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Running without an AOS Cayman 2.7

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My 987 2.7 ate 3.3L of oil in 3000km.
The tail pipe is black from the burned oil and it sometimes smokes  when cold with the AOS connected.
I also got a check engine light caused by one of the cats going below efficiency limits because of so much burned oil.
A few times I took the oil cap off as the engine was cold and I noticed that the smoking was less or none at all so I concluded that the AOS must be bad.
I had a look and there was oil on the outside of the AOS and its hose connecting it to the intake as well as in the intake tract.

A few days ago I disconnected the AOS from the intake and plugged the intake connections towards the AOS.
I linked a hose from the AOS down under the engine and it vents to the atmosphere.
I expected that the AOS is putting oil into the intake and there will still be oil in the intake that needs to be burned off so i drove the car a bit hard for a day.
I start the car every other day to monitor the oil burning situation and oil level.
Now with the AOS venting to the atmo. the car produces a thick mist of smoke and vapour at every cold start up and keeps smoking/vaping for a minute or so until it warms up.

Trying to figure out if there is something else wrong with this engine other than the AOS.
I took it to Porsche service and they told me two of the coils are missfiring, so i replaced them but absolutely nothing changed.

When cold the engine idle is not perfectly smooth(feel the engine shudder a tiny bit), and I think it should be very smooth for a 6cyl.
Also when warm the idle needle does not stay still...jumpy idle.

I have to do a bore inspection but i'd like to know if there are other checks i can do myself to find the root cause of the oil consumption.

Opinions and experience are welcome.

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i do not see in your post the mileage on the car.....this is really needed. rough idle can be caused by many things ...among them aos,slow o2 sensors ,high mileage spark plugs ....get it bore scoped/leak down compression test and change aos.....what oil are you using ? 

Edited by extanker
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Car is now 172.000km old.

Using Motul 5W40 x-cess.

It was on Mobil 0W30 when I got it not long ago.


I'm more concerned about the oil consumption rather than the jumpy idle or slight shuddering at idle when cold.

Aldough they may be related or coming from the same reason.

It feels like its running/idling on just 5cyls or less in the first minute of running.


Porsche Budapest said the 3rd and 5th coil give out slight missifres.

I chose to replace the coils myself for both the costs and the fact that I like learning about this car and working on cars.


I used this information to identify the 3rd and 5th cylinder and replaced the coils with new ones but nothing changed in terms of oil consumption or cold behavior.


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have you pulled a spark plug and looked at one ? 172 k is not too bad it depends on how they were driven. scope/compression leak down and aos.....if that does not fix it.....get a for sale sign 

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Car was a showroom car and then a dealer car for 150.000km

Interior is really good, exterior as well, original paint, all things working right, so a nice car to keep.

Even if the cyls are scored I think I'm going to sleve it...but lets hope it does not come to that.


I haven't pulled the spark plugs yet, I don't have such a long connector...but i'll get it this weekend.

Thanks for the replies guys.

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I pulled all the sparks and did a leak test today.

Cold engine, no throttle pressed, all plugs out, was fueling as I did not pull the fuse.


Like the Porsche service said, two of the cylinders are not running so well with both spark plugs black and one of them with considerable amounts of burned oil deposits.

They suspected the coils but I think there may be more to that.


One of the cylinders that was labeled as slightly misfiring has a lowest compression as well.

The other one that Porsche said its misfiring has a similar compression like the other cyls.

The difference is not so big between the first 5 measured but the last cyl I tested had an unusually high number, and I tested it twice...perhaps it filled with fuel by the time I got to it.

The numbers are really not bad, especially for a 170.000km engine and cold testing. I am a bit worried about the lowest and the highest numbers.

With the mean value being 15 bar, 1.5bar being 10% allowable differences 3bar is 20% between the first and the last.


Since the numbers are so high I suspect no broken rings or significantly scuffed pistons/cyl walls.

This engine has a slight knock when cold that completely goes away after warming up,  the knock is coming from the left bank(bank 2), I suspect a stuck valve guide and/or a bad seal.

That would explain the lower leak reading, the deposits on the spark plug and the oil consumption...perhaps together with the AOS failure its become a significant problem.

As for the highest reading...I have no idea...i'm open to opinions.




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well the plot thickens........knocking,oil usage.......by the way,valve seals and guides DO NOT knock. If this was my car i would sell it before i put the thousands of dollars it may need in it . If this is a gen 1 [older] you may want to drop the sump and cut open the oil filer looking for chain tension-er bits  and/or IMS seal bits . what year is this car ?

