Vacuum confusion

[ericzacha]

I posted this earlier:
I am a fairly new MB owner: 1988 300 SEL. My CEL came on recently, and the car has had a rough idle at start and stops. I took it in to the shop and they told me that I needed a new air mass meter. It seemed like a simple enough job so I ordered the part. Then my cruise control began to fail. It doesn't turn on at all now. After some research it looks as if my problem might be one of three things:
1. throttle actuator
2. air mass meter and separate cruise control issue or
3. some sort of vaccum leak.

It seems that the only way to figure out whether one of the first two is bad is to change it out. But the vacuum lines I can check first, I just don't know how. I have some experience with cars ie: change out the occasional carb, or water pump, but nothing too dramatic.
My question is: how do I check vaccum lines? Which ones would be important to check in this instance?

And do I need to see if I can cancel that air mass meter order and start with the actuator? Thanks for your help.


Now, after reading some posts I'm a little confused. It seems that some people say they had similar problems caused by vaccum issues. Then I just read a post by Steve Brotherton, who seems to have an impressive wealth of knowledge, that "The only vacuum pump on a gas 126 is the one for the door locks and it is next to the spare tire." Does this mean that the problem can't be a vacuum problem if the only vacuum system in the car is for the locks? How does the cruise work in a 1988 300 SEL??
thanks
eric zacharias

88 300 sel
64 ford falcon
94 isuzu rodeo

[sbourg]

All vacuum assist functions other than door locks utilize engine vacuum in the gas MBs. The diesels develop little engine vacuum, so a separate pump supplies vacuum for brakes, climate control, etc. Both types have a separate pump just for door locks.

In the gas models, vacuum leaks occur most often in the idle air hoses and interconnects. These are the larger diameter hoses between the intake manifold port, pcv orifice, airflow meter plenum, and idle control valve.

Steve

[ericzacha]

I spent a good hour or two this morning sucking on different vacuum tubes. They all seem to be okay except for two that exit the engine right below the air mass meter. One exits there and heads toward the valve cover and the other goes to a relay and on into the driver side front panel. I took off both hoses and there were no leaks, but when I pull out the distal end (leaving them attached under the AMM) then there is an air leak. Is this normal? Is there a direct connection between the AMM and these hoses, and if so how is a vacuum ever created.

[sbourg]

The engine is a huge vacuum pump. With any restriction in the intake, a significant vacuum will be 'pulled' in the intake manifold. That restriction is a nearly closed throttle valve during idling conditions, or a small opening throttle during cruise conditions. However, the airflow meter is located ahead of the throttle plate, and the metering flap should offer only minimal restriction. You will find very little to hold a vacuum at that point. Any hoses feeding to the rubbery airflow meter plenum will be air supply hoses, not vacuum hoses. Vacuum 'supply' hoses will attach to the intake manifold casting, AFTER the throttle valve.

Steve

[ericzacha]

That makes sense. Now the odd thing. I checked all of my vacuum tubes this morning and didn't think that I had found any obvous leaks. Put everything back together, started it up, and presto-- no rough idle and the cruise control worked perfectly all day. I don't really feel like I fixed anything, so I have a sneaking suspicion that it is going to come back. At least now I have a better reason to more thoroughly rule out a leak with wd-40.