Prefilter in the shutoff vac line - PeachParts Mercedes-Benz Forum

JHZR2 · #1 02-11-2004, 09:56 PM

Hi,

In my searching of the archives, Ive seen reference to placing a prefilter inline with the shutoff valve vacuum line. This supposedly prevents oil from getting stuck in the rubber connections at the key cylinder, etc., causing them to soften, etc.

Does anyone have info on such a thing? It seems to me that a prefilter has awful big connections to mate up easily with the vacuum lines.

Also, does it go on the line between the vacuum pump and the key, or between the key and the shutoff valve?

Thanks!

JMH

dmorrison · #2 02-11-2004, 11:56 PM

You want a filter that is clear. Mine has a yellow/orange paper element inside. This will fit into the rubber hose fitting easily and you can see any oil that may be leaking into the vacuum sytem from the vacuum pump.
As the large black vacuum hose goes to the brake boost pump. It has 2 outlets just before the booster. The one that branches off to the tubes going into our firewall is the branch you want to install the filter into. If your vacuum pump develops a leak you will see it in the filter. It will help prevent the oil form being drawn all the way up to the ignition as well as everything inside the cars vacuum system.

Dave

dmorrison · #3 02-11-2004, 11:59 PM

Here's the kind of filter I have.

Dave

A link to a google search site.

http://www.************/shop.asp

JHZR2 · #4 02-12-2004, 12:27 AM

wouldnt having all that air volume in the shutoff line cause a lag in shutoff time? When the car is on, the air in that line is at 0" Hg, i.e. atmospheric pressure. When it turns off, my car pulls about 11" Hg on the shutoff valve.

Pulling vacuum on the negligible amount of air in the shutoff valve line is much easier than pulling it on all the volume in a prefilter. It seems to me that this would cause a lag. In your experience, does it?

Thanks,

JMH

lrg · #5 02-12-2004, 12:53 AM

It might, but I believe that's a factory stock setup (I have the exact same item in my vacuum line) so the Mercedes engineers obviously thought it was a good idea.

#6 02-12-2004, 01:31 AM

Quote:

Originally posted by JHZR2
Hi,

... placing a prefilter inline with the shutoff valve vacuum line. ...JMH

It does not prevent anything but warns you that the diaphragm in the shutoff valve at IP is faulty if there is evidence of oil in the filter. You do not want the oil to reach the ignition switch which will cause more shutoff problems.

I've learned this add-on feature from the help of this forum.

dmorrison · #7 02-12-2004, 01:41 AM

When the engine is running the vacuum system is evacuated as you drive along. The system consists of the pump. The various lines and a reserve tank that "stores" the vacuum. This reserve tank is about the size of a 2 liter coke bottle. Maybe a little larger. So your system had a lot of "reserve" ability. When you let the car sit for a week. The reserve tank holds the vacuum. The amount of force that your door or shutoff valve needs is quite small.
11 inches of mercury is a tremendous amount of force. the atmosphere is only 30 inchs of pressure. So your using 33% of the atmospheric pressure to help you PER every square inch of the actuating unit. Whether it is the shutoff valve or a door actuator. That really is a lot of force.

Now if you have a small tear or slit in on of your actuators then you allow the atmosphere to leak into the vacuum and it may make your system ineffective depending on how fast and large the leak is.

So to conclude. The cubic inches of the filter is really nothing IF your system is air tight. Your vacuum pump haas the ability to quickly evacuate quite a large system.

Dave

wolf_walker · #8 02-12-2004, 06:51 AM

Quote:

Originally posted by dmorrison
IF your system is air tight. Your vacuum pump haas the ability to quickly evacuate quite a large system.

Dave

On these cars, that's a pretty serious IF there..

That reminds me of another post I wanted to make..

Rick Miley · #9 02-12-2004, 10:14 AM

Quote:

Originally posted by dmorrison
As the large black vacuum hose goes to the brake boost pump. It has 2 outlets just before the booster. The one that branches off to the tubes going into our firewall is the branch you want to install the filter into.

I would like to respectfully disagree with Dave. If the filter is in this location, then oil showing in it would have already gone through the key switch. Since oil ruins the rubber connectors at the switch, you really haven't gained much.

Dave having the filter in this location also explains why it is always under vacuum, so there is no lag for him.

If you're going add the filter, it should be in the line from the firewall to the shutoff device. That way you would (hopefully) see the oil before it got to the switch. But in this location, the filter would not be under vacuum while running, so there might be a small lag in shutoff.