Car Shudders When Ignition is Turned off

A few possiblity here
what u describe sounds like dieseling
--- chk timing. Maybe too advance
--- high mielage. Do this try super high octance fuel if it fix the problem then it means u have too much carbon build up. Well. Easy. Do a decarbonization on ur engine.
Let me know how it come out and keep me update.
Good luck man

I am hoping that the problem won't be timing because my car was only recently serviced. I have noticed that this problem only occurs if I do not turn off the cars air conditioning which seems to make the car idle faster if that is of any further help. I know I could end the problem by ensuring the a/c is off everytime but it shouldn't be happening at all should it?

How do I decarbonize my engine? Do I just purchase some kind of carbie cleaner and throw that thru the fuel system? Not down the carburator directly, thats bad right? And with regard to the high octane fuel, should I refill with that when my tank is close empty or anytime at all? Am I best of continuing to use a high octane fuel in future? Sorry to be coming hard and fast with seemingly supid questions but to be frank the only knowledge I have about cars is that my barbie doll had a pink car with 4 wheels. Again many thanks for your help!

I had a service almost a month back but I've only recently been using the a/c which is why I think I have only just noticed the problem. I raced out this morning and got some Valvoline Fuel Injector & Carby Cleaner before refueling with a high octane fuel from Shell. I've done about 100kms (60miles) since putting it in the tank and I can't say there has been a whole lot of difference in its performance. The shuddering isn't taking quite so long (from about 3 seconds down to perhaps one). But again it does only occur when I have the a/c on. Do you think I should put the car back in for servicing? Yet again I REALLY want to say thanks for the help you're giving me. Anyhow I hope I am providing helpful information to my query. All the best

I don't know where u at. But here the GM old engine decarbonizer works pretty good. But we use machine in the shop.U must fill the tank with high octance fuel to verify the problem. This should be done after u verify ur timng. Sounds like u might idling a little high.I have time I'll chk if it can be adjust or not.I'm not at the shop now. Good luck man

This is not a us model. Can't look up in my alldata. But i'm a L1 tech no sweat. Can't be that hard. Let me know what size engine and I'll find a similar us model to chk out the sytem. Later. Go go go. Heeeee hawwww.