1990 300E Vacuum Questions W/Pics - Mercedes-Benz Forum

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post #1 of 2 (permalink) Old 11-20-2009, 09:53 PM Thread Starter
BenzWorld Junior Member
Date registered: Oct 2009
Vehicle: 1990 Mercedes 300E 119k Mother-in-laws old car, no service records of any kind.
Location: Austin
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1990 300E Vacuum Questions W/Pics

I'm trying to check for vacuum leaks on my 300E to isolate a rough idle and hard starts after after sitting warm for more than 5 minutes. The car idles at 700rpms 85-90 degrees and holds 15hg vacuum. The economy gauge, which from reading these forums I understand to be a vacuum gauge, sits about 1/8 to 1/4 from the left edge of the gauge. At 1000 rpms the needle is pegged full left and vacuum reads 20hg. At idle the needle has some slight fluctuations seemingly in time with the rough idle.

I've used a manual vacuum pump on the hard opaque lines that come from the intake manifold and the hard black plastic one at the base of the throttle body. All hold 20hg plus. I checked one on the back of the fuel pressure regulator that connects to the breather vent tube. There are only two areas I can get to fail and I'm not sure if I'm even checking them right. One is a red line behind the brake fluid resevoir shown in the pic. It connects just above the Y connection in the picture and, from what I can tell, runs across the engine compartment, under the battery box and into the right front fender.??

The other area, also depicted, has multiple lines. one from the intake manifold through the green and white valve thingy and into the base of the Y connection. The Y then splits and connects to the top of two seperate electrical sensors. A line then comes out of each sensor, one through a white and black plastic valve and over to the egr valve. The other is blue striped and goes to the left side of the engine(the a/c compressor?).

The green and white valve thingy (a little help here) was broken so i removed it for the test. If I connect to the base of the Y I can pull a vacuum. If I connect to the line exiting either sensor I can't get vacuum no matter what i plug up. This all would be suctioning away from the Y.

What is that green plastic valve that is broken? What does it do?

What are those electrical sensors that the Y connects to?

Should I be able to hold vacuum on the EGR valve?

What is the blue striped line?

What is the red line that runs into the right fender?

Am I even doing these test right?

I label the pics with arrow so you experts can give me some references.


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1990 300E Vacuum Questions W/Pics-mercedes-300e.jpg
tysmith747 is offline  
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post #2 of 2 (permalink) Old 11-21-2009, 07:00 AM
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Date registered: Apr 2007
Location: NC
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The green thingy I believe to be a check valve. It will only hold vacuum on one side.
orange911 is offline  
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