Evaporative emission system leak detected (gross leak/no flow) any ideas what is causing the problem

Could be as simple as forgetting to replace the cap after filling gas tank. Barring that, a defective cap will cause same. If neither of those are the culprit, you'll need to look over the EVAP system plumbing for cracked hoses, loose connections, etc. This is the system that routes fuel vapors from the tank to the intake manifold to be burned rather than just vent to atmosphere like the old cars did. A good repair shop will have equipment called a smoke generator. They will hook it up to the system and it presurizes the plumbing and fills it full of smoke. Finding where smoke is escaping will pinpoint the offending leaks. Good luck!

Thanks for the info, off to the mechanic I go.