AOS: What? How? Which? All that stuff ....

[zcacogp]

Chaps, 

I've just changed the AOS on my 987.1c (which was a very easy job, keeping me amused for no more than 20 minutes. All done from above with no need to go under the car, despite what I've read on t'interweb about it). And I am curious about what it does and how it works. Here's a photo of the old one, with the four ports numbered: 

 

I understand that these are: 

1 - Intake of oily air from crank case

2 - Return of oil to crankcase

3 & 4 - Return of air to inlet manifold (but I am not entirely sure that this is correct)

My question is how the separation of oil and air happens and what happens when the part fails? I understand that it leaks oil into the air intake system, and my intake pipes were thickly coated with a layer of oily gluck which I assume is down to failure of this part. However the car also smelled really strongly of hot oil/petrol before the part was changed and this smell is diminishing now. 

I also noticed that one pipe from the AOS to the nether regions of the left hand side of the engine (I didn't investigate to see where it connects to) had cracked and was falling apart. I think that this was the pipe on Port 3 in the picture. What does this pipe do? I didn't like the design of it at all and it was very brittle where it went over the engine so I replaced part of it with some robust wide-bore rubber fuel hose, which I hope will last longer. Given that this pipe would have allowed a lot of whatever it was carrying out (or possibly a lot of air in) then what effect would this have on the engine? 

All answers to my rambling questions welcomed .... thanks!

[eob]

I can't answer your questions, but I am interested for my own benefit.
What prompted you to change it? Any symptoms that would tell you?

Edited November 14, 2022 by eob

[zcacogp]

Hi eob, 

The symptoms were that strong oily/petroly smell and the fact that the oil filler cap was hard to pull off when the engine was idling, which also caused the revs to drop when it was removed. Plus there was nothing in the car's history (120,000 miles) to say it had been changed, although that's not always definitive. 

(If you DO have a go yourself then don't be put off by some of the descriptions online. Removing it was a matter of undoing a couple of M6 bolts from above, plus unclipping the pipes on Ports 3 and 4 (which needs to be done carefully but is not hard.) There was a jubilee clip on the pipe on Port 2 and then the whole thing lifted out, and the new part dropped in. All done from above, although some on-line instructions talk about doing it from below. It can't have taken more than 20 minutes to do, although that would perhaps be 40 minutes if you include removing the engine cover, drinking some tea, reassembly and clearing up afterwards.) 

[Paul P]

I think the AOS is a "version" of a PCV valve - this explanation seems to cover the why's and how's - I know the AOS has a diaphram so I assume this kind of applies.

 

https://www.gatestechzone.com/en/news/2021-06-positive-crankcase-ventilation

 

 

[TROOPER88]

The fact that you had a jubilee clip on the rubber bellow indicates that it has been changed before if the same as an 86.

From factory, it would have a spring clip which is the main reason access is needed under the vehicle and the most awkward part of the job.

 

The pipes that come from 3 and 4 are vacuum lines and become brittle with age and split on the corrugated sections. 

 

I can not comment any more as I do not really work on that many 87's tbh. 

 

Paul 

[phazed]

Also interested as I have a new one on the shelf.

I know if you use a continuous amount of high revs, (track days) the AOS can get overwhelmed. This apparently sucks a lot of oil vapour that collects in the AOS and even though all might have been well before, causes plumes of white/grey smoke on restart which is worrying.

A short drive, all disappears and all is well.

You can fit an upgraded Motorsport one but I believe they are about £700-800!!!

Apparently they occasionally fail which is why I will change mine as I have it.

[map]

14 minutes ago, phazed said:

You can fit an upgraded Motorsport one but I believe they are about £700-800!!!

Plus the upgraded pipework too - say another £150 or so.

[zcacogp]

8 hours ago, Paul P said:

I think the AOS is a "version" of a PCV valve - this explanation seems to cover the why's and how's - I know the AOS has a diaphram so I assume this kind of applies.

 

https://www.gatestechzone.com/en/news/2021-06-positive-crankcase-ventilation

 

 

Thanks Paul - that's a very interesting article. Although I don't quite understand the failure mode; if the diaphragm breaks then it seems that the air intake can bring in air directly from atmosphere and not from the crankcase, although I don't understand the consequences of this. As the article says, lambda will compensate for incorrect AFM (or AMM) readings so the engine should run OK. 

[zcacogp]

7 hours ago, TROOPER88 said:

The fact that you had a jubilee clip on the rubber bellow indicates that it has been changed before if the same as an 86.

From factory, it would have a spring clip which is the main reason access is needed under the vehicle and the most awkward part of the job.

 

The pipes that come from 3 and 4 are vacuum lines and become brittle with age and split on the corrugated sections. 

 

I can not comment any more as I do not really work on that many 87's tbh. 

 

Paul 

Thanks (other) Paul. By '86' and '87' I presume you mean '986' and '987', non? 

I was pleasantly surprised to see a jubilee clip on the outlet pipe downwards as it made the job a lot easier. Whether that indicates that it has been changed before (quite possible, given the age) or not I don't know. 

[TROOPER88]

10 hours ago, zcacogp said:

Thanks (other) Paul. By '86' and '87' I presume you mean '986' and '987', non? 

I was pleasantly surprised to see a jubilee clip on the outlet pipe downwards as it made the job a lot easier. Whether that indicates that it has been changed before (quite possible, given the age) or not I don't know. 

Yes, 986/7.

As I say, you are fortunate to have had the jubilee clip.

[bally4563]

13 minutes ago, TROOPER88 said:

Yes, 986/7.

As I say, you are fortunate to have had the jubilee clip.

if you have the spring clamp a pair of these are your friend!

