Guess it's my turn to have a vacuum issue - Mercedes-Benz Forum
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post #1 of 15 (permalink) Old 04-07-2008, 03:39 PM Thread Starter
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Guess it's my turn to have a vacuum issue

This problem manifested itself on the '83 300D last February. I was out of the country but went back for some things, flew out again and came back just this March so I thought little of it and attributed it to the month long the car was just parked. Unfortunately that's not the case as it seems.

I used the Mity Vac to measure at certain points, I think the diagram I made best explains my findings. However, some symptoms include:

1. Door locks not holding vacuum, one cycle and that's it - fuel door remains open but the rest locks very slowly.

2. Maybe the climate control has a problem but I can't correlate the vacuum issue with it, yet.

3. Shutting off is a hit or miss. A few months back it would take half a second for the engine to properly shut off. Now it takes slightly longer, and it doesn't do it gracefully anymore and clatters to a stop.

4. When I checked the vacuum lines in the engine bay (by the way, I've removed the EGR vacuum lines) I hear a fluttering air sound at shutoff the moment I release the Mity Vac from its tap to the second port on the main vacuum line. Does this mean it's the vacuum pump, or the valve that sits atop the pump?

5. Pumping the brake with the engine running makes the Mity Vac needle drop from 20ish mmHh down to around 5mmHg. This is while the MV is tapped into the second port.

6. The second port holds vacuum with the engine OFF, but the first port does not. Normal? I'm guessing if this is normal then that means the first port links to functions when the car is running (transmission, EGR, etc), while the second port is for accessories for when the car is on or off (climate control, central locking, shut off?).

7. Since I mentioned the central locking as probably having a relation with the vacuum issue I measured the vacuum at the yellow check valve before it splits and enters into the firewall. The reading was 3.5mmHg with the engine OFF, but it holds.

8. With the engine running, tapping into the first port gives a reading of 23-23.5mmHg, while the reading on the first port is 21mmHg. However, the first port makes the needle fluctuate a bit, while the second port does not and is just steady.

Please see the attached diagram I made. I hate to do vacuum pump work on this thing, primarily because I have to remove the belts, which I never liked.

Also does anybody know what size the allen screws are on the vacuum pump? I'd like to know so I can grab spares in case I strip any. Thanks.
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post #2 of 15 (permalink) Old 04-07-2008, 05:35 PM
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Test the brake booster directly.
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post #3 of 15 (permalink) Old 04-07-2008, 06:05 PM
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Patrick, sounds like a possible complicated fix! Other than taking it to a mechanic, check out this great repair manual from mercedes source. Good luck.

Diesel Vacuum Source Troubleshooting and Repair by Kent Bergsma | MercedesSource.com
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post #4 of 15 (permalink) Old 04-07-2008, 07:39 PM
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You might also look through the car at all the lines instead of concentrating on the Pump end for some sort of obvious damaged lines from Rodents or something. A sitting car developing a problem can often be such vermin and unexpected areas them little buggers can get into and chew stuff. Maybe you'll get lucky and find something like that which just needs new lines or something.
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post #5 of 15 (permalink) Old 04-07-2008, 08:06 PM Thread Starter
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The garage is clean, rodent free and I've never had problems with infestation before. Thanks for that insight. Most parts of the vacuum system under the hood have been replaced except the vacuum pump, the vacuum valve atop the pump housing, the brake booster (component of vacuum system), the vacuum control valve, and the shut off valve at the ignition switch in the cabin. I have a relatively new (2-3 years ago) main vacuum line, new rubber lines, new yellow check valve, new dashpot, new 3/2 way valves (but bypassing the EGR vacuum lines disregards these two parts), and inline filters as a modification.

Douglas, thanks yes I'll test it directly, I found a link somebody posted over on Mercedes Shop Forum (was that you?).

Ron, thanks. The link Diesel911 posted over on MS was for the service manual, I'll go with that first. I know Kent Bergsma probably has quite a bit of experience dealing with MB diesels so I'll consider his advice as my last resort.


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post #6 of 15 (permalink) Old 04-29-2008, 03:37 PM Thread Starter
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Just an update:

I checked the vacuum system for anything that doesn't work, and noticed that even with the engine running the fuel filler actuator did not lock the fuel lid so I disconnected the central locking by removing the yellow check valve from the 4-way fitting, and now it shuts off properly. At least I know it's in the central locking system now...

When I lock the car though, only the fuel filler actuator doesn't lock the lid. The rest does lock, albeit slowly, but they do. Does that confirm that the fuel filler actuator is the culprit? When I tried to bypass it, I felt some moisture on the two 3-way fittings. Does that confirm that either the fittings have gone bad or the actuator itself needs replacement?

Thanks for your insight. I hope to get the system up and locking properly again at least I don't have to humiliate myself each time I shut her off.


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post #7 of 15 (permalink) Old 04-29-2008, 04:52 PM
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That fuel door actuator is expensive and a pain in the ass... it's very easy to overlook. Condensation, or other on the moisture issues? It's very strange that water would be found in the system, as vacuum should clear that. In any case, check that no water is entering your trunk through either the rear windshield seal, the trunk seal, or the indicator seals. Then, replace your fuel door actuator and check the vacuum reservoir fitting/integrity (maybe the reservoir is sucking in water???).
...Then, prepare yourself for when the rest of the actuators start failing... they usually fail around the same time.
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post #8 of 15 (permalink) Old 04-29-2008, 05:14 PM Thread Starter
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I've already thought about the rest of the actuators failing simultaneously so I blocked off the central locking system first. I don't know why there's water, but when I touched it the water was outside the system (not in the vacuum lines, but just covering them).

Looks like a PITA too, I tried bypassing it first but got tired after a long day. The simple act of bypassing it with a straight vacuum line to each of the yellow lines was hard enough!

I guess there's no real way to tell if that's the culprit unless I replace that, and install everything back as they were supposed to be and from there try it out.

At least I got the car to shut down nicely

I just checked Autohaus AZ, seems like $20 isn't that bad for a new actuator. I'm curious, and this is slightly OT: why are there two variations of the master vacuum switch on the driver door? What's the difference?

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post #9 of 15 (permalink) Old 04-29-2008, 05:24 PM
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I had the exact problem a few years back (around 2003) with my 300D. Pretty much the same symptoms, I ended up unplugging the fuel filler actuator and putting golf tees in the vacuum lines. That fixed the problem for me. Door locks, trunk lock, A/C and everything else vacuum-related worked once I did this.

Somehow your post just reminded me that I still haven't ordered the replacement parts...guess I should do that!
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post #10 of 15 (permalink) Old 04-29-2008, 05:46 PM Thread Starter
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Darren, that's funny! Did you just bypass the lines and routed them directly (yellow-red to yellow red, yellow-green to yellow-green)? I got impatient and wasn't able to check if that did the trick for me though, I'm just happy I got to shut the car down

The A/C works fine in my car though I felt a slightly better response from the brake pedal when I came to a stop earlier. I'll dig in some more and see what I find out.


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