I hope one or more of the over 200 members now viewing this forum will be able to help me with an answer: How does one determine whether the check valve on top of the W116 diesel engine vacuum pump is good or bad? I am not getting enough vacuum to shift properly, lock my doors, or shut off my engine. A vacuum guage reading looks like the guage has a bad case of jitters and the needle oscillates back and forth at a very rapid rate never climbing above about 17 inches of mercury. I have the check valve off the pump. If I blow into the check valve from what would be the top as it comes out of the vacuum pump, I get a small "pop" sound, then air freely passes through it. From the bottom side, no air at all will pass through the check valve. I have a used spare from my parts drawer that allows air to freely pass through from the top to the bottom. I suspect the popping -- which is felt more than heard -- is the cause of the back and forth oscillation of the vacuum guage at test. Any help? Thanks in advance.

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1980 300SD 146K miles.

You might want to measure the vacuum acumulation at the pump. The general pump may need a kitting. Sounds like your check valve was okay unless it is sticking open in service.

Good tip, and thanks. She who must be obeyed has my undivided attention for the next couple of hours, but I will check the output at the pump and post back here later in the afternoon. Would a re-kit require removal of the pump?

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1980 300SD 146K miles.

It's possible that your car has a double diaphragm pump. Some of the 116 SD's had them. If so, it's possible that one of the diaphragms is leaking and the other isn't, resulting in the fluctuating gauge.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I just came in from testing the vacuum. With the replacement check valve in place, I am now reading a steady 21 inches of mercury on the vacuum guage. I hope this implies that I will now have enough vacuum downstream to take care of the vacuum needs of all the systems requiring it. I intended to check the vacuum at the brake booster, but had a severely skint knuckle that needed the bleeding staunched. With that done, I have taken to the recliner until the morrow. I would appreciate suggestions and observations from any and all as to the next step in the event that I have a major drop in vacuum at the brake booster and no improvement in engine cutoff, tranny shifting, etc. Thanks all.

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1980 300SD 146K miles.