I need to replace the vacuum control valve on my 1987 300TD. This would be the VCV type on diesel engines 1985 and older as shown here: https://www.peachparts.com/Wikka/OM603TransVCV2

If I am able to obtain a used VCV is there a bench type test that would verify it isn't leaking internally?

There isn't much to the VCV. Just a chamber, an arm, and a rubber flapper. The most common failure is the rubber flapper coming apart and clogging the vacuum inlet. They can be (CAREFULLY) taken apart and a new rubber piece installed in the flapper. There isn't really a bench test since they're a controlled leak, they leak more volume than a hand vacuum pump can supply.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

If you have a vacuum source and a gauge, do this:

https://www.youtube.com/watch?v=WZzY6t2InrY

Thanks, i'll give that a try

I was fiddling around with the vacuum system last night and ran another measurement with the VCV still on the car. After replacing a few more vacuum connectors the measurement came up to about 9 inches. This is with the VCV clocked all the way to the right on its adjustment points.

I'm thinking maybe I've missed a few leaks in the system. I'm going to replace some more connectors and hoses and see what happens.

Have you checked the basic vacuum amount from the pump ? .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

yes, the vacuum pump is outputting at over 20 inches at the pump and is the same measured at the VCV input

Thinking about this a bit, it has two hose nipples so maybe you could connect them with a 'Y' connector then apply vacuum with the Mity-Vac and see if it holds ? .

I gave away my spare injection pump so I don't have a spare VCV to experiment on but only 9" indicates a pretty serious leak somewhere .

You could also disconnect the green and yellow vacuum pipes under the hood and see if that helps ~ the yellow's are for the central locking and the green is for the HVAC .

Please keep us posted .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better