If you look at the rear of the injection pump I think you'll find a white plastic valve body, with a couple of vacuum lines (plastic pipes) connected to it. I think this is the fuel shut off. All the vacuum lines are fed from what amounts to a vacuum manifold by the brake booster, there's a thick black pipe that comes through the first firewall to this area from the vacuum pump on the front of the engine.

Because the vacuum lines all arrive at this manifold, any one of them leaking, almost anywhere under the hood, could cause the shut off not to work. I think what you'll have to do is start examining the rubber Y and T pieces that join the pipes together until you can find the culprit. I don't think there's a much more scientific approach than this.

I'm assuming that you have a 300SD, later than '81?

Kevin

__________________
'85 300SD