Engine "run on" on Audi Coupe

[Nick45]

I feel I've known the answer to this in a previous life or something but can't bring it to mind . . .
what generally causes the engine to not shut down immediately when you turn off the key? All suggestions gratefully received as I try in vain to keep the car sweet long enough to sell the bloody thing!
Gracias, Nick.

[agent006]

Which engine type?

[Nick45]

2.0 16v - series 3 '91 coupe.

[workshy fopp]

The timing being too far advanced will make it run on.