Running on - cause?

[underwhelmist]

Alfa Romeo 156 2.0 JTS (petrol).

I took the scenic route to and from the supermarket the other day just to get the car up to temperature, it's not been used much the last few weeks. On returning home I switched the ignition off but the engine kept on running, very slowly and lumpily, for about 20 seconds. Just to see if it was a one off, I started the engine again and it did the same thing again when I turned it off.

What should I be investigating to find the cause of this? Could it just have been a freaky one-off?

[tapkaJohnD]

There can be a number of causes of running on, but your jistory points to using the car too little and not for long enough. A short trip after a pause in use will not let the engine warm up completely, so the ECU may have still been enriching the mixture when you turned off. The rich mixture will have deposted carbon in the chambers that will be glowing and igniting any mixture than can get through, even with the ignition turned off. You need an "Italian Tune Up", a blast at high engine speed for long enough to thoroughly warm the engine, burn off any carbon and clean the chambers!

For a more detailed discussion of running on, see: https://www.howacarworks.com/ignition-system/curin...
Good luck!
John

[Cliftonite]

This used to be a fairly regular occurrence up to what, the 1970s.

So I am told!

De-coke, anyone?

[underwhelmist]

Cliftonite, yes I had heard of it happening on carb engines but not fuel injected. And John, thanks for the advice, that’s all the excuse I need!

[Penelope Stopit]

There could be a problem with the ignition supply not cleanly switching off, something keeping a voltage on the supply