Air oil separator replacement on a 2.9.

[AndrewT]

I'm a competent and experienced DIY mechanic so I'm wondering how straightforward it is to replace the air oil separator on a 2.9 987.2 Cayman? Any advice/instructions welcome.

The reason I'm asking is that my car occasionally emits a cloud of blue smoke on start up. The OPC found no problem when the previous owner reported it because they couldn't make it happen in their workshop. The bores have been inspected with no problem found. I know that the most common answer is that "they all do that sometimes", however it would also appear that a faulty/worn air oil separator can cause similar issues. I though that for the relatively low cost of the part I might as well replace it anyway. I haven't yet phoned an OPC for a price on the part but I've seen them here for £61.00 delivered.

 

[Motorhead]

Andrew,

Here's a procedure for a 986 Boxster which may help:

[link=http://www.pedrosgarage.com/Site_3/Replace_the_Air_Oil_Separator.html]http://www.pedrosgarage.c...Air_Oil_Separator.html[/link]

and

[link=http://www.planet-9.com/reviews/service-items/p359-porsche-factory-aos-2f-vos-replacement.html]http://www.planet-9.com/r...f-vos-replacement.html[/link]

My recollection is that a faulty AOS can produce clouds of persistent white-ish smoke on start-up. As you're no doubt aware, blue smoke in the exhaust usually indicates burning oil (worn valve guides/piston rings, etc.) but if it clears quickly then it's most probably due to a small residual amount of oil collecting in the bores getting burnt off when you start the engine. One "feature" of the flat-6 boxer engine layout I'm afraid.

Jeff

 

[AndrewT]

Jeff, thanks for the links. I have read elsewhere about white-ish smoke being emitted but if the AOS is letting a small amount of oil into the air inlet wouldn't you expect that to burn with blue-ish smoke? It only happens occasionally on my car, maybe a dozen times in 4.5k miles but as I'm contemplating selling it privately to buy a 3.4 S (PDK) I'm concerned that a new owner would be unduly worried if/when it occurs for them.

 

[Motorhead]

Andrew,

The AOS exits into the inlet tract downstream of the MAF sensor, so the oil mist is mixed with fuel before combustion; I presume that's why the exhaust smoke is white-ish in appearance rather than having a blue tint which would be the case for neat oil collecting in the bores.

Before you do anything else it would be worth checking the air inlet tract (and the MAF itself) for oil contamination to give you some idea whether or not the AOS is faulty.

Jeff

 

[Buddy]

I agree with Jeff, it's quite easy to just take a look at the intake rather than changing the AOS, since it is an occasional occurrence it's obvious that the AOS has not failed as such even if it is giving rise to the occasional smoke. Some smoke on start up is as you say normal but not always visible, I've even noticed it on cold mornings when queueing in traffic in my gen1 S but oil consumption was nothing, it's not always easy to recognise the colour of exhaust smoke as it is drifting against a background which has it's own colours, white is distinctive if thick but from what I've heard of AOS failure it's difficult to even see behind you when they fail for thick blue smoke.

Check the intake after the MAF as Jeff suggests and if it is clean as expected, save your money.

 

[AndrewT]

Thanks Jeff and Kevan, I'll check the 'intake after the MAF' which I guess requires removal of the engine cover which I've done before. I should have said that although my car has done 72k miles it's used no oil since it was serviced 4.5k ago.

 

[stephenbailey]

Jeff

Thats a great link: pedrosgarage

May look at that myself in the future

Thanks for posting

Steve

2006 Cayman S

Lancs

 

[Motorhead]

Andrew,

You've got an AOS problem if your car smokes like this:

[link=https://www.youtube.com/watch?v=u1tmbR20KEY]https://www.youtube.com/watch?v=u1tmbR20KEY[/link]

Jeff

 

[AndrewT]

Hi Jeff, thanks for the link, it's a shame it wasn't filmed for longer as I'd like to know how long the smoking went on for but mine certainly doesn't last as long as that. Mine doesn't smoke when I try to film it

 

[Buddy]

Andrew, a failed AOS would continue to smoke like that until the engine management shut it down with error codes popping up off the Lambda sensors or hopefully spark plugs sooting up before it ran out of oil! Has anyone ever run an engine on in that state?

 

[Motorhead]

That's right Kevan, and the main reason to get a faulty AOS fixed is that oil in the exhaust damages the cat converters.

Jeff

 

[AndrewT]

Thanks again Kevan and Jeff. Do you think that there's a point at which an occasional puff of smoke could be attributed to a developing AOS fault rather than the constant smoking due to a catastrophic failure of the AOS? The people selling the AOS which I linked to in my first post say on their web page "When the AOS starts to fail it cannot separate all of the oil out of the mist and some liquid oil is sent to the air intake. A few drops of oil is enough to generate the GREAT SMOKE BOMB for which our cars are famous for. A big smoke bomb once in a while is completely normal, due to the architecture of the flat-6 engine, which may let a drop of oil flow into one of the combustion chambers sporadically."

I'll follow your earlier advice anyway and check the inlet tract first.

 

[Motorhead]

I think that an examination of the inlet tract for oil traces should be your first port of call Andrew. If it's clear then I think that you can assume that the AOS is OK and that the smoke is more likely to be caused by something else, as mentioned above.

Of course if you are still concerned, it shouldn't be too difficult or expensive to change the AOS to rule it out completely as the source of the problem. After that you're going to be into compression tests to check the piston rings and valve guides, but if you're not burning any oil that may be a bit extreme.

Jeff