AOS, oil filler tube, or something else?

Over the summer the transmission was rebuilt, IMSB swapped for a LN dual row, RMS was replaced, and the AOS was replaced. At that time it was filled with Motul rather than the M1 it has used for the rest of its life. They put the wrong crush washer on on and about 50 miles after the work I came out to a puddle of oil under the motor. Since then we noticed that the oil level had dropped, topped it off, and it's back down again (maybe 1000 miles since the work). There is no obvious signs of loss (e.g. Nothing making it to the ground and nothing obvious that I can see). In 15 years and 70k this car has never burned oil. Maybe 300 miles ago the plugs and coils were also replaced. At that time I had the top end scoped and there was no sign of bore scoring. The last couple of times my wife drove it she reported a burning oil smell after downshifting at speed to accelerate. I took it out tonight (first time since she mentioned it last week), but even flogging it a bit I didn't notice any odd smells (though my sinuses are messed up so that isn't conclusive). None of the other signs to suggest bore scoring (no smoke or ticking). Could the new AOS be bad? Is the filler tube was cracked, how does that present and could it lead to these oddities? Other ideas?

Yes, an AOS can fail out of the box. Not saying this is your problem but there's only one way to verify it. https://rennlist.com/forums/996-foru...l#post12617976 Check for oily gunk at the outside bottom of where the rubber sleeves join the intake plenum. If that area is oily, too much oil is sucked into the intake via the AOS.

No oil in the coolant...I hope.

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Thanks Ahsai. I'll take a look at that later this week when it warms up some. Nope and given the propensity for short trips that barely get it warm, the filler tube is rather clean too.

What exactly does the AOS do? When should it be replaced?

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AOS = Air Oil Separator, returns oil vapor laden air to intake while removing most of the oil. It should be replaced when you have the transaxle out (due to ease of replacement) or when it fails.

Don't want to hijack thread, but why does the AOS have cooling water routed to/through it? Thanks

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My WAG is in colder climes to prevent the water vapor that also is in the crankcase fumes that pass through the AOS from freezing and blocking crankcase fume flow through the AOS. In warmer climes the heated AOS helps with the removal of water vapor from the crankcase fumes. While the AOS surfaces are not hot enough to boil water they are quite warm and keep the water droplets that may form/collect on the AOS walls/surfaces warmer and in the lower pressure present in the crankcase can boil/turn to vapor again so the water as vapor can pass on out of the AOS. If the water remains in liquid form it can run down and back into the crankcase and the water content of the oil then can increase to a dangerous level. (I note my 2002 Boxster does not have a heated AOS and after just 4K miles of winter usage when I had the oil analyzed the water content was around 7%. That's nearly half a quart of water.) An additional benefit could be the AOS surfaces that collect the oil droplets as the crankcase fumes swirl around inside of the AOS as the fumes flow from the crankcase to the intake are kept warm and the oil collected thus drains away/down rather than linger and build up to a point it possibly gets pulled back into the stream of air that is flowing on into the intake.

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Mac: Ok that is logical. These are really just fancy PCV's correct? I miss the "old" days! I.E. pull the hose off, pull device out of valve cover, wash out with solvent so the internal shuttle rattles and reverse the process. Makes me feel old!

As Ahsai said the AOS could be bad. You also need to get the car in the air and carefully inspect it for any leaks. I would hazard a guess the AOS is fine -- no guarantees! -- but there is an oil leak probably from a camshaft cover that is dripping oil down on the exhaust. This leak only happens once the engine is running and while it may continue a short while after the engine is shut off the oil falls on the hot exhaust and is vaporized. In severe cases not only can you smell this -- hot/burning oil smell -- you can see the oil vapor wafting up maybe through the engine compartment vents. Ideally you would want to lift the car with the engine/drivetrain up to temperature and the engine idling but you have to be really careful when under a hot car with a running engine. This is really a job for a professional auto tech.