Engine shut off failed
 

On my 1985 300D the engine shut off went from working perfectly, to a slight hesitation to total failure in just 2 or 3 stop start cycles. As best as i can tell, the vacuum (measured on H.Fright vacuum squeezer), is about 7 or 8 inches of mercury. On my 240D, it is about 15. All the other vacuum related functions, are about the same as they have been since before the t.Rump regime. Shifts OK, brakes are fine, door locks semi work as always. Not sure how to diagnose. All the vacuum lines seem tight. Replacing the vacuum pump seems to be a nightmare. Is there anything else to check before contemplating the vacuum pump? Thanks!

Your wallet might think so, but the job itself is pretty easy.

I would try applying vacuum directly to the shut-off valve.

Vacuum pump failure is a critical engine fault, right? It's failure typically is gernading ball bearings onto the crank case.

I think you have a vacuum leak. Semi working door locks to me means that you have a leak so where.

Disconnect and block the vacuum line feeding the door locks. Does it work?

If you measured the vacuum at the line at the shut off valve on the fuel injection pump it could be the vacuum switch on the steering Colum lock or something internal to it or simply the rubber tube has cracked that connects the plastic tube to vacuum switch.

Get a vacuum reading from the main line but since your power brakes are working OK my guess is that it is OK. But, better to check it.

Using a Mityvac test the shut off valve on the injection pump. Apply vacuum, using the Mityvac with the engine running. If the engine does not shut off you have found your problem - a faulty shut off valve. You should see 15-19 inches of vacuum being provided by your vacuum pump.

Where is the shut off valve?
 

There is a gizmo just in front of the oil cooler housing, and above the back end of the injector pump. It has a vacuum hose on top and another going into the side of it. I am assuming that is the shut off valve. I tried vacuuming each of those vacuum ports, and the engine kept running (with the key turned off).

I assume you mean the oil filter housing. It sounds like you found the shut off valve. If applying vacuum to both sides of the valve and the engine did not shut off you have found the problem.

Actually, it sounds like the part that was "found" was the transmission vacuum control valve. The shut off valve would have a single, brown vacuum line attached.

The shut off actuator is located on the extreme rear of the injection pump. Cylindrical in shape, with a hose nipple pointing straight up.

https://www.bing.com/images/search?view=detailV2&ccid=q7k3Ns0i&id=9F442E65CFEF5461C834C98ACA706E2906E78066&thid=OIP.q7k3Ns0ibC2_ MjUtSyj8wAHaGP&mediaurl=http%3a%2f%2fwww.peachparts.com%2fshopforum%2fattachments%2fdiesel-discussion%2f79848d1271647942-79-300sd-fuel-cut-off-solenoid-picture767b.jpg&exph=600&expw=712&q=om617+shut+off+valve&simid=608050377412315525&selectedIndex=14&a jaxhist=0

Why does everyone call the vacuum piston/diaphragm/actuator on the IP for shutoff a valve? It's incorrect and quite confusing. The shutoff valve is on the ignition barrel unit.

Pelican Parts refers to it as diesel shut off valve”.

You know you have low vacuum available. Pump up your mitivac to the vacuum you have measured while hooked up to the shut off. It probably will not shutoff.

Now increase the vacuum until it does. That is the value you have to provide or exceed. With your Mitivac for it to work. You should be able to find out why you lack the vacuum to that point if you do.

Four pounds of vaccum is really not very much. If you orally suck on a vacuum gauge you can easily exceed that amount. Actually it sounds like you have been a little confused. The vacuum line to the part you want to test goes to the injection pump. If it will not shut off or hold a vacuum if the part is bad.

Start the engine, plug your Mityvac on the nipple of the shutoff valve, and pump up the volume. If the engine goes off, the shutoff valve is bad.

Easy first test.

When I have confronted the situation that you describe, sun tortise, on my 617.952 engines...
1. Ready MityVac.
2. Clear mind of assumptions about anything.
3. ISOLATE and TEST *every* single vacuum sub-system that is connected to the engine shutoff circuit. What DO you mean? Within the engine compartment...
A. Disconnect the green line (climate control sub-system), connect the MityVac to it, pull vacuum, watch gauge for at least 30 seconds. Vacuum hold or drop?
B. Do the above for the solid yellow line (locking sub-system), yellow line with gray stripe (reservoir sub-system), blue line (seat back locks sub-system - coupe only).
4. As others have stated, pull the brown-with-blue-stripe line from the shutoff mechanism situated on the rear of the injection pump. Use the MityVac to draw vacuum on the mechanism. Vacuum hold or drop?

