Crankcase Suction - Normal?

[sullivas]

I took off my oil cap while the engine was warmed up and idling (there was a hissing noise coming from the tube) and I covered the fill tube with a small piece of

cardboard and the crankcase suction held the cardboard like it was taped on. Note that when I took off the cap the engine

faltered a little and RPMs dropped a few, when I put the cardboard on the fill tube the idle came right back up. Is it

normal to have suction this strong? I read in another thread that this suction is not a good thing and it should be looked into.

On my DD's (volvo's) I can put pieces of paper over the fillers and it barely holds them on. One of the

cars has no suction at all, in fact it has a little bit of pressure pushing out.

Thanks

[JFP in PA]

There should be a low vacuum under the oil fill cap; these are low tension ring motors and require a slight vacuum in the cases to help control oil and blow by gasses. If the vacuum becomes very high (hard to nearly impossible to remove the oil cap while idling), it is a sign of a failing AOS.

[sullivas]

No the cap came off fine no problem.

Will there be a check engine warning if the suction gets too high?

Possibly an AOS problem?

[Loren]

If you can get the cap off with the engine running chances are your AOS (and bellows) are fine.

[JFP in PA]

No the cap came off fine no problem.

Will there be a check engine warning if the suction gets too high?

Possibly an AOS problem?

When your AOS fails, you do not need a warning light, just look in the rearview mirror: if it looks like you are spraying for insects behind you, your AOS is gone.............

Excellence magazine recently had an article about the AOS and showed how to test it with a manometer; which while all well and good, most knowledgeable techs just try to remove the oil filler cap while it is running

[Loren]

Yeah, I know a Porsche shop foreman (won't mention his name) that took a new oil filler cap and added a vacuum pressure gauge to it. Seemed like over kill as obvious as most AOS failures are.

Perhaps he thought he could detect a small crack or split bellows?

[sullivas]

Thanks Guys, have to check out the Excellence article.

BTW, how many miles is an AOS good for?

[JFP in PA]

Somewhere between one and one million miles. Seriously, there is literally no rhyme or reason to when or how these things fail. The early ones were more prone to crapping out than the later ones, due mainly to a redesign by Porsche. We recommend that customers replace them at opportune times when the process is quick and simple, like when the gear box is out to do a clutch.