I have a 1995 Mercury Cougar with 83,000. miles on it and they can't get the check engine light to stay out?

Thank you, I got the check engine light fixed & took it to get smoked & it failed the test. They said the emissions were all perfect but now California law requires some kind of snap smoke test. I guess they snap the throttle & if a tiny bit of smoke comes out, it fails the smog. Is something plugged up now or do you have any idea what would cause that if the emissions are perfect ? What would cause the smoke. It doesn't smoke any other time just when they do that test. The guys that fixed my emissions said they didn't see any smoke. I don't want to have to pay $125.00 for a diagnostic test if it could be just a plugged hose. Any help you can give is greatly appreciated.