94 S320 Dieseling

[m4411b]

I have a 94 S-320 6 cylinder thats been running great until today. Each time I stop at a light or stopped it diesels between 5 and 10 on the odometer. I checked for vaccum leaks but found none. Checked the air filter and it was ok. Did notice that a switch located on the pipe connected to the air breather that when I disconnected it with the car running the engin did not die as it had been doing in the past when disconnected. Can anyone tell me whats wrong or what has gone bad?