OK, here is my cautionary tale... it's been uncharacteristically cold (and dry) here in Oregon lately. I went out yesterday to drive to work, and thought I would warm up the car a bit first (the windshield was covered with ice). It was about 25 degrees F out. I did a long glow cycle (30 secs), and fiddled with the idle knob while that was going on. I stepped on the fuel pedal first, to take the tension off the cable, and then backed the knob out fully to set it for maximum idle. When the glow finished I turned the key to start, and the engine started fine. The idle seemed a little high, but not roaring. I got out of the car and went in the house to get a package that I had forgotten, and when I came back out a minute later the engine was running at a pretty high speed - not screaming, but 3000 rpm at least.
Fearing the worst, I dropped everything, ran to the car, turned the key off... no effect! I popped the hood, and jammed down on the stop lever... this brought the speed down to a slow idle, but WOULDN'T STOP THE ENGINE! I stood there holding the stop lever down feeling kind of stupid... what next, cut a fuel line? With what? Call a neighbor with the cell phone in my pocket and tell someone to bring me a screwdriver to loosen a fuel line clamp? It finally hit me that the idle knob was keeping me from being able to push the stop lever far enough to kill it. I zipped into the car for a moment, turned the idle knob in, and then the stop lever worked.
There are probably a couple of things going on here: evidently the linkage was sticking due to the cold weather (my 240D has the old-school bell crank and multiple threaded sticks that go over the valve cover). My IP vacuum shut off is usually reliable, but not in this instance; usually when I shut off the key I can count to 1, 2, stop before it kills the engine. I have had the experience where if I run the car for a minute or two, and then decide to shut it off that the vacuum shut off struggles to accomplish its task - it's like the vacuum system has not built up to the full vacuum yet.
I threw the MityVac into the back seat today, and I will test the shut off manually when I get home today. I drove the Ford Ranger yesterday after that scare
Kurt