1984 Mercedes 300D Turbodiesel Won't Stop

I have a 1984 Mercedes 300D that runs like a Panzer. It rumbles, but she’s still a gem. Trouble is, she won’t turn off when I pull the key out. I admire her dedication, but it’s really annoying having to open the hood and hit the stop lever every time I park. The electrical system turns off, but the diesel keeps puttering away. The key clicks on the exit and I just can’t figure her out. Any ideas?

The solenoid that operates the fuel shut-off might be bad, or there’s something physically preventing the shut-off from operating.

Since you can operate it manually I’m guessing it’s the solenoid.

I believe it can just be that the accelerator linkage is out of adjustment. I seem to recall there is some sort of procedure involving special Mercedes-Benz protractors. Otherwise my vote would also be for the solenoid.

The shut off is vacuum operated.
Like most (all?) MB diesels, the 300D is shut off by the fuel shutoff valve, which is a vacuum actuated device located near the injector pump. The key “ignition switch” is the control valve to port the vacuum to the shutoff valve. The most common “failure” mode is a vacuum leak which does not allow actuation of the shutoff valve. You can troubleshoot the shutoff valve with a MityVac (hand vacuum pump from Pep Boys,NAPA etc.) to see if it will work with vacuum. Then work upstream or downstream from there.

Are you having other vacuum related problems, like doors not all locking? The MityVac is an essential troubleshooting tool for these cars.

Likely culprits from my experience are a vacuum line accidentally knocked off in the engine compartment. Check by the oil filter on the drivers side firewall. Especially true if you’ve just changed the oil filter or worked on the throttle linkage.
Next culprit is a leaky shut off valve which is located on the end of the injection pump. But really any vacuum leak stresses the shut off system.

So do a cursory search for unhooked lines first, then use the Mity-Vac for further diagnoses.


A diesel will run as long as fuel runs into the cylinders. The fuel cutoff is failing. Benzman basically is saying find the fuel and stop it.

The locks don’t work either. That’s a great point… I’ll try the MityVac. Thanks for the lead!