It's the solenoid in the EVAP system that discharges gas vapor into the intake. In and of itself, it should not cause a performance issue. But a vac leak in the system could cause the car to run poorly. Generally an erratic idle or O2 code accompany a vac leak. It is worth noting that the discharge line rots where it goes under the plenum.
Do you have a test light or DMM?
Please describe the poor running condition with more detail.