Major looking Failure, AOS or something Worse?
 

1998 Porsche 996 C2 manual.

I was driving on a long run a couple of days ago in the rain and as the rain stopped I could see some blue oil smoke coming from the rear of the car. There was nowhere to pull over so I check the engine instruments (Water temp normal, oil pressure normal) and carried on at approx 25mph a few more tens of yards. As I slowed to pull over the smoke went from a small amount of blue smoke to great billowing clouds of white smoke.

As I stopped, the check engine light flickered and I noted that temp and pressure were still normal just before I switched off. I checked the coolant level and found it to be normal, but there was very little oil left. I checked the underneath and found no leaks. At the start of the journey and part way into it I checked the dashboard digital oil level indicator and both time it indicated normal (at least 3/4 full). I ran my finger around the exhaust and it was dripping with oil.

I've been investigating the faults today and found each plug to be very oily. A borescope check has so far revealed no holed pistons. In the front right cylinder there was about half a cup full of oil. The rear left cylinder was full of oil, such that a cup full drained out when I removed the plug. With all the plugs removed I turned the engine over with the starter to pump out the remaining oil. When doing this there were no unusual mechanical noises.

My question is this:
Is this symptomatic of a major failure of the AOS, or could it be something else?

I believe that huge clouds of white smoke that were generated as I came off the throttle were due to the extra vacuum generated at idle sucking large amounts of oil down the inlet manifold tracks. Remember all the plugs were oily so it looks to me like the failure is common(i.e. upstream) to all cylinders.

I guess I'm looking for reassurance that if I change the AOS I'm likely to fix this, however I would also like to know if it's likely to be something else(barring bent con-rods caused by hydraulic locking of the pistons/cylinders full of oil).

Many thanks for any input/guidance in advance.

I would remove the air filter housing and then remove the throttle valve. A bad AOS will leave pools of oil inside there. If you see pools replace the AOS, and clean as much of the oil out of there as possible.

HERE is a link for throttle body removal

HERE is a link for replacement of the AOS

Thanks Casey, I'll do that and report back.

gee
that is a LOT of oil to end up inside a motor.. I'm keen to hear your results.

If the AOS fails it can literally pump oil into your intake.

Don't be surprised if you have to clear out the throttle body and replace the MAF as well.

Glad you didn't try to restart -- I've seen a bent rod due to an AOS failure.

Remember that your exhaust is probably oily as well -- this will take some time to burn off.

Mike

There is more than one mode of AOS failure. The one you are looking at could indeed pull oil up as far as the MAF and airbox and can create excessive crankcase vacuum where even the RMS can get pulled towards the case creating a leak. It all depends on how long it was driven after the failure. This is measured in seconds. You may also find a lot of oil in the exhaust...

Cheers

I've just pulled the airbox and throttle body and as Casey predicted, there are large pools of oil in the inlet manifolds. I'll start working through the Pelican guide to replacing one's AOS later today.

For those that are interested, I did try to start it a few minutes after first stopping as a result of the failure but it only turned half a revolution without firing because of what we now know to be a rear left cylinder filled with oil. Because it didn't fire I'm pretty confident the rod won't have bent at that point, but I'm not confident a rod didn't bend just as I was shutting it down the first time. The reason for attempting the restart was because I was stopped still partially on the highway in poor weather and the car is silver so doesn't stand out. The chances of another vehicle driving into it were pretty high.

I shall probably make a measuring stick and put it in each of the cylinders and measure how far up the cylinder each piston goes. If they all come up the same amount then I should be OK to carry on with the AOS replacement. If there is one that's got a shorter stroke than the others then I know it's time to talk to the bank manager.

Best wishes on your exploratory work. Please keep us posted.

FWIW, in my case the engine with the bent rod would not turn at all.

Once disassembled, there was no other damage -- I did a complete rebuild anyway, as that is what I just did with those engines at that point.

Mike

Well the expansion tank proved to be a real pig to remove, even with the engine lowered. I also left the level sensor in the tank rather than remove it prior to removing the tank. The sensor broke during the wrangling to remove the tank. - A lesson learned for next time. At least the tank itself is in good shape.

I now just have to remove the last manifold bolt right at the back of the engine and I'll be able to look my AOS nemisis right in the plastic.

BTW, if you're wondering why progress is so slow, it's because I'm in the UK and am working on the car outdoors with no source of light. My only daylight is an hour prior to starting work in the morning and an hour at lunchtime. Roll on the weekend.

Well the manifold came out ok, but I could only release one of the hoses from the AOS, so I'm about to drop the engine and trans out to try to defeat the hoses with proper access. At least removing the engine seems to be a pretty easy job.

It turned out that persuading the engine to drop down enough to make it easier to remove the AOS was also very challenging. I simply couldn't get the water hoses off in order to allow the engine to drop down.

After hours of battling I managed to get the AOS out, however to my horror the oil drain pipe that goes into the crank case snapped off in the hole. Another hour of breaking up the remains with a screw driver and carefully checking that what I fished out enabled me to reconstruct all of what I had snapped off finally left me with a car without an AOS.

To my astonishment the new AOS virtually fell into place. I ran out of daylight to fit it, and storms are forecast tonight and tomorrow morning, so I'll only have a couple of hours of weekend left tomorrow to get it back together. I reckon I won't be firing her up until Wednesday, and only of all goes well.