Would removing your fuel cap extremely change your engines running ability.

[chasinthesun]

Ive replaced both fuel pumps ,replaced the tank and f filters but cant get the 87 190e to idle ,when I remove the gas cap and start the motor it sounds like its running on 3 cylinders ,put the cap back on and runs but no idle ,stalls out.Its a 4cylinder cosworth and has good compression , runs smooth at 1500 rpms and revs with no stumbles.