Now then, if the above still has not identified vacuum loss, there are certainly more things to do on a 1985 300D (that I have indeed done) to locate vacuum leaks. From this point forward, it just gets a bit more complicated.

If you do the above, what are the results?

Benz Mantra, I like! :D

If you get a separate length of vacuum line and attach it to the shut off valve/shutoff servo/gizmo you can simply suck on that line to see if it will shut off the Engine. If that does not shut the engine off then you have a the shut off valve/shutoff servo/gizmo problem.

Although as the other member said you should be getting more vacuum for the line going to the vacuum switch on the steering colum lock.

If you replace the shut off valve/shutoff servo/gizmo read up on doing that because there is 2 possible places to stick the bent tab into If it goes into the wrong place when you start the Engine it will run away/go to max fuel.
As a precaution most people like to be ready to block off the air inlet incase the engine runs away.

NOT fuel shut-off valve.
 

I found the blue/brown vacuum line and the fuel shutoff valve, which is below the other gizmo that i thought it was. In front of the oil FILTER, not cooler housing. When i applied vacuum, it smartly shut off at about 5 or 6 on the vacuum pump gauge. There is only about 3 coming off that blue brown tube, when i turn the key off. So i will follow the other excellent suggestions here to track down where that vacuum is getting lost, and report back later.

1. Congratulations on achieving this first step. You're on your way.
2. With all due respect to previous posters to this thread, the above results are PRECISELY why it is folly (or worse) to get into the nitty gritty of part such-and-such-failing-and-how-to-replace-it-and-cost-and-blah-blah-blah UNTIL the fault(s) in a system has/have been definitely isolated.

Narrowed down to yellow line with grey stripe
 

I rechecked the vacuum and found it to be nearly 15" with only the vacuum pump and brake booster in the circuit. There is a 3 way connection that goes to a brown tube, a green with yellow stripe, and a yellow thing that looks like a check valve or filter of some sort. Off the back end of this comes a yellow line with grey or black stripe. With the key off but engine running, i can make it stop by pulling the yellow line off the yellow filter looking thing, and blocking it with a finger. It takes about 3 or 4 seconds for it to shut down. So hopefully i am on the way to finding the problem, downstream of that yellow /gray tube, which is for the locking circuit (?). Weird thing is that the lock have always been iffy, and are about the same. All these tubes go thru a rubber thing in the firewall, so i am hoping to find it on the other side and see where the break is. Or maybe just block it off, who needs door locks?!


Thanks for all the advice; you fellow Mercedes victims always come thru for me!

15" of vacuum?! Why that's a sprightly vacuum pump on your engine.

"yellow thing" = check valve.
Off of said check valve:
A. Yellow with gray stripe ends at the vacuum reservoir located on the ceiling of the trunk. By having it connected to this check valve, a properly functioning vacuum circuit will retain vacuum when the engine is off.
B. Solid yellow line - the locking sub-system. First stop of this line is in the driver door. From there (for the sedan), it makes its way to the rear left door, passenger door, rear right door, fuel flap, and ends at trunk lock.

Be careful with those yellow lines: One has the stripe and one doesn't. Know with certainty which one you are dealing with at any given moment. I suspect that what you wrote above (if I am reading it correctly) is erroneous on this point. While the vacuum reservoir has been known to be the source of leaks, it is FAR more common for the locking sub-system to have leakage.

In theory, in summary, and based upon what you wrote above: If you take the solid yellow line out of the vacuum circuit at the firewall (read: block the port to which it slides into [so no vacuum leaks]) and the engine shutoffs off properly, the conclusion would be a leak(s) in the locking sub-system (since you have not indicated that the green circuit [climate control sub-system] is faulty). On the other hand, what I just wrote is from my mind, an entity that produces errors from time to time. I encourage you to continue to work with facts as measured.

The door locks will still work without vacuum, just not automatically. And the driver door lock is not automatic in the first place.