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Old 08-19-2006, 09:16 PM
Jeremy5848's Avatar
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Join Date: Mar 2006
Location: Sonoma Wine Country
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Vacuum gauge fun

After reading some of the many threads having to do with the convoluted Mercedes vacuum system (especially my California version 1985 300D), I thought it would be fun to play with all of the hoses and such. Besides, I might just learn something.

I got out my beatup old vacuum gauge and pulled apart a fitting that looked innocent: I think it was for the EGR valve. Since that line has had a BB stuck in it since before I had the car, I knew I wouldn't harm anything. I discovered that the vacuum read between 10 and 12 inches of mercury and went away when the engine was shut off.

Was this normal? I realized that, on the pump side of the various check valves, there probably would not be any vacuum with the pump not running. The vacuum level, I saw from reading a few more threads, was more or less normal. I had expected much more, somehow.

The next thing I learned was that the little plastic barb fittings on the main vacuum line are fragile. Yes, I broke one of the little suckers. Fortunately, I was able to epoxy the broken end so that I could drive the car while I waited for a new line. Expensive lesson (they usually are -- but you remember those lessons better). See the first two pictures.

I had purchased a pair of inexpensive vacuum gauges on eBay -- they were surplus from a motorcycle carburetor synch manufacturing operation, the seller said. I wanted to measure vacuum in the engine compartment, as near to the pump as reasonable, and also in the trunk, next to the reservoir. That way, I could check vacuum with engine running and also see what happened when the engine was off. How much is lost every time you unlock the car, how much is lost just sitting, etc.

I didn't need to see the vacuum while driving (hence the inexpensive, non-illuminated gauges with hose barb fittings). One gauge was mounted in the engine bay and the other in the trunk. After installing the new main vacuum line, I replaced an existing three-way rubber junction piece with a four-way junction. The extra leg was for the vacuum gauge. There were lots of places the gauge could have been mounted. In the end, I put a cable tie base on the back of the gauge and cable-tied the gauge to the vacuum line. This is shown in the third photo.

In the trunk, I recycled the three-way junction piece removed from the engine bay to split the line going to the "egg carton" vacuum reservoir and installed the second vacuum gauge. This is shown in the fourth photo. I haven't yet decided how to secure this gauge; right now it is simply flopping around.

With everything buttoned up, I started the engine and watched the gauges. Since the system was at atmospheric pressure, it took a few minutes for the vacuum pump (the original? I have no idea) to evacuate the system. Eventually, the engine gauge read about 15 inches of mercury and the trunk gauge about 17. That was somewhat better than before -- perhaps my ancient test gauge has a leak or maybe I accidently fixed a leak while working on the system.

I will watch the system to see how it performs. My fuel economy will of course improve. Do your vacuum numbers look anything like mine? Some models have an electric pump (under the rear seat?) -- what vacuum does it pull? By the way, I noticed that the engine vacuum gauge reads lower as engine speed increases. Anyone know why that is?


Attached Thumbnails
Vacuum gauge fun-vac01_4677.jpg   Vacuum gauge fun-vac02_4678.jpg   Vacuum gauge fun-vac03_4669.jpg   Vacuum gauge fun-vac04_4671.jpg  

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Old 08-19-2006, 09:26 PM
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Originally Posted by Jeremy5848 View Post
By the way, I noticed that the engine vacuum gauge reads lower as engine speed increases. Anyone know why that is?

Simulates manifold vacuum on a gasser. Tells the tranny how firm/soft to shift.
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Old 08-19-2006, 09:30 PM
sailor15015's Avatar
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Join Date: Apr 2005
Location: Norman, Oklahoma
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I believe its only the SD's of those years that had the electric vacuum pump for when the car is off. I've only checked the vacuum right out of the main line but I got 22" at idle. Never checked it anywhere else.

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
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Old 08-20-2006, 10:51 PM
Join Date: Dec 2005
Location: CA... No. of S.F.
Posts: 890
I applaud your desire to understand these...

various vacuum systems and sharing your findings with this FORUM. For me much of my initial learning about my diesel MZBs has been learing about these systems and how IMPORTANT they are for proper operation of these vintage cars.

Since yours is both a 1985 model and CA car, I challenge you to begin the process of learning about the vacuum system that is mostly located in the engine compartment and controls the tranny [ I'm assuming you have an automatic as most do]. Check out the links at: ........... and

I think yours is the latter. Make a copy for future reference of whichever is yours. Eventually I will have to take a closer look at my father-N-law's '85 300SD and I think it is the same as yours. Not looking forward to troubleshooting such a complicated vacuum system with so many components... included the dreaded "Control Unit"! I have started a THREAD about how "Critical" it is to understand the use of restricted orifices is to this vacuum system... and your's has 3+ such orifices. I started another THREAD on the vacuum lock system as well and ended up putting together a "user-friendly" diagram that some have found very useful.
Happy motoring,
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Old 08-21-2006, 09:49 AM
winmutt's Avatar
85 300D 4spd+tow+h4
Join Date: Apr 2006
Location: Atl Gawga
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Originally Posted by ForcedInduction View Post
Simulates manifold vacuum on a gasser. Tells the tranny how firm/soft to shift.
Does this have any relation to hard shifting at start?

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