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#1
07-06-2010, 08:40 AM
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Vacuum Drop When in Gear 380sl
Hey everyone,
Thanks again for the help getting my car running again. I'm still working out a few bugs here and there. I've wondered about this vacuum happening for awhile now. When I put the car in gear, the vacuum drops quite a bit. At idle in park, I have a steady 20 inhg, but when in gear it drops quite a bit, and to different levels, sometimes as low as 15 inhg. I tried plugging the modulator vacuum hose and it has no effect on the drop. Is there another vacuum source to the transmission? Is the drop just related to the engine drop in speed from 900 to 700rpm? Even the economy gauge needle comes to about the halfway mark. It doesn't seem right. Thanks 
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#2
07-06-2010, 12:51 PM
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That isn't normal. There is some kind of leak, and, unfortunately, there are about 100 places the leak can come from. This is one kind of repair where I actually think "shotgunning" a problem is a good idea, because if through some miracle, you happen to find the particular leak that is making the problem serious, there will be 10 more in waiting, given the age of the rubber at this point. OTOH, there could be a lot of small leaks that add up to a big one.
Some common points of leaks (some found on my 380) include: --all vacuum elbows and connectors - (some weren't gripping well) --idle hoses (all) - (mine were VERY bad) --injector seals - (I suspect that mine are bad) --the vacuum lines themselves (strangely, mine weren't bad) --the o-ring at the fuel distributor Even the doughnuts between the halves of the intake manifold, and the "boot" connecting the air meter assembly to the throttle can develop cracks. To do this THOROUGHLY, I've resigned myself to removing the entire intake system and "starting over" with new stuff.
__________________
86 560SL With homebrew first gear start! 85 380SL Daily Driver Project http://juliepalooza.8m.com/sl/mercedes.htm
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#3
07-06-2010, 10:34 PM
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Your foot would be on the brake pedal when you put it in gear, yes?
Maybe the brake booster is contributing to vacuum loss. I'm going through this issue currently.
__________________
http://www.benzypalooza.com/index.htm
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#4
07-07-2010, 09:37 AM
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This took a little math ...
There actually is a spec for the economy gauge. It should be all the way to the left at .585 bar/17.32" Hg and just at the beginning of the red at .4 bar/11.84" Hg. So it sounds to me like your gauge may be reading a bit low - at 15" it should be about 3/4 of the way to the left.
Your vacuum numbers sound good to me. If the car is running and idling well and the trans is shifting OK, I wouldn't worry about it.
__________________
Chuck Taylor Falls Church VA '66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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#5
07-07-2010, 09:46 PM
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All good ideas guys.
All of the vacuum hoses have been replaced in the last year or two. I've checked every connection and everything seems good as far as hoses and connectors. I've checked the vacuum with two different gauges and both read 20 inhg at idle in park. I've put it in gear without using the brake, and the vacuum still drops. I don't have another 107 for reference, so was wondering if others cars did the same. Does anyone's car lower the vacuum this much when put in gear? I guess it's no big deal since it runs well?
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