Vectar Pre-Ignition Problem

Over last couple of weeks I have noticed the occasional slight run on after
the ignition has been turned off, have not made any changes & have also
changed petrol stations to see if that made any difference, but didn't!
Has anybody else had any experience of this & how to cure?

1996 1.8GLS Vectra Petrol

 

What sort of driving do you do? If driven locally, there may be a carbon
build up in the cylinders, if you do drive it locally only, give it a good
thrash down a fast road for 20 mins which will help burn off any deposits.

The fuel injection will not inject any fuel in when the engine managment
system knows you have turned off the ignition.