Vacuum Leak Repair

I noticed when I bought the car that the door locks were kinda slow, and the egine didn't shut right off with the key. At first, I was happy to have functional door locks at all! Lately I've been noticing the locks don't seem to unlock themselves after sitting for a few hours, so I decided to look into finding a leak.

The frist places I tested for leaks were at the locks themselves. I removed the carpet from the drivers side floor in the front and the black tunnel on the right where the lines run. I found the lock and unlock lines (Yellow with green or red stripe) and their 3-way connections splitting off to the left and right side of the car. I pulled off each line one by one and applied a vacuum with my hand-powered vacuum pump. Each one seemed to be fine so I had to look at the vacuum source.

This picture shows the source of the vacuum and all of the other vacuum powered items under the hood.

The first thing I did was to tap into the line running from the vacuum pump to see if the pump was functioning correctly. My guage showed 7.5 inHg. I wasn't sure what normal should be, so I disconnected all the lines and took a reading at point 1 on the picture above. It was 24 inHg! I definately had a problem. I started looking at the lines running into the cabin of the car. I fumbled with them for a while then I got the idea to connected the line running to the locks and storage tank directly to the 24 inHg vacuum source. I started the engine and let the vacuum built for a little while. When I shut the car off, I locked and unlocked the doors 4 times! I couldn't believe it! Since I now knew that the storage tank and door locks were fine, I started looking at the lines running toward the engine. I found that line 3 (yellow) held almost no vacuum at all and line 2 (green) on the fuel pump wasn't much better. I checked all the other lines running from the black box on the engine. The one running to the right side of the car seemed ok along with all the other things it connected with. The black line running from the box to the injection pump didn't hold any vacuum at all and it still doesn't now as far as I know.

The next step was to figure out just what that black box did. I looks like it might be a complicated thing I'd regret opening, but inside all I found was a little swich with the three lines connected to it! The switch is pressed by a cam that moves with the throtle linkage. I removed the swtich, attached my pump, and found that it leaked real bad! I called the local dealer (Mercedes-Bena of Morristown) and we figured out what it was and, of course, that is was $36! That seems like a alot for a little silly switch with not clear purpose, but it fixed most of the problem!

Before running to the dealer, I removed the vacuum control from the injection pump (number 2) and took it apart to see if I could adjust it and if it was any good. The guy at the dealer said it should hold a vacuum at the idle speed position and release it was the throtle was increased. The one I had leaked a good amount at idle and ALOT at any other position. I called him back and told him I'd need one of those too, then he told me it was somewhere around $160!!! Since I haven't begun work for the summer, I decided I could live with the minor leak for now. I adjusted it so the leaking was at a minimum at idle speed and reinstalled it.

After replacing the swtich (above, MB calls it a Vacuum Valve) the gauge reads about 18 inHg with everything connected! A BIG improvment! The locks work much much beter than before and the engine now shuts off as soon as the key is turned.

I'll replace the other "Vacuum Control Valve" sometime this summer. I already checked out the junk yard, but all the 240D's had no engines. I'll keep my eyes open for any deals, $160 is alot of money!

If you have any ideas where I can find a good used "Vacuum Control Valve" for my car, email me! Thanks!

Thank you to all the people on the Mercedes Veterans mailing list for your help with my leak!

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