Car will not turn off

Hey everyone

I have a 1985 Nimbus (old car). Recently it has been idiling way to high. This is an autoamtic car, when in park it is idiling at 2500 RPM apposed to a normal 1000 RPM. It is also idiling to high in drive. When I turn of the car, the engine pistons still rotate very slowly. The car seems to knock very slowly and will not stop. The only way I can stop this, is by putting the car in drive, getting the revs as low as possbile then stall the engine, this only works half of the time.
I have also noticed that there is a vaccuum in the fuel tank. When I uncap the fuel tank, air rushes in. I am using Unleaded fuel, and adding lead substitute.

Any help is much ablidged.

Thank you

Adrian