AOS unplugged (pictures!)

[White, Walter]

Hello Forum. My first post. New owner of my first Porsche, an '04 Boxster Basic.
When my Boxster started smoking white smoke after start-up, I already knew exactly what to do since I did a lot of research on Boxsters before my purchase. I unplugged the hose connecting the AOS to the intake plenum and saw some oil in that tube. So I ordered an AOS (Air Oil Separator) and changed it. At first I was very disappointed when it continued to smoke sometimes at start-up, but it cleared away in a couple days. I guess that it takes a few miles to clear out the oil from the intake system.
Anyway, I was curious as to how this AOS worked, so I cut open the old one and tried to figure it out.


Here is the AOS from my '04 986 with the cover cut away from the diaphragm area. The diaphragm appears intact except where my screw driver cut it trying to remove the cover.


I cut out the diaphragm here. The spring holds the diaphragm away from port B, allowing air to flow from port A to Port C.


This shows the flow through the AOS. Port A plugs into the crankcase, and port B connects to port A internally. Port D connects back to the crankcase (I think) and this is where the oil that has been separated from the air drains back into the crankcase. The red arrows is air/oil vapor, the green arrows is oil particles. It appears that as long as there is not too much vacuum on port C, air can flow through port C into the intake manifold. But if the vacuum on port C becomes too great, the diaphragm is pulled over the opening at the center of the diaphragm area, closing off the intake manifold from the AOS.


At first I thought the diaphragm was intact, except where my scew driver cut it when I was prying off the diaphragm cover. But by flexing the rubber material, a couple of hairline cracks appeared that were not visible on original inspection. This diaphragm, made of a rubber like material, was incredibly weak and was very easy to tear. I think the oil vapors just weaken the diaphragm until it breaks under the smallest amount of pressure.

[jwgn777]

Nice job of inspecting. When I was very young I started taking things apart. That's when my troubles started. lol

[63buglover]

Good deal! I had saved my old one with the intention of dissecting it, but haven't got around to it. I was also curious how it worked and exactly what the point of failure was.