I am working on my 1985 300D to find a vacuum leak. I have isolated it to the central door locking system. For now I put a golf tee in the hose which comes off the first vacuum check valve which splits off for the reservoir and the central locking system. With the tee in place where the plain yellow line fit in, my engine now shuts off right away. Before, I could not get it to shut off even with the emergency shut off lever under the hood. (This was embarrassing at times. One day I thought, the heck with it, I'll just lock the car and let it run. It was then that I discovered that when I locked the driver door, the engine shut off. How convenient. Quirky but convenient.)

I had the panel off the driver side door and all of the hoses looked fine. I also inspected the trunk actuator and that looked ok too. I found the attached diagram and it states that the 3-way and 4-way fittings under the front carpets could be the culprits. This will probably be my next move as they seem to be easier to access than the line to the fuel door.

Does it seem like I am on the right track?

Attached Thumbnails

 

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85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

I will study this later. I need to log off now and prepare work for a large job tomorrow. I have the same issue, poor shutoff, brake assist not good, and my door locks don't work worth a damn, the latter leading me to think that's where the problem is. I like the info you gained with the golf tee. In a few days I can get back into my vacuum leak hunting adventure - '84 300D.

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1986 300SDL, 362K
1984 300D, 138K

I think that the only way for this to happen is when a "locking circuit" is somehow connected to the engine shutoff.

Which is very weird and I would focus on fixing that first.

EDIT: This is wrong

Get a ‘Mity Vac’ tool it is indispensable for finding the problem. Available at your local Hazard Fraught Tools location. But get the genuine Mity Vac not the generic one they also sell.

This will let you search for the leak. Start where you unplugged the line under the hood.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Yup, trying to track down a leak without a Mity-Vac is like shooting in the dark. Yeah, you might get lucky and hit something but not necessarily what you are aiming for.

Many of the vacuum systems are interconnected and a leak in an unrelated part of the system can cause low vacuum throughout the system. Unless the vacuum reservoir reaches a minimum amount the you will have intermittent systems glitches.

The first place to start is supply. Isolate the main vacuum feed, including the brake servo assist and see exactly how much vacuum the pump is achieving.

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Uberwasser vacuum system troubleshooting
http://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

The same guy used to post over on benzworld.org. Can't remember that far back bu my impression is that the benzworld post was more detailed.

When I did mine one of the issues was the main 3 connector valve on the drivers door.

When I took up the plastic molding at the bottom of the front doors I found that all of the rubber connectors were like new. Unless water pooled frequently under the carpet there is not much to deteriorate them.

With the right left lower door moldings off and the carpet up you can troubleshoot the color codes lines from there to find out which component is causing the issue.

I found it was one of the rear door actuators. There is 2 places they can leak. One is the exterior boot which there is a repair for and the other is the inside rubber diaphragm which was the issue in my case..

A member of one of the forums said they fixed the crack in the actuator diaphragm with diving suit cement. I bought a new actuator.

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84 300D, 82 Volvo 244Gl Diesel

If you're doing this without a mity vac, you are taking a journey to Mordor on all fours.
Buy one, and be done with this in an afternoon.

It's theoretically possible for the plastic vacuum lines to crack, or the rubber connectors to lose their grip, but 19 times out of 20, it's an actuator diaphragm –again, easy to isolate with a Mity Vac.

Oh, and I've used several generic vacuum pumps, all of which had compromised internal seals and would leak down over a period of hours. This makes trouble shooting a pinhole leak impossible. Just pony up and buy the name brand as I did. Invaluable bit of kit.

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1981 240D 4sp manual. Ivory White.

I should have mentioned that I do have a MityVac. I put it on the line to the reservoir and was able to create a vacuum that held perfectly, so I eliminated the grommet on the reservoir as the source of the leak. When I put it on the yellow supply line going to the driver's door, I could not create a vacuum. So, I believe it is downstream from there.

It occurred to me today that since the problem somewhat corrected itself when I turned the key to lock the door (opening the green or lock side of the vacuum lines), this points to the problem being on the red or unlock side of the door lock system. This tells me that my next step should not be to get to the fuel door, which only has a green (lock) vacuum line.

I will not be able to get back to this until probably Thursday. I am hoping to find a cracked connector under one of the front carpets.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

My understanding is that the entire vacuum system is interconnected. The engine shutoff mechanism uses vacuum to my knowledge.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

This guide is superb. Thank you!

I noticed that the diagram I attached describes the lock side of the lines having a green stripe and the unlock lines having a red stripe. The troubleshooting guide describes them as opposite, that is, red stripe is lock and green stripe is unlock.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

Saying that the vacuum system is interconnected is like saying that the electrical system is "interconnected"

To put it in abstract terms "a wiper switch shouldn't open the windows"

Yes the engine shutoff uses vacuum and when it gets vacuum it shuts the engine off.

The vacuum shouldn't be getting from the locking circuit to the engine shutoff.

Some person colored in the chart that is near the front of the thread. When I did mine I did not have that colored chart.

Nor did I recently read the fixit guide which I put into my notes at what must have been 8 years ago.
I just remember it seemed like a good guide.

The fact is that once you remove the bottom door plastic molding and get some carpet up you can see the direction the Lines go and you will see what color they really are.

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84 300D, 82 Volvo 244Gl Diesel

I think what is happening is: When I turn the key to lock the door, the valve in the door switches to the circuit without a leak and therefore the vacuum in the entire system can build up again and has enough pressure to actuate the shutoff mechanism for the engine. I think you are implying that the two circuits are isolated. I do not know. I am just presenting the symptoms and trying to deduce the problem. This is all new to me.

Since you mentioned "a wiper switch shouldn't open the windows", when I move the switch to adjust the passenger side rear view mirror in my 84 300D, the wipers turn on for one cycle. My understanding is that this is a grounding problem, but electricity is not my forte.

__________________
85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

Yes, that indeed can happen. Bottom line is that ya need to start from the vac source and work your way around the car testing with the Mityvac. You have a vac map, all ya gotta do is test the individual pieces, preferably in the biggest parts of the system possible. So... test an entire branch of the system instead of ripping each door open to test individual components

Ok now that makes sense.

With the ignition key off and switching to lock closes a vacuum leak.

My bad

That would mean that: there is a massive leak in the unlock circuit and that there are no vacuum "restrictors" to prevent that from happening

It's