fizzling after ignition turn off
 

I have had a sporadic vacuum leak for the past several months. Some days the engine shuts off right when I turn the key off; some days it takes a few seconds for the engine and some days it takes up to 5 seconds. I have noticed that when the engine does not shut off right away, there is a fizzling sound coming from the key ignition area. It seems pretty obvious that there must be a vacuum leak somewhere close the key. Also the transmission shifts perfectly when the leak disappears and not nearly as well when the leak is present.

You should always state the model and year here when asking questions.
That being said most of the MB diesels I’ve read about here have a vacuum switch as part of the ignition switch assembly
I’m not sure of the easiest way of looking at it. Probably either removing the interment cluster or looking from underneath. Any way I’d look for a cracked vacuum line before replacing the unit

You can get to they key unit by simply taking off the plasic underdash panel. Start by just replacing the rubber connecters for the vac lines on the switch and see if that helps. Easy and cheap to do.

Just for the heck of it, is your ACC system on different settings with the problems occur?

Try this first:

experiment with shutting off the car with different buttons pressed on the climate control (the main 5 only). See which function allows the car to shut off quickly and which one is making it leak.

Check for oil in those ignition vacuum lines....

Sorry, I have an 83 300sd. ACC doesn't affect the shut-off.

Fizzling sound might indicate liquid, and if your IP shutoff valve is allowing oil to get sucked into the system, it goes right for the ignition switch.
Ask me how I know.

Did you get any oil dripping inside the car? Also, did the oil have any effect on your glow plug relay (at the ignition) connection?

Jimmy Joe is right. Check the vacuum line (brown) from the shut-down valve (bolted to the rear-most of the injection pump) to the steering lock/ignition switch. If it has oil in it, then you can rest assured the diaphragm in the shut-down valve is starting to leak and allowing oil to enter the vac line. This symptom will get worse as time goes on until the engine will not stop running ever unless you open the hood and press the shut-down lever! The solution is simply changing out the shut-down valve. It is a little tight getting to it between the injection pump body and the oil filter cannister but it can be done with a little patience and by tilting the valve upward after removing the 8mm bolts.