Ive got a 2004 CTS that over the past 5 years has had same issues on and off. when turning the…

Ive got a 2004 CTS that over the past 5 years has had same issues on and off. when turning the ignition to start the car it will not start but even with hands off the ignition key to turn, the engine continues for 15 plus seconds to turn before stopping. Car will be taken for service with no evidence of problem. currently has not started for 2 weeks. normally the Purge valve is the problem. I simply change it and problem solved. not the case this time. please help

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