Pointing fingers at AOS..? or not this time
 

Background: At 70k miles, my car got a rebuilt gearbox, along with a new clutch and DMFW and so far has never burn or consumed any oil.

Now, 5k miles later, at 75k I had to add about one quart on these 5k miles…. and I assumed that the AOS was heading south (the second one since the car was new) and was creating the oil consumption, but when I removed the small hose on top of the AOS it showed some oil inside the hose but not too much.

And looking around the AOS, I found a small puddle of oil underneath a hose connector, this connector joins the oil hose from the filler cap on the rear (Cayman) to a flexible tube that goes on top of the engine (about 1 ½” diameter) However, these hoses where not fully seated with each other (with the plastic locking ring loose)... it seems that these hoses worked themselves loose while lowering the engine to remove/install the transaxle.[/B]

The question here is: Would a loose hose at this connector, allowing the crank to vent to the atmosphere could create the oil consumption?

Your comments will be greatly appreciated, thank you.

PS; today I drove the car to work after properly reconnecting the two oil hoses and the engine seems to be running somehow better (placebo effect).. ?

that's the oil fill tube. it'll leak a bit of oil when throwing the car around, but you'll usually smell it as the oil gets onto the engine and burns off. any addnl vibration since the new dmfw was installed - could an unbalanced situation cause seepage past the rms? i'll get a weep at the rms when at the track and running the car consistently at/near redline (presume more vibration at this rpm).

I noticed an oil leak from my oil filler tube and corrected it....just previous to this I noted suction on the oil filler cap as I tried to remove it with the engine running. These things indicate vacum where you don't want it.

It seemed that the AOS was on its way out. I pulled the plenum cross tubes and found oil pooling in the right side plenum while the AOS tube had just a film of oil. I just changed the AOS.

A failing AOS could still be the cause of your oil consumption. Check suction on your oil fill cap or better yet get hold of a slack tune manometer as described on pelican.

Though you may not have had noticeable oil consumption previously, you're well w/in P-car specs (for my 3.8L) of ~ 1.5qt per 5k miles. Even then I don't worry unless it's sudden usage as (noticeable) consumption has much to do w/ driving style. I notice more on long road trips or hard back road runs and DEs where moisture/fuel in oil is more readily burned off (due to engine being driven harder and hotter). Whereas w/o long or hard runs, contaminants replaces the consumed oil so you're none the wiser.

That said, each engine also has a sort of intrinsic oil fill level regardless of what an electronic or manual dipstick might show. My 997 seems happiest settling around 3 bars on e-dipstick while my Boxster liked 2 down from top.

Keep an eye on it but do not feel it's of much concern right now for you.

Good luck :)

The short answer is no. Oil does not significantly evaporate into the air. It goes on the ground or out through the tailpipe. Do you notice any oily sooting of the tailpipe? What year car and how many miles?

Hello Topless,

I believe that the vacuum generated by the AOS also helps the piston rings to seal properly. Now, if the crankcase has an open passage (a disconnected hose..) then the AOS vacuum gets lost, therefore burning a slight amount of oil through the now non properly sealing rings...

However, this is my theory and for that reason I wanted to share it here, hoping that someone more knowledgeable than myself will share his (hers) comments, thank you.

Regards,
Gilles
'07 CS with 75k