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

MBeige · #1 04-07-2008, 07:39 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.