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Old 05-16-2016, 03:41 PM   #1
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Aos question

Friend just got 2001 boxster s with 99 911 motor. Upon start up, smoke comes out of the pipes more than normal and then stops. Previous owner changed oil upon pickup of the car and the oil level is very high. I know we need to get some out. When idling, when you take the oil filler cap off the engine starts to run rough. Sound like an aos issue? The car was not run for several months prior to pick up. New owner has put about 30 miles on it going home. Any thoughts appreciated.
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Old 05-16-2016, 04:00 PM   #2
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Momentary white smoke on startup is normal and it should clear very quickly.

I'd make sure that the oil level is correct and then check the tube that runs from the AOS to the intake. If there is more than a misting of oil residue in that hose, then I'd replace the AOS.
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Old 05-16-2016, 04:07 PM   #3
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So the rough running when oil cap off means nothing?

Thanks for responding.
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Old 05-16-2016, 05:18 PM   #4
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that's normal

Quote:
Originally Posted by DWBOX2000 View Post
So the rough running when oil cap off means nothing?

Thanks for responding.
No, that is normal, you are actually creating a vacuum leak by removing the cap
and engine will run rough.
It happens the same when:
running without oil cap (or non sealing one)
broken/cracked oil fill tube
broken dipstick tube
broken/cracked aos tubes
etc. etc.
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Old 05-16-2016, 05:28 PM   #5
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Thanks. I will suggest lowering the oil, running for a while and see what happens. Still excessively smoking, bring back to mechanic that sold it to him.
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Old 05-16-2016, 08:40 PM   #6
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Quote:
Originally Posted by DWBOX2000 View Post
So the rough running when oil cap off means nothing?
The intent of the oil cap "test" is to check the relative amount of vacuum, not how well the engine runs, when removing the oil cap.

A good AOS will be somewhere in the range of 4"-7" of vacuum whereas a failed AOS will be in the range of 9"-12+". Thus, one way to check the AOS is to remove the oil cap with the engine running to see how hard the vacuum makes it to remove the cap.

The general saying is that you should be able to lift off the cap with a little pressure on a good AOS and it would take a good pull to get it off on a failed AOS (due to the greater vacuum that occurs when the AOS fails). The problem with this test is that it is somewhat subjective and can be hard to tell the difference between the higher and lower vacuum levels - a little pressure to you might be a good pull to me or vice versa.

This is why I suggested checking the oil vapor return hose. If the AOS is failed, then there will be a lot of oil vapor going into the intake.
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Last edited by thstone; 05-16-2016 at 08:45 PM.
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Old 05-17-2016, 03:29 AM   #7
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Got it. Thank you.
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Old 05-18-2016, 03:15 PM   #8
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Rough running when oil cap removed is normal. Positive crank case I believe they call it


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