Re: If it's not the AOS... ?

At the track last month I had a giant puff of oil smoke out the tailpipe during a redline 3rd-to-4th upshift. Everything else about the car felt and sounded fine, but it happened again a lap later, so I parked the car.

Someone there suggested that it was almost 100% certainly the AOS, so I drove it to the shop the next day and had it replaced. Everything has been normal driving to work and back in the last month. Now, I haven't been back to the track to really prove that the problem is gone under the same conditions, but I have had a couple of opportunities to do a 3rd gear redline shift on a freeway onramp, and I thought I saw some smoke one time, and last night trying it again I thought things look hazy out the back for a moment there, although it was too dark to be certain. So I'm afraid the problem may still be there.

If the new AOS is good, what else could be causing this if it's really still happening?

Thx,
Trygve

... scavenge tube on one of the oil pumps is cracked.
Very common on tracked cars.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Edited 1 time(s). Last edit at 12/29/2010 08:53AM by Pedro (Weston, FL). (view changes)

When my AOS went (big plume of smoke), I replaced it and there was still some degree of smoke, especially on full throttle for a couple hundred miles.

I would have someone follow you to see if your still getting smoke during the shifts, its really hard to see whats happening while doing a red-line shift into third. you could clean the throttle body as there may still be some oil there from the replaced AOS. The next thing to do would be a compression test/leak down test. Have you missed a shift lately? how are your fluid levels?

Thanks so much, guys. I'll look into all those things.

Pedro, I've searched online and in the parts catalog for the scavenge tube, but cannot find reference to it. Is it something coming off each of the "oil separator" units in the sump/pan area? The parts catalog shows 996 107 080 54 for the separators but it's unclear to me where the scavenge tube is (and how it leads to oil in the intake/exhaust).

I will try to check the throttle body to see if maybe the mechanic did not clean it when replacing the AOS. Al, everything like fluid levels is OK, and no missed shifts. The car has seen a lot of track and race miles but is still going strong in most respects.

There may be a more frequent occurrence of the traditional "startup smoke" thing happening, but I'm not sure about that yet.

...valve) and the intake manifold inner surfaces and so on. Just about every place, surface that the intake air comes in contact with and then every surface the exhaust gases contact will have an oily film (or worse).

Even with some clealing of the TB and the cleaning of the manifold area where the AOS hose connects to the intake manifold and as far into the manifold as one can reach, there is quite a bit of intake and engine surface area that can accumulate some oil film.

It takes time for this oil to be reduced by air flowing past it and by the combustion process and the heat of the exhaust to burn away all of this. The oil doesn't evaporate, is not thinned by any contact with gasoline (at least upstream of the injectors) so all you have is gravity and air flow. In cooler weather of course the oil's going to be less fluid and thus some oil can remain for a long time.

Upon a high speed shift from say 2nd to 3rd there is a tremendous amount of air flowing through the intake and during this period some oil can be dragged along and into the combustion chambers and sometimes a bit of smoke will appear at the exhaust.

Whenever an AOS goes bad and after it is replaced I like to -- though I haven't always done it -- run at least a bottle or two of Techron through the engine as per directions on the bottle to at least ensure the fuel system and injectors are clean and some valve, combustion chamber, exhaust system deposits are removed.

The new AOS is not perfect either. It can allow under some conditions some oil vapor to make its way through the AOS and into the intake system. Some engines generate a bit more blowby (even though they still run good) so an AOS in one engine can have more work to do that one in an engine that generates less blowby. Also, engines can differ in their valve stem seal performance and a bit of oil smoke at various times is the result.

Plus the type of oil you use in the engine, how many miles it has on it. The more miles means more contamination and dilution with fuel and water and this oil then is more prone to generating oil vapor (the oil aerates more readily than fresh oil).

Next some engines are more sensitive to the oil level too. I have come across some S owners who have found oil smoking on the track to be reduced if they run the engine with the oil down a bit from full (when measured hot). However, at one S driver reported that to avoid the flicker of an oil warning light he had to run his Boxster engine with its oil level a bit overfull.

Might point out that Porsche really doesn't support these engines on the track. They are just not up to it, though there are many owners who track these cars and experience no problems. But because the way the engine is designed it is just not able to scavage oil adequately under track conditions and oil smoke once in a while is going to happen. And this activity will put no little amount of oil on the intake manifold surfaces (and other surfaces which I covered above) and even after one has left the track and is driving on the street under some conditions oil smoke has to be expected.

Sincerely,

MarcW.

That is a mountain of good information to consider. I will use try some Techron on the next fill-up. Also hoping Pedro or someone can expand on the cracked scavenge tube thing and what that part entails.

The oil was changed with the AOS repair, but then that means any AOS "spray" was not fresh oil.

Having accumulated probably over 300 "hours" of track time on the motor, and beyond 100,000 street miles on top of that, I don't think it's fair to say that these engines are not up to being driven on the track. Those "supported" GT3 Cup engines have scheduled rebuilds based on hours, and I think it's under 100 hours, and I think the rebuild cost alone would pay for 2 or 3 new M96 motors.