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Old 12-05-2014, 01:01 PM   #1
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AOS assessment

So I understand an indication of a failing AOS is an increased vacuum in the oil fill tube - how much is okay and how much vacuum is too much? I can easily remove the oil cap when engine is running but you can clearly hear the vacuum and once the cap is off, the cars runs poorly.

How do you determine if the vacuum is too much and indicates a failing AOS?


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Old 12-05-2014, 01:12 PM   #2
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My understanding is that if you can easily remove it with your hand then you are OK. It is when it is very very difficult to remove by hand that you have a problem. Once it is off (even with a normal AOS) you will hear a lot of swooshing type sounds.

The car will start to run poorly because of the large vacuum leak that just happened when you took the cap off.

There are a couple of threads in here about measurement methods of a vacuum that you could also use. Search for "slack tube manometer", here is one thread:


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Old 12-05-2014, 01:16 PM   #3
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^^^ Steve beat me to it as I was typing ^^^

As long as its easy to remove the oil fill cap, you (normally) don't have a failing AOS, but if you want to sure, remove the throttle body & AOS to manifold vacuum line and check for pooling oil.
The engine will run poorly with the cap removed as you are introducing a big vacuum to the system....

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