1980 300SD Wouldn't shut off--even after pressing the STOP lever under the hood.

[HoundFables]

Hey guys,

I've been working on my 1980 300SD for the last several weeks. Things have been going well, and the car is getting closer to being in "safe" driving condition. I went on my first long drive last night after completing the shocks and rear sway bar end links. It was harrowing, but I got the car up to 75 on the highway for a bit--spent about half an hour or fourty minutes driving around, which may be as long and as far as the car has been driven in the last 10+ years.

Anyways, I'd been on several smaller drives prior to this, and the car has been turning off with the key as expected every time. I'm pretty sure I'd even tested the STOP button at some point to see that it worked, but I'm not 100%. After I'd returned home from my journey, the key failed to kill the engine. Even after raising the hood and depressing the STOP button, the engine lowered its RPM and began to chug, but still would not quit. At this point, I was panicking, lol. I began Googling and someone on a forum suggested removing the connecting rod that went down to the injection pump throttle lever and pulling on that manually, which also did not work.

I tried blocking the air intake at the air cleaner, but it was too leaky and that did not work either. I eventually decided to remove the fuel hose at the pre-filter, and that eventually did the trick after MANY seconds, THANKFULLY.

The car has some vacuum issues, definitely with the door locks and probably the HVAC, so it's not unreasonable that the key would be spotty--but this does not explain why the emergency stop lever would not kill the engine (or does it?).

I decided to test out the vacuum shut off valve today, and it held vacuum with my hand pump. I managed to get the fuel lines purged and the engine running again today, and the switch was able to kill the engine by manually actuating it with a vacuum pump. I did not test the STOP button today, but will eventually for sure.

So this leaves me wondering--what could have caused my random shut off issues last night? I was almost beginning to worry that it was beginning to diesel on its' own engine oil or something. I've spent hours Googling today, and even though I found a small collection of threads where people had this issue, there never seemed to be a resolution.

Thanks!

[daantjie]

https://www.youtube.com/watch?v=Yx2f22l5bZ0&t=78s&pp=ygUkTWVyY2VkZXNzb3VyY2UgZGllc2VsIGVtZXJnZW5jeSBzdG9w

This might help.

[HoundFables]

Regarding the MercedesSource video, I did find that one the other night. I have checked the little spring loaded plunger, and the spring appears to be compressing alright. My concerns are that my problem may be what he's talking about when he says that the problem may be "inside the pump", which would be unfortunate.

I did have the car going today in my driveway and I was able to shut the engine off with the key and with the stop lever several times. The key did feel a little slower than usual. Could this intermittent stopping issue be the fault of the injection pump? I'm not really sure what's left to check, tbh!