[TROOPER88]

13 minutes ago, bally4563 said:

if you have the spring clamp a pair of these are your friend!

Had a pair and they broke the first time I used. them.

With the lower AOS clamp, the issue is that you cant see it AND touch it / work on it at the same time. 

This is the beauty of a jubilee clip. Easy to get a ratchet on it and you don't need to be looking at it as you loosen or tighten. 

 

[phazed]

19 hours ago, phazed said:

Also interested as I have a new one on the shelf.

I know if you use a continuous amount of high revs, (track days) the AOS can get overwhelmed. This apparently sucks a lot of oil vapour that collects in the AOS and even though all might have been well before, causes plumes of white/grey smoke on restart which is worrying.

A short drive, all disappears and all is well.

You can fit an upgraded Motorsport one but I believe they are about £700-800!!!

Apparently they occasionally fail which is why I will change mine as I have it.

just saw a Motorsport one on the web, 911 I believe. £1,500!!!!

[zcacogp]

44 minutes ago, phazed said:

just saw a Motorsport one on the web, 911 I believe. £1,500!!!!

£1500 for a 911? I'm not sure I'd want to pay that much for one .... 

[phazed]

49 minutes ago, zcacogp said:

£1500 for a 911? I'm not sure I'd want to pay that much for one .... 

From Design 911

https://www.design911shop.com/Porsche-9871-Boxster-Cayman-Oil-separator-Motorsport/prod138162/?showtax=true&gclid=CjwKCAiAjs2bBhACEiwALTBWZWKGgcorMUQ0zfabMGUmzDtL7RTfB-jBfI3t1yqaqx2aqxxJOAwEKRoC_qIQAvD_BwE

[map]

5 hours ago, zcacogp said:

£1500 for a 911? I'm not sure I'd want to pay that much for one .... 

 

4 hours ago, phazed said:

From Design 911

https://www.design911shop.com/Porsche-9871-Boxster-Cayman-Oil-separator-Motorsport/prod138162/?showtax=true&gclid=CjwKCAiAjs2bBhACEiwALTBWZWKGgcorMUQ0zfabMGUmzDtL7RTfB-jBfI3t1yqaqx2aqxxJOAwEKRoC_qIQAvD_BwE

Take those part numbers and check with your local OPC - having just been through this loop for my 986 I can assure you that Porsche UK are likely to be the best value for these parts. 

Edited November 15, 2022 by map

[bally4563]

10 hours ago, TROOPER88 said:

Had a pair and they broke the first time I used. them.

With the lower AOS clamp, the issue is that you cant see it AND touch it / work on it at the same time. 

This is the beauty of a jubilee clip. Easy to get a ratchet on it and you don't need to be looking at it as you loosen or tighten. 

 

Never had a issue, just to get them off and yes when they go back.    The Jubilee clip is your friend, only on a 986 much easier on a 987 , you can actually see what you are aiming for

[horror]

As fittings: Jubilee (worm drive) and the spring clamps perform different functions.

A Jubilee Clip is a constant torque clamp: if (say) a radiator hose expands and contacts with heat, the Jubilee Clip stays at the same physical size so it’s actual clamping force varies. The Jubilee Clip if checked after use will often appear to need subsequent tightening, followed potentially by over tightening. Not so much a problem with older engines with cast iron, copper or aluminium outlets but not good with plastics.

The common types of spring clips are “constant tension band clamps: and will move, i.e expand and and contract with heat cycles, maintaining a constant clamping force. Factory/OEM will (often) specify different ‘strengths’ of clip for different applications, with (say) oil cooler lines being stronger than water hose or vacuum lines. Different OEM part numbers for the same sized clamp can be the only way to distinguish one from another.
 

[phazed]

I like that.

[Boxer boy]

2 hours ago, horror said:

As fittings: Jubilee (worm drive) and the spring clamps perform different functions.

A Jubilee Clip is a constant torque clamp: if (say) a radiator hose expands and contacts with heat, the Jubilee Clip stays at the same physical size so it’s actual clamping force varies. The Jubilee Clip if checked after use will often appear to need subsequent tightening, followed potentially by over tightening. Not so much a problem with older engines with cast iron, copper or aluminium outlets but not good with plastics.

The common types of spring clips are “constant tension band clamps: and will move, i.e expand and and contract with heat cycles, maintaining a constant clamping force. Factory/OEM will (often) specify different ‘strengths’ of clip for different applications, with (say) oil cooler lines being stronger than water hose or vacuum lines. Different OEM part numbers for the same sized clamp can be the only way to distinguish one from another.
 

But the coefficients of expansion of the metal jubilee clip and hose ( what ever material) and ferrules ( what ever material) are all different .ie even the boggo jubilee clip expands as does everything else .Indeed the none metallic have a lower coefficient of expansion , far lower than the metal.

Could it be the spring ones simply 

1- cheaper …..think bean counters work ! 
2- quicker to fit ..”.     “.          “.           “ 

3- less moving parts = better reliability.In terms of hundreds of thousand or millions used in totality in the VAG group of which Porsche is one marque a move from 0.002 % failure rate down to 0.02 % is a hundred times or 100 less 9issed off customers with there new car springing a leak in week one .Wether a Seat or Lamborghini or anything in between .

 

[phazed]

I believe the spring clip gives a more even pressure on the hose than a Jubilee clip would.

[½cwt]

58 minutes ago, phazed said:

I believe the spring clip gives a more even pressure on the hose than a Jubilee clip would.

Possibly, but the main reason they are used is they are easy and reliable to fit in manufacture as they will always close properly.