Dieseling on shut down

[1986 930]

As far as I know I have a stock CIS System, Euro Fuel System. After running the car for a bit, the car diesels when I shut it down. ( turn off the key and it continues to run for a bit, very roughly) When I run the car around town and start and shut it down it only does it on the first shut down. Otherwise it runs great. So I have two questions:

1) What is causing this?

2) Is this bad and I am destroying my car little by little?

Thanks,
Mark

[jwasbury]

There is something called the ignition delay relay. This maintains power to the ignition system for a few seconds after you shut off the key. The fuel pumps power is cut, but the car runs on for a few moments to burn off the last bit of fuel in the injection system. From your description, its possible that what you are calling dieseling is actually the normal ignition delay.

[Helmsy]

It could be the injector(s) not holding pressure enough after shutdown and leaking.

Check your fuel pressures. Raising the system pressure can stop it occuring however if it isn't the usual delay-relay shutdown then more than likely a leaky injector.

[1986 930]

Thanks for the replies.

If the injector(s) were leaking, wouldn't I smell fuel?

[RarlyL8]

No you won't smell fuel because they are leaking into the cylider. The delay relay is in place to allow any unburned fuel to do so, meaning that if the fuel system is working perfectly there will be no run-on.
Unfortunately the only way to find a leaker is to pull the injectors and test them. If this is not something you want to do the relay can be jumpered/disabled. There has been a few threads on this phenomenon lately and one has the ID# for the relay and which ports to jump.

[1986 930]

Is it something to worry about beyond just annoying?