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Oil Separator Replacement
 
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Pelican Technical Article:

Oil Separator Replacement

Tom Morr

Time:

1 hour1 hr

Tab:

$30

Talent:

****

Tools:

5mm Allen, flathead screwdriver

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

New oil separator and hoses where applicable

Hot Tip:

Order new hoses before you start

Performance Gain:

No more smoke

Complementary Modification:

Change the oil

The engine air-oil separator (referred to as the AOS) is an emissions device that is responsible for collecting residual gases and vapors contained inside the crankcase and funneling them back into the intake manifold where they can be burned in the combustion chamber. This reduces the overall emissions of the engine. Due to the location of the AOS directly above the exhaust manifold and attached to the valve cover, all the hoses associated with the AOS have a tendency to dry out and become very brittle. It is a good idea to carefully inspect the condition of the hoses before you begin the job and order any hoses that you think might break or fail while performing the work. The hoses on our project car where only a few years old and one was already broken and another cracked during removal.

The AOS is the black box attached to the top right side of the valve cover. It has three hoses attached to it. The AOS to valve cover hose; the air intake hose and the bottom oil return line.

Begin by removing the AOS to valve cover line. Gently wiggle it off the oil separator and the valve cover. There is a small vacuum line attached to the bottom of the hose that assists in returning oil to the crank, remove this hose.

Next remove the air intake hose. This hose also has a connection for the vacuum line to the brake booster. The hose on our project cars was broken at the brake booster line and needed to be replaced. The other end of the hose attaches to the lower section of the air box.

Use a 5mm Allen and remove the two bolts holding the oil separator to the valve cover.

Lift the AOS from the engine and remove the oil return line on the bottom.

The separator is just a piece of plastic that uses vacuum and centrifugal force to cause the air and oil to separate. The oil, which is heavier than the air pools down into the lower hose and is returned to the engine. You can clean the oil separator out with solvent and reuse it, but it is a messy job and takes a fair amount of time. I find it is easier and not that expensive to just order a new one. If you do clean it out make sure the solvent you use does not damage the plastic and that you flush it well as you do not want a bunch of solvent running through your system.

Installation is the reverse of removal

The AOS is the black box attached to the top right side of the valve cover.
Figure 1

The AOS is the black box attached to the top right side of the valve cover. It has three hoses attached to it. The AOS to valve cover hose (green arrow); the air intake hose (red arrow) and the bottom oil return line (yellow arrow).

Begin by removing the AOS to valve cover hose (yellow arrow).
Figure 2

Begin by removing the AOS to valve cover hose (yellow arrow). Gently wiggle it off the oil separator and the valve cover. There is a small vacuum line attached to the bottom of the hose that assists in returning oil to the crank, remove this hose (green arrows).

Next remove the air intake hose.
Figure 3

Next remove the air intake hose. This hose also has a connection for the vacuum line to the brake booster (yellow arrows). The hose on our project cars was broken at the brake booster line and needed to be replaced. The other end of the hose attaches to the lower section of the air box (green arrows).

Use a 5mmAllen and remove the two bolts (yellow arrows) holding the oil separator to the valve cover.
Figure 4

Use a 5mmAllen and remove the two bolts (yellow arrows) holding the oil separator to the valve cover.

Lift the AOS from the engine and remove the oil return line on the bottom (yellow arrow).
Figure 5

Lift the AOS from the engine and remove the oil return line on the bottom (yellow arrow).

The separator is just a piece of plastic that uses vacuum and centrifugal force to cause the air and oil to separate.
Figure 6

The separator is just a piece of plastic that uses vacuum and centrifugal force to cause the air and oil to separate. The oil, which is heavier than the air pools down into the lower hose and is returned to the engine. You can clean the oil separator out with solvent and reuse it, but it is a messy job and takes a fair amount of time. I find it is easier and not that expensive to just order a new one. If you do clean it out make sure the solvent you use does not damage the plastic and that you flush it well as you do not want a bunch of solvent running through your system.

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Comments and Suggestions:
Wile E Coyote Comments: Where does the oil return line hook into on the engine? Mine is connected at the AOS but not to anything on the bottom / other end.
August 20, 2016
  Followup from the Pelican Staff: It connects to the engine oil dipstick tube. - Nick at Pelican Parts  
Med Comments: Hi ,
Do you know why there is water in the AOS bos for my slk 230 ?
All i know its that I drove fast in a big water paddle, could some water got in to air system ? Inside the AOS and the pipes are full of white oil/watermix. :
Thanks
February 9, 2016
  Followup from the Pelican Staff: The water is likely moisture from the engine. This can happen when the engine doesn't get driven enough / warm up to prevent / remove the moisture. - Nick at Pelican Parts  

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