Hey, I have 98 sunfire gt and when the car is cold it works perfect until it gets hot then it dies out. When i hit the key the pump makes a loud sucked out screaching sound. It acts like as if every time I start it up after it stalled, she gets pumped up with fuel then it burns it and stalls out again. Also when I take of it will surge and stumble and sometimes buck. But when i hold the throttle none of this happens. Just when its idleing and when i go to take off. It isn't throwing any codes but my scanner tells me that the EVAP system isn't responding.

What harm could a non-responding EVAP system do? and how could I fix it?