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Auto-Trend Repairs and Service Information Forums -> 1979-1991 Mercedes-Benz R107 + W126 -> Mercedes Benz W126 + R107 Vacuum Test Instructions for V8 420 + 560 SEL / SEC / SE / SL
Post Info TOPIC: Mercedes Benz W126 + R107 Vacuum Test Instructions for V8 420 + 560 SEL / SEC / SE / SL
SELLC


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Mercedes Benz W126 + R107 Vacuum Test Instructions for V8 420 + 560 SEL / SEC / SE / SL
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One of the first things to check when having problems such as a high idle, hesitation or drive-ability problem should be the vacuum. It's true most all V8 Mercedes have a built in "Economy" gauge that is in essence a "vacuum" gauge but that's just not meant for diagnosis. If one finds that their economy gauge is not sitting all the way to the left upon idle it's safe to say you have a problem.

Next step would be to purchase a vacuum guage. Snap on and Matco sell some nice units but there are less expensive kits out there from Harbor Freight if money is tight.

Next would be to hook the vacuum gauge up in-line of the MAP sensor. The MAP sensor is the vacuum hose that goes to the EZL (Ignition control module) located on the left drivers fender well on both R107's and W126's. This allows you to get an accurate sample without taking any of the systems off-line.

When the engine is warm you should see about 17-20 inches of mercury at an idle. The needle should stay steady. If there is a vacuum leak or late timing it will show up as a low reading below 17 I.O.M.

A vacuum test can also reveal many other problems that can be identified by watching the gauge. For example a quick "goose" of the throttle should result in the needle swinging down to 3-5 inches of mercury with it returning to 17-20 inches of mercury upon idle.

You can get more details about vacuum gauge readings at this link- http://www.secondchancegarage.com/public/186.cfm - They seem to have a nice little guide that covers the various scenarios.

Here is what late timing and various small vacuum leaks looked like on a 1989 560 SL with a stretched chain and fractured guide.

Here is a video



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Rastus


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Yo SELLC,

The use of a vacuum gauge has got to be one of the best ways of optimizing the tune of your vehicle, and discovering its internal conditions and current tune settings etc etc ! It's a shame that their use has become less and less over the years, as something as simple as this item has the potential to really get the most value and performance out of your engine. Complicated electronics and computers give us all a "false sense of security", where as the fitting of this gauge will tell you exactly what's going on in real time ! Yes it does take a while to learn how to interperet and determine the readings, but you just can't beat it for the "real-deal" of what's going on inside your motor. You would have never thought that the fluctuations on the dial would tell you things from worn-piston rings to rich-mixtures, late ignition timing to sticky vave-springs, and everything else in-between ! This is possibly one of the best features to be informed about on this whole forum, yet I wonder how many people will bother to even check it out ?!!!

Cheers,

Rastus

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SELLC


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Yes, and I am proud to say that the 560SL as shown above went from 15 Hg (i.o.m) to 19 with the change of the timing chain and repair of various brittle vacuum lines! We were both very happy to see that however the main and most dangerous problem of a loose timing chain guide was fixed at the same time.

Recently we did some exhaust work to that SL shown above and all I can say is WOW! From 80 to 120MPH in what seems like a blur. Amazing these vehicles are, especially the M117 560 in the R107. Such grace and power that was even more exciting than a newer 2005 SL500 that I drove! It's just amazing! Of course this kind of experience does not come cheap, as the roadsters were and still are very expensive.

I have since returned that POS Harbor Freight Vacuum gauge and got myself a "Snap-On" unit. I swear SnapOn is like the "Mercedes" of tools.



-- Edited by SELLC on Monday 27th of May 2013 02:47:45 PM

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