Originally Posted by yogarda

I'm sorry for not specifying... This is a newly-acquired '83 300D Turbo, and I have inherited vacuum problems in a variety of places, which I am slowly pinning down each one thanks to the wealth of info here.
One of my three lines goes back to the lower nipple on the VCV atop my IP. One is a direct input via a couple of 3-way splits from the brake booster line, and the third heads off to the EGR. So not everything here is isolated to EGR function, though I do have my EGR line plugged to help me diagnose the rest of the system.

I was actually not intending to get into my own specific problems here since it appears there's plenty of existing threads that descibe my problems and diagrams to help me... But in the process of t-shooting my two 3/2 valves, it occurred to me that I'm not sure how they're *supposed* to work. I can get them to hold vacuum on the one port but, as described, they quickly leak their vacuum when the button is depressed. Knowing that vacuum venting is a part of this whole process, I wasn't sure if these things vent vacuum when depressed, or are supposed to transfer vacuum to the other two nipples in a leak-free manner.

Thanks for your quick reply.

Originally Posted by Brian Carlton

With the 300D, it makes the most sense to eliminate all the lines to the valve cover because the 3/2 valves only control EGR.

We can do that for you, if you wish.

The function of the valve is as follows:

When the valve is "closed": The consumer (EGR) is connected to the vent line.

When the valve is "open": The consumer is connected to the vacuum supply line.

Originally Posted by dawsonj3

Brian, I have a difficult time accepting that the operation is as simple as you describe. A purely binary operation (vacuum present or vacuum absent) would appear not to require the usage of two different valves which are both influenced by throttle position. These valves almost certainly have a specific function as it pertains to the shifting behavior of the transmission- which I don't currently understand.

Also - on the 1985 model, there is an electronic replacement for these valves - a device which is also influenced by throttle position.

I would be very interested to hear an answer from a Mercedes engineer who is familiar with the function of the device in question.

Originally Posted by dawsonj3

Perhaps each of the two valves serves as a switch - allowing the EGR to operate between a certain range of throttle positions. As the throttle is actuated, perhaps the EGR may function above a certain RPM above idle, but then is turned off again to allow maximum engine power once the throttle has opened past a certain point (indicating full throttle acceleration). Just speculation on my part.

Originally Posted by dawsonj3

Brian, I have a difficult time accepting that the operation is as simple as you describe. A purely binary operation (vacuum present or vacuum absent) would appear not to require the usage of two different valves which are both influenced by throttle position.

Originally Posted by dawsonj3

These valves almost certainly have a specific function as it pertains to the shifting behavior of the transmission- which I don't currently understand.