1981 300d wont start

The engine stop device is a vacuum powered bellows in the aft part of the injection pump. This little bellows wears out and gets oil logged. Suggest you look at the aft end of the injection pump for a brown?? plastic vacuum line attached to a silver dollar size & shape device and detach the vacuum line therefrom. If there's any lube oil in the vacuum line the "shut down device" (bellows) is holed and must be replaced. You can temporarily plug that vacuum line and use the car. But one must then stop the engine manually by pressing the STOP lever on the engine's throttle linkage.

If you want to replace the shut down device yourself be forewarned that its not hard to do but IS tricky in that one needs to hook the end of the bellows onto the end of the fuel rack (which it pulls to OFF when one turns the key to OFF.)

Hope this helps you get underway.

Here's how the stop thingy works: when vacuum is applied to it it pulls a rubber diaphragm inside which causes a lever to pull a rod inside the injection pump which cuts the flow of fuel.

When the thingy fails it gets a hole in the rubber and all the suck in the world won't make that lever move. That is, when it fails you con't turn the engine off.

It's really really really unlikely it's failed stuck in the off position which would require vacuum to be asserted, well, pretty much forever.

I take it the car has sat awhile? Not uncommon and you have two choices:

1) crank and crank and crank forever till you fluch the gum and crud out or pull it to 35 mph and shove it into drive and it'll start. The latter is what I've seen usually dont and I've never seen one that failed to start this way.

I agree with that analysis. If it's a vacuum/bellows problem, it should fail in the constantly on mode, not off, since vacuum is needed to move it to the off position.