# Sucking through oil cap??



## tangapants (Oct 10, 2012)

3.2 v6 2007 TT

Should the engine be sucking air through the oil cap when the engine is running with the oil cap off?

Thanks


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## brittan (May 18, 2007)

Yes; any blow by gasses are returned to the air inlet stream for the engine to consume, rather than vent them direct to atmosphere. It's the reason for the 'O' seal on the dip stick.


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## tangapants (Oct 10, 2012)

ok thanks for the reply

when i mean sucking i mean excessive sucking with a rough idle. I also have a rough idle at cold start.

i read and hear so many conflicting comments online and from garages about air sucking through the oil cap when off [smiley=bigcry.gif]


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## brittan (May 18, 2007)

The rough idle is because the air flow through the oil cap is not being measured by the MAF and therefore the air/fuel ratio is wrong. Probably worth a scan for fault odes ref the cold start rough idle.


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## TT-driver (Sep 14, 2010)

I'm not totally up to speed with the naturally aspirating V6. But so far my experience with naturally aspirating engines, the blow by gases are blowing out when the oil cap is removed. If the engine is sucking air through the oil cap opening, then I'd be surprised. Surely the air inlet of the engine should be less restrictive than the small carter ventilation pipe that passes blow by gases into the inlet manifold? So why then is it sucking air while at the same time cylinders are producing blow by gasses?

With turbo engines things might be different: passing blow by gases into the manifold while idling and into the turbo inlet while the turbo creates pressure.

Is there a PCV valve in the V6?

Just asking, not disputing.


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## Hoggy (May 8, 2002)

Hi, Crankcase emission system are "closed" & & under some vacuum from induction system, so should always be some suck there. Removing oil cap allows air to be sucked in upsetting the induction system whether turbo or N/A.
Hoggy.


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## TT-driver (Sep 14, 2010)

OK, I see. Perhaps things were slightly different on my 1974 Peugeot 204....

What then is the role of the PCV valve on the 3.2?

Apparently is can go bad, causing rough idling:

http://www.audi-sport.net/xf/threads/a3 ... nt.211026/


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## McEnroe (Oct 7, 2016)

I think your "gasket" may well be torn in the breather valve / Pcv -- or what even Audi call it- 
Poor idle - and or excessive smoke at idle from exhauste are tell tale signs- 
I have also seen the guide in the link you posted


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## Hoggy (May 8, 2002)

Hi, Years ago crankcase emissions just vented to atmosphere, nowadays recirced back into induction system & reburnt.
PCV controls the emissions flow from crankcase to induction system, so if it fails it upsets the induction system & can cause rough idle on all & a whistling sound on the MK2
Hoggy


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## TT-driver (Sep 14, 2010)

Even on my 1974 Peugeot 204 a hose ran from the valve cover to the air filter. So it burnt blow by gases. On a 1958 Peugeot 403 this wasn't the case.

But if all blow by gases need to be burned, then what is there to control with a valve? I'm asking in terms of a n/a engine.

For the turbo it's clear: boost inside the manifold shouldn't end up in the area of the engine where the blow by gases come from. So from idle to boost, the blow by gases need to be rerouted from intake manifold to turbo inlet.


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## bmcc (Mar 11, 2015)

The PCV system is pulling crank case vapors into the air/fuel mixture so the valve is there to add some control on the amount pulled in at idle/low/high throttle (high low vac conditions). You would want minimum vapors at idle for smoothness of idle & deceleration and max vapors on full throttle. It's like a controlled air leak but one that you know will occur and can be handled in the fueling etc.


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## Hoggy (May 8, 2002)

TT-driver said:


> Even on my 1974 Peugeot 204 a hose ran from the valve cover to the air filter. So it burnt blow by gases. On a 1958 Peugeot 403 this wasn't the case.
> 
> But if all blow by gases need to be burned, then what is there to control with a valve? I'm asking in terms of a n/a engine.
> 
> For the turbo it's clear: boost inside the manifold shouldn't end up in the area of the engine where the blow by gases come from. So from idle to boost, the blow by gases need to be rerouted from intake manifold to turbo inlet.


Hi, Whether Turbo or N/A there will always be crankcase vapour/emissions, but on a healthy engine these should be minimal anyway.
"Blowby" gases usually means engine whether Turbo or N/A is in a poor state of health.
Hoggy.


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## TT-driver (Sep 14, 2010)

thanks Hoggy and BMCC.


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