Bypassing the blowby gas system to a catch can?

Richard Tanimura rictan302 at
Wed Jun 2 01:38:38 EDT 2004


I have an S2. Being a turbo, it runs higher pressures
and has more blow by (probably). I don't know if you
do but it also has a MAF.

I am going to put a catch tank in my car. I might also
try and put some negative pressure in there :-)

I will route the gas back into the intake tract down
stream from the MAF since it has already been metered
once. I don't know how much volume it will be but it
will ensure that I am running pretty close to the what
my MAF is metering. 

As you have understood, venting to atm. means you lose
some metered air. But if the volume is small, it may
not matter.


--- Will Ng <wng-audi at> wrote:
> The oil in the intake system, the constant vacuum on
> the rear main seal 
> causing that nasty screeching noise, and that mushy
> oil-soaked rubber hose 
> just waiting to burst, have me wanting to fab a
> catch can setup to route 
> the blowby gases to it instead of back into the
> intake. I would probably 
> vent to the atmosphere after the catch can. (don't
> tell the EPA)
> And plug up the opening at the rubber air horn.
> Besides removing the oily vapors that can cause
> detonation, it'll stop the 
> rear seal problem. I'm probably running lean because
> of a vacuum leak