This is a problem with the canister purge valve. Here is a picture of the system
The purge valve should have power on the red wire then when the PCM grounds the GY/Y wire this activates the solenoid so vacuum can then be applied to the vapor canister.You will alos need to check and make sure you are getting vacuum to the canister purge valve.
You can take and hook a hand vacuum pump to the solenoid the go to the canister and hook a vacuum gage to the line there. Then turn key on and make sure you have power to the canister purge solenoid on the red wire(if not this is your problem and you will have to trace this wire) ,if so then ground the GY/Y wire and pump vacuum to the solenoid and you should read vacuum at the canister hose.
If this is ok then you problem is most likely in the GY/Y wire between the solenoid and the PCM. You can then go to the PCM and on pin 67 and ground the wire and repeat this test. If it works then your problem is the PCM itself,however this would be rare.
Here is a pic of the connector at the PCM