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I've been trying to work out where my oil is coming from (see prior posts about oil pooling on top of the engine block). I've changed my AOS and looked for leaks with dye but I can't find any.


Instead, it seems the oil is leaking from the plenum seals after it builds up and the air pressure allows it to slowly get out. Perhaps the shop that did the AOS didn't do the plenum clamps up tight, but they are now.


I know some oil in the plenum is normal, so to test I cleaned it and did a 25mile highway drive. Even after this short drive, the plenum has a fair amount of oil inside and not a haze. Is this normal? What else could cause more oil than normal? Vacuum leak? 

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A failed AOS will dump a bunch of oil in the plenum and it is very difficult to get it all out.  It pools in the nooks and crannies of the intake plenum, thus the first sign of AOS failure is exhaust smoke..think like a gigantic smoke screen behind the car.  Unless you think the new AOS has failed (which happens...BTW did you put in an original Porsche OE AOS?), I would just monitor this for awhile.  Watch your oil level carefully.

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Thanks for the feedback. I changed the AOS at an local independent and drove it for about a month but then noticed oil pooling on the block and rough running again. I looked around for other leaks with dye but couldn’t find any. The oil pools were directly below the plenum seals so believe once the oil built up in the plenum it leaked thru under pressure.

I figured the AOS may still be broken, so changed it again myself. Used official Porsche part and fully cleaned Plenum. Did a 25 mile freeway test journey and now the Plenum is oily (thick layer on finger) already. Since I fully cleaned the plenum I don’t think it is residue.

I’m wondering if a vacuum leak or something similar might cause problems like this. At this point no smoke except the usual shirt period of light smoke on startup.

Sent from my iPhone using Tapatalk

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Two thoughts come to mind since you are saying "oil pooling on the block".  I thought you were talking about oil inside the plenum.  Have you checked your oil filler tube for cracks?  There is also the AOS crossover line that goes from the AOS to Bank 2 that becomes very brittle from heat and then cracks.  Both of these can cause rough running.  Maybe you should consider having your engine and intake smoke tested.

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  • 3 weeks later...

Many thanks for the suggestions. I put it back together and so far no new leaks. So I think it was leaking out of the plenum seals, I assume that was because the Indy shop didn’t put them back right.

Sent from my iPhone using Tapatalk

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