Oldsmobile/Buick Repair: code po440, olds alero, purge valve

Question
2001 olds alero 2.4 / evap system malfuntion/ Have replaced gas cap/smoke tested 3 times/ replaced the canister selenoid valve/ The service eng. soon light comes back on when it runs the monitors. We are trying to get it threw emissions... I also work at a full service automotive shop we are stumped on this one.
Thanks for any suggestions.

Answer
Hi
Watch the data while smoke testing. Make sure the FTP (Fuel Tank Pressure sensor) mirrors the "H2O or the Vacuum on your machine. If it does not, does it list the fuel level accuratly (closely accurate) compared to gauge? If not suspect the FTP sensor.
Make sure the system holds pressure.
Do you observe any smoke?
Top of tank?
Check the purge line in engine compartment.... make sure it did not rub belt.
Is purge on all time?? It should NOT be purging at all times. If so it will set 440 AND no smoke will be visible.
(take line off purge valve underhood, and make sure there is not vacuum going to tank at all times.)
This is where I'd lead y'all to start first.
IF all this checks out, you VERY well may have a faulty PCM, I have seen them do this EXACT same thing. ELIMINAT all other items first, though.
I Hope this helps
Pawl