your amps might measure lower with the door off.
Yes I was checking amps with door off and then on. With the door off i got about .3 or .2 amps less. If my meter wasnt at work ide check my blower now.
The more air the fan moves, or work it does, the more amps it draws. When they rate a motor it is under a specified load. It would be my guess that, getting back to the original post, the motor is not being loaded to its perfect world load to draw the nameplate amperage. Cap is fine, doesn't need replacement.