My turn to have a vacuum issue - '83 300D - PeachParts Mercedes-Benz Forum

MBeige · #1 04-07-2008, 06:39 PM

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.

Diesel911 · #2 04-07-2008, 09:29 PM

I sent you a private message.
Do you have a Federal emissions car or a California or other type emissions car.
Mine is federal and the hare Vacuum line with that goes to the brake booster is different than the CA type cars. So maybe the Vacuum readings at the 2 outles inbetween might be different.

Also there is more than one type of vacuum pump and there is a vacuum valve back where the steering wheel lock is at the end of your ignition switch (to control the vacuum to the vacuum shutoff on the IP).
Check this section to check the Brake booster and ect.
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Index/Groups/43BrakeBooster.html

tangofox007 · #3 04-07-2008, 10:20 PM

Quote:

Originally Posted by MBeige

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

Nothing in your post suggests that there is a problem with the vacuum pump. (I am assuming that you meant "in/Hg" and not "mm/Hg.")

You might try plugging the vacuum supply to the door lock system at the yellow check valve. Then see if that improves the shut-down operation.

MBeige · #4 04-07-2008, 10:50 PM

@Diesel911 thanks, I do have a California version but for '83 that's no different from a Federal, right? I had a suspicion that the vacuum valve at the ignition switch is bad so I already have the part ready.

How do I know what type of pump I have? Thanks for that link, I'll test the car tonight.

I forgot to mention that the main vacuum line was replaced maybe two or three years ago (Cohline I think was the brand). All rubber vacuum lines in the general area of the IP, main vacuum line and corresponding branches have been replaced.

@Tangofox007 yes sorry I meant inches Hg. The reason I mentioned the pump is because I hear a hissing sound like I mentioned so I don't know if that's normal or not. I was thinking of temporarily plugging them and see which one removes the symptoms, then I guess I found the leak if I chance upon it. The yellow check valve has been replaced already along with the dashpot. I also installed inline fuel filters as a vacuum line filter like many have done on this forum.

So you're telling me you think the vacuum pump is in good order? Does the fluctuating needle mean anything when it's tapped into the main vacuum line (with engine running)?

By the way before I got home tonight it seemed that stepping on the brake before shutting the engine off made it shut off quicker. I'll probably have to do more runs and see which is which. Thanks.

tangofox007 · #5 04-07-2008, 10:55 PM

[QUOTE=MBeige;1817513
So you're telling me you think the vacuum pump is in good order? Does the fluctuating needle mean anything when it's tapped into the main vacuum line (with engine running)?

[/QUOTE]

The vacuum pump is of the reciprocating variety, so rapid needle fluctuation would be expected. If you are getting over 20 in/Hg, it sounds like your pump isn't the problem.

MBeige · #6 04-08-2008, 12:58 AM

But why is it that the first port has a fluctuating needle reading while the second port is steady? Is there a valve between the two?

From experience I've worked with several vacuum pumps before (non vehicular - more like laboratory pumps - belt driven, direct drive, etc) and the vacuum gauge hooked up to them usually show steady readings, not fluctuating. Hence my question. The pumps I worked on are rotary with two rotors, two vanes, stators, and a center shaft that turns all these via either belt/pulley combo or direct drive into motor shaft w/ bushing. That's why I was asking if the fluctuating needle is a bad sign. But you're right I read on the pages that 20+ in/Hg is a good measurement.

The ports I'm talking about are the two nipples on what seems to be a valve on the main vacuum hose:

First would be the one to the front (left), the second port would be the one closer to the brake booster itself (right).

MBeige · #7 04-29-2008, 06:31 PM

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.

mb300d4me · #8 09-24-2008, 11:36 PM

I see when you tested the vacuum the pressure dropped from 20ish to 5 when breaking. I'm also having a soft pedal and when testing I see this same drop when pressing the break. Is this pressure drop normal? When tested I had the vacuum gage plugged into the port closest to the pump.

Cr from Texas · #9 09-25-2008, 12:20 AM

Even in the first post, it looked like you'd identified the fuel door pod as the problem. You could just leave it blocked off for awhile but with the price of diesel you may need the lock fixed as a high priority.

Mine had similar symptoms several months ago and replacing the fuel door pod solved the problem - well for awhile anyway, now I've got a bad pod under the dash.

argus445 · #10 12-17-2008, 07:08 PM

what should the vacuum output be idle or at above idle i get something like 8 at idle and about 15-20 at 2000 rpm how do i check the vacuum pump make sure it's not the cause and not a lot of tiny leaks causing issues?