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Discussion Starter · #1 ·
Hi,

I've done a search and some reading on this topic, but couldn't seem to find a clear/definitive answer.

I have a 2009 Boxster S (9A1 engine) with about 23,000 miles that I bought last fall. From the service records I have, it looks like the AOS was NOT replaced.

My question is: How do I know if the AOS is on its way out?

I've read that if I remove my oil cap and feel a vacuum, that's a sign that it's on it's way out.

I do not (think) that I'm having any problems that would point to AOS concerns. No excessive smoke (some on start up for a little bit).

But, being a bit of preemptive kind of guy - today after going for a bit of a ride (just 15 miles or so), when I got home and the boxster was idling (so that I could check my oil level) I decided to go, take the oil cap off and see if there was vacuum. As I removed the cap, I could feel some resistance to its removal, and then hear some air rushing in. Also, at that time, the car went from a rock-steady idle to a "lumpy", uneven idle.

I've read in some posts that the sign the AOS is on its way out is if there is "excessive" vacuum, - which suggests that some vacuum is normal.

Since I haven't had the car all that long, i don't know what may be a "normal" or "excessive" amount of crankcase vacuum. I can confirm that I do have some crankcase vacuum, and that when the cap is removed, it will result in unsteady idle.

So, with all that said - I ask the experts here - what say ye? Do I have an AOS problem on my hands, or is what I've described "normal"?

Many thanks!!

Eric
 

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You should have crankcase vacuum. Excessive vacuum may indicate possible AOS issue when compounded with other problems, like smoking, oil leak, oil burning etc. From what you describe it seem you have no issues. The difference in the vacuum feel from the cap is quite significant but if you want to be on the safe side you will nave to measure it. Vacuum gauges are not expensive. The values for normal and excessive vacuum are out there - just do a search, I do not remember the numbers.
Yeah, preventive maintenance is nice but in many cases not really needed. FWIW in my ownership of 21 cars and working on them I have ended up with a box of new parts for different cars in my garage which I never used.
 

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The 987.2 AOS and oil system are quite different from the 987.1 My 2010 S has always had a reasonable amount of vacuum in the crankcase: I think it was like 12 inches of water when I measured it. I doubt you have a problem.

The unsteady idle (and possibly even a CEL) is normal if you take the cap off: you're basically introducing an air leak into your intake system.
 

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There's an article on Pelican about measuring crankcase vacuum. For the 987.1 desired is 4-7" water - no idea about 987.2. I built a gauge with some tubing and measured mine at 4.4", so stopped worrying.
 
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