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Old 04-15-1999, 09:22 AM
Moses McKnight
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Hi, I'm having some problems with the vaccum system in my 81' 300D. How can I tell for sure if the vaccum pump is bad or not? When I disconnect the pump from the rest of the system and put a guage on it, it reads about 22 in Hg and does not go up or down based on engine speed. When I shut off the engine the vaccum leaks back down to zero within 30 sec to a minute.
Also there is a little oil in the vaccum lines. Can this oil come from some other place than the pump or is that the only possible way to get oil in the lines? I haven't yet done a leakdown test for the rest of the lines but if there is a leak somewhere else I wanted to know how I could tell if the pump might be bad as well.

BTW: Where can I get a diagram of the vaccum system? Will that be in the chassis or engine manuals from Mercedes?
Old 04-15-1999, 02:38 PM
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The books that you can get at any parts store have the vacuum diagram. Are you not getting any vacuum at all? What kind of oil did you find in the lines? If it is brake fluid then your power brake booster has possibly gone out. If the diaphram goes out in the booster, it allows brake fluid to get sucked into the vacuum line. And if this is the case, the brake fluid can have a caustic effect on the vacumm lines going to all of the vacuum powered accesories (power locks, vents, ignition shut off).
Take a good look at the fluid that was in the lines. The vacuum is not too complex on the vehicle but can be an expensive repair. Fortunately, there is a rebuild kit for the vacuum pump. If it is the brake booster, THAT could be a very expensive repair.

83 TDT 110K
Old 06-14-1999, 11:12 PM
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I have a '85 300SD. There is a vacuum shutoff (for engine) unit attached to the injection pump. If the vacuum diaphragm leaks vacuum, it will suck fuel to the vacuum line. The shutoff unit may still shut off the engine with a leaking diaphragm.

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