I cut open my AOS! - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Rodger · 05-19-2007, 12:51 PM

Let's see how it works!

....and why it fails!

DISCLAIMER: I am not a mechanical engineer, so my explanation could be wrong on some accounts. Feel free to correct me. Also, I would not recommend doing this, as there are spring-loaded parts inside, as well as oil vapor, which could cause a fire. Used motor oil is known to the State of California to cause cancer and birth defects. I took all necessary precautions while cutting this open (gloves, eye protection, fire extinguisher nearby, etc.). Please keep in mind this is an AOS which, to my knowledge, had NOT failed in any way.

First, a little lesson in how a Positive Crankcase Ventilation (PCV) System should work.

With the engine idling, there's high intake manifold vacuum (suction) and low crankcase pressure (due to the low cylinder pressures, and consequently low crankcase blowby pressures). This causes the PCV system to draw very little crankcase emissions into the intake, since very little exist.

At cruising speed, intake manifold vacuum is lower, but crankcase pressures are higher, due to the engine actually moving a load. This results in a medium amount of crankcase emissions being sucked into the intake.

At wide-open throttle (WOT), there is close to zero intake vacuum, but crankcase blowby will be relatively high. This results in the maximum allowable crankcase emissions to be sucked into the intake.

During an intake manifold backfire, the PCV valve acts as a check valve to prevent the vaporized oil in the crankcase from igniting, which could cause a crankcase explosion. Intake manifold pressure (the opposite of vacuum) forces the valve closed, as the intake would have higher pressure than the crankcase in this situation.

Now let's see how the AOS accomplishes this:

As shown in this pic, crankcase air enters the small-diameter passage (the one with the orange o-ring at the very bottom) and enters a large chamber. As the crankcase oil vapor enters the large central chamber, the pressure decreases (due to the significant volume compared to the tube). As the pressure drops, so does the velocity. Both the pressure and velocity reductions contribute to the condensation of the oil vapor back into droplets. These droplets would fall harmlessly to the base of the large chamber and would eventually drain back into the engine. These same droplets would have a difficult time making it past the tube at the top of the center chamber, if they were lucky enough to remain suspended in the airstream.

The top two arrows show the direction of the oil-free crankcase pressure as it leaves the chamber.

The arrow on the bellows shows a second path for crankcase pressure (possibly that which is developed in the cylinder head valvetrain area?) which is entirely seperate from the crankcase ventilation path. As the pressure enters the bellows, the only escape path becomes the tube outlet to the left. Above this tube outlet, inside the AOS body, the path suddenly narrows, giving the pressure nowhere to escape. This may induce a small amount of turbulence into the exiting stream. Also, because the narrowed path is directly inline with the bellows, I'm concerned that bellows "blow-out" may occur to due pressure spikes under certain conditions. Could this cause a weakened bellows to fail? The pressure that exits the bellows path takes a convoluted tube somewhere else (presumably the intake).

Let's move to the top half of the AOS, which contains the diaphragm to control the escape of crankcase pressure.

Crankcase pressure enters through the rectangular hole at the base of the diaphragm housing. Intake suction is drawn through the hole in the center of the housing and out through the tube connection to the intake pipes near the throttle body. Access from the rectangular hole to the suction hole is restricted by a pair of spring-loaded plates and a rubber diaphragm. You can see one of the springs (this happens to be the large one) in the pic.

Shown here is the underside of the removed diaphragm housing lid. Don't mind the tear in the rubber diaphragm, I did that during the removal. You can see the large plastic disc attached to the diaphragm. This plastic disc is supported by the large spring in the previous pic. The plastic disc does not seal against the body of the disphragm housing, only against the diaphragm itself. Also shown in this pic is the small spring which actually controls the passage of pressure through the housing (along with a small amount from the diaphragm itself).

Rodger · 05-19-2007, 12:53 PM

Here, the small spring is shown compressed. In this position, the plastic in the center of the spring seals against the rubber center of the diaphragm, which allows no pressure to bypass in the event of an intake manifold backfire.

Here, the small spring is shown in the relaxed state, as it would be during idle, cruising, and WOT.

How the AOS Fails:

Most failures are probably due to the rubber diaphragm tearing. It's not very sturdy! When it tears, there is no longer a tight seal inside the diaphragm housing. This seal failure allows significant suction to be developed through the AOS by the intake. This suction results in large quantities of oil to be drawn in and ejected as smoke from the exhaust. Remember, the release of crankcase pressure is supposed to be regulated by the spring-loaded plastic disc.

The diaphragm is responsible for sealing three sections: Around the circumference of the diaphragm housing, the central small spring outlet, and around the face of the large plastic disc. The latter of these appears to be how the oil gets through when it fails.

Look closely at this pic. See the thick circular rubber in the center of the diaphragm? Around it are four darkened ares in the shape of rectangles (one is obscured by diaphragm deformation).

Those four darkened areas make contact with the similar holes in the large plastic disc shown above. When intake manifold suction is high and crankcase pressure is low, the diaphragm is supposed to flatten against these holes to prevent intake suction from reaching down into the AOS housing and drawing in oil droplets. If the diaphragm tears, these are left uncovered and you get massive oil suction during engine start and deceleration in gear. It only takes a small hole to allow suction into the AOS central chamber.