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  • Founder

Have you thought about cleaning the heads up? I’d have them off and check valve alignment and seating.  At this kind of mileage I’d think about an overhaul.  Depends how confident you are.  


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imho these motors do not take to kindly to inexperienced hands in them,there is a rebuilder here in the states that will not give you any core credit if it has been taken apart or if it doesn't even qualify for a rebuild .   

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Most of the times people get scared of stuff that's actually so simple and inexpensive and they sell good cars.

I know guides don't knock, but a partially gripped valve guide and shot valve stem seal will pour oil into the engine.

I like this car, it feels fresh and new. Engine pulls strong and sounds great.

Thousands of dollars is not something scary, especially if the result is a perfect Cayman.

Even if I have to take the engine appart and rebuild it, I think it would be a much better deal than to sell it for a lot less(I cant hide the problems to any buyer) loose money on the sale and then try my luck with the next car which may be the same or worse. 

These are engines like any other engines..nothing special about them.

With the right documentation at hand and proper tooling its an easy job even for a DYI mechanic, but if I were to take the engine appart I'll have Porsche service do it.

I will have the heads off and cleaned, you're right, an overhaul will have to be done fairly soon if I plan to keep the car for a long time, and I do, I really like the Cayman, it reminds me so much of the 924, very good balance and fun to drive.


I drove the car today after letting it warm up a bit, it drives perfectly with the AOS disconnected and plugged.

No smoke.

I'll keep it like this for about a month and monitor the oil consumption and if its substantial i'll have the heads removed and cleaned.

Does the engine have to come out for that job?

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Oh, and I did cut the oil filter when I got the car and did the oil change, nothing special in it, the usual few and fine sparkling bits, sump was nice and clean with nothing but some factory sealant bits.

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  • 2 weeks later...

As a report, car is running great without the AOS.

Meaning the intake ports normally connected to the AOS are plugged and the AOS is venting to the atmosphere(under the car) trough a hose.

Cleaned the plugs and driving it like this for 1k kilometers. Until now it smoked and vaped every morning or evening i'd start the car...but this morning was the first day without smoke.

Oil level has not dropped but I have not done more than a couple of hundred Kms.

I'll keep driving it daily and note what happens.

If the oil does not drop and the smoke is less or at all, its clear that the AOS was the only faulty part.

The compression/leak test results are ok as well...so maybe its just the AOS.

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Second day with no smoke in the morning. 

Ordering AOS.

Noticed some moisture in the water pump area, will check it out when I install the new AOS.

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Some oil is still being burned and I do a bit of sprinted drive every time I drive the car, I never drive it without at least a few dozen seconds of high revs.

Car burns some oil for 30 seconds flat every time is started from cold and clearly it burns oil during operation as well.


Ate approx. 400g of oil in 500Km, so average of 800g for 1000Km.

At 173.000km this is not that bad but I guess it could be better even with my style of driving.


I'm going to try an oil detergent to clean up the inside of the engine(try to free up the maybe sticky tappets when engine is cold) and will also try an intake and combustion chamber detergent like seafoam to try to remove any deposits from the valve seats.



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Strangest engines ever...the most sensitive engines i have ever seen.

Anyway, rant over...so yea...sometimes no smoke or very little, sometimes a s**t load of smoke even without the AOS connected.


AOS was also spewing oil into the intake so that contributed to the oil consumption and smoking but was not the whole story.


Seems like it depends on how long the car has been sitting and something else..what exactly i'm keen to find out.

Will do a bore inspection and try to get a leak down tester as well before taking the car to a specialist to have the engine out and problem fixed.



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  • 2 months later...

Just got the new AOS replacement and i want to install it this week.


Used the car for weekend drives and even for daily drives to work.

Current status is without the AOS connected, the intake ports that were connected to the AOS are plugged and the AOS vents to atmosphere trough a hoose clamped down near the gearbox.

I've been running like this since August and the smoke problem and oil consumption decreased considerably. Now i want to install the new AOS and re-evaluate the situation.

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As far as I'm aware, the AOS operates much like a PCV valve and I suspect you would see increased oil consumption without one (especially without engine vacuum to assist its operation).  The only difference being that a PCV valve keeps pressure out of the crankcase and the AOS keeps vacuum out of it.


I would imagine that, without the AOS, your vacuum in the intake manifold would leak into the crankcase, start throwing oil everywhere and the fuel vapours will no longer be removed.  Thus the high oil consumption... It's very easy to simulate this by simply removing the oil cap - the engine will idle like crap.

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  • 2 weeks later...

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