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.

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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.