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Old 06-04-2019, 01:16 AM
JamesDean's Avatar
Electrical Engineer
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,034
OM602 Vacuum Pump Questions

Hi All,

I'm tinkering with my 91 300D and I had some vacuum leak issues. I've buttoned up the leaks I had but I was checking the pump per the factory method as shown:

I get vacuum fine at the booster. If I put my finger over the line at the booster (or connect it) I do get 15-16" at the secondary connection at the pump.

Going by the test, my pump is bad but it doesn't appear to be from a logical stand point--at least mine lol.

Does the vacuum pump have two separate "stages" or "chambers" that should function independently and by failing this test, the separation is compromised?

What do you guys think?


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Old 06-04-2019, 12:08 PM
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Join Date: May 2016
Posts: 6,023
The vacuum pump has a couple of disc style check valves in it that can fail or crack and allow leaks. A good pump will pull 21-22" of vacuum easily.

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