1994 300d won't shut off. I checked all elements which all appear good. (I have a hand vacuum pump that works properly.) All other parts of the vacuum system, like door locks, work OK. Here is what I have done:

1. verified that vacuum pump on engine is working OK. Good vacuum is present, when engine running, at vacuum line offtakes from main line to brake booster to vacuum system plastic tubes.

2. shutoff gizmo at rear of injection pump. With engine running or not running I cannot draw a vacuum at the shutoff diaphragm with hand pump. But: with engine running I removed the existing line and rand a bypass line from the shutodd gizmo to one of the oftakes at the main line to the booster, mentioned above, where I knew vacuum was present. It shut off the engine immediately. That led me to suspect thaty the shutoff gizmo was working OK.

3. valve at ignition switch with brown and brown/blue plastic lines. I replaced this in light of apparent successful test of gizmo, above. New valve failed to solve problem. The brown/ brown/blue tubes were verified to be correct installed. I even tried reversing them as experiment. No success either way. I removed valve, still connected to tubes, from ignition and started engine. The valve is operated by a push rod in it which is driven by a cam in the ignition switch. With engine running I pushed the rod with my finger, but engine did not turn off. I concluded problem was not this valve and that I had wasted my money on this new valve. Therefore, I had to conclude the problem was also not a cam failure in the ignition switch.

4. vacuum tubes/lines and connectors: With hand vacuum pump I verified that all plastic tubes and rubber connectors were intact and holding vacuum. I concluded I did not have a leak in this part of the system.

5. Conclusion: I am driven to conclude that, despite my success in activating the shutoff diaphragm by putting a tube directly between the shutoff diaphragm and the main vacuum line, the problem must nevertheless be the shutoff diaphragm at the injection pump. This is for two reasons: (a) everything else works OK, and (b) I could not get a vacuum using the hand pump with engint running and not running even though I could stop the engine as described above. What am I missing? Where else should I look?

Advice and insights are solicited. Thanks.

__________________
Douglas
1959 M-B 220S cabriolet

correction: 1989 300D (123 series)

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Douglas
1959 M-B 220S cabriolet

The diaphram may be bad. You say your hand pump wont activate the shut off valve. Could your hand pump have a leak. If the valve shut off when you ran a bypass hose then I dought it is the shutoff valve.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

For what it is worth, the 617 engine manual says that the shut-off valve should be replaced if it fails to hold vacuum. Yours is a 1989 123?

Wasn't '85 the last year they made w123's?

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

My car is a 1984 300D (123 series).

The hand vacuum pump is working OK. It holds a vacuum nicely.

The hand vacuum pump would not draw/hold any vacuum when connected to the shutoff diaphragm both when the engine was running and when it was not running. Same behavior both ways.

But, when I connected a bypass hose from the shutoff diaphragm to the offtake on the main vacuum line between the engine vacuum pump and the brake booster, where I knew there was vaccum when the engine was running, the engine shut off immediately.

That is my conundrum. Why does the bypass trick work and the vacuum pump not? What else am I missing in this analysis?

Thanks.

__________________
Douglas
1959 M-B 220S cabriolet

The shut-off actuator no doubt is leaking. The vacuum from the booster line is probably adequate to overcome the leak. But the vacuum level from the normal source is lower, and not adequate to actuate the valve.
I would sugget that you confirm the vacuum level in the line to the shut-off actuator, just to make sure there is no problem there.

If you decide to repalce the shut-off actuator, make sure you know what you are doing. Improper installation can result in a runaway engine when you start it up the first time. (Which will be the last time for that engine.)

I'm in agreement with Tangofox.

When you apply vacuum directly from the pump, you apparently have enough flow to draw a vacuum at the shutoff diaphragm, despite the fact that the diaphragm is leaking.

However, when you go through the normal channels, via the ignition switch, the flow is probably reduced considerably due to restrictors and/or smaller tubing and a valve at the switch, so, when vacuum is applied to the shutoff diaphragm, it simply bleeds off and the engine cannot shut down.

If you cannot use a Mityvac to shut the engine down, it's time to replace the shutoff diaphragm on the IP.

And, make sure you read up on the proper procedure to change this diaphragm and, additionally, assume that you are going to do it incorrectly and the engine is going to run away. Make sure you are properly prepared.
Read up on this in the archives.

Thanks for the replies. All reinforced the conclusion I had reached preliminarily--that the shutoff gizmo was the culprit despite my being able to shut it off with the bypass line.

I am well aware of the possible problems installing the actuator; that is, the runaway engine. I have read the other threads' discussion this issue, and I knew about it from before. I once did the job on a 240D and had been warned then too. I prepared myself to react immediately should that happen.

On the 240D the failure was different: the diaphragm leaked oil into the lines. Rather it tried to but I had a clear fuel filter in the line to protect against oil intrusion. Many years back there was an article by the late Frank King in the Star magazine about the benfits of installing these filters in the vacuum lines. I did so and, coincidentally, two weeks later engine oil showed up in the one serving the shutoff gizmo. It had failed, leaking oil. Then a month later the main vacuum pump itself failed and leaked oil. The second filter I had installed in the lines intercepted that. And because the filter is clear plastic, I could see it immediately. I was very fortunate. Oil did not get into the vacuum line system which would have been extremely destructive. I cannot recommend those filters more highly.

My problem this time is that (a) there is no oil leak and (b) I could get the actuator to work doing the bypass. That is was made me pause, take stock, and ask for help analyzing. Many thanks.

__________________
Douglas
1959 M-B 220S cabriolet

I agree. My Mityvac shuts the engine down with one pump of the handle, and it holds the vacuum applied for > 1 minute - no leak here.

__________________
Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.