3Sq

What is the purpose of the 3.5mm vacuum hose that connects the plenum with the tb?

First I thought the hose was used to draw in crankcase fumes during idle. But the icv and tb must provide enough suction for that.

Then I noticed the vacuum hose was filled with oil as with the plenum. (The hose was deteriorated so I replaced it, along with all the other hoses). Could the plenum be used as an oil catch can, and the hose, a drain?

There is also a one-way rubber valve in the plenum. Is this connected to the 2 large crankcase hoses or the vacuum hose? If it's for the vacuum hose, it only allows flow from the tb to the plenum - strange.

Any thoughts on all this?

Thanks,
3Sq

moribundman

Make sure that the opening where that litte vac hose goes into the TB is not clogged with oily crud.

From what you've said I can tell that you pulled the plenum off the throttle body. Did you make sure that the two big breather hoses that went from the plenum to the valve covers were clean? No oil is supposed to collect in there or in the plenum. You should also make sure that that the thick TB-to-plenum gasket makes a tight seal, because it's a major source for a vacuum leak.

Maybe it's to remove the throttle body and the ICV for a thorough cleaning. ;-)

3Sq

moribundman,

I did clean the two large hoses and the plenum. I also poked a paperclip into the tb and plenum nipple. No obvious clogs. Not bad after 90k miles and 7 years (96 A4QM).

You mentioned that no oil should collect. The two large hoses dripped drops of oil when tipped down, but the plenum just had an oily film. Does this sound normal considering age? Also, is there a baffle or catch can in the valve cover?

You're right - I should clean and check the tb and icv too. The plenum gasket seemed OK - it was somewhat snug and I did not notice any change in idle while jiggling the plenum with the 2 bolts loose.

Thanks,
3Sq

moribundman

it's not like a catch can. It looks like the crankcase fumes and oily residue seperate in the plenum. The vapors mix with air that comes from the air filter and the get sucked right in the TB. If you take a close look, you'll see that there is a chamber in the plenum, in which the oil residue settles. That oil is sucked into the TB via the little vac hose you mentioned.

I would say it's absolutely normal if a little oil drips out of the breather hoses. As long as there is no blockage or leaking O-ring, everything should be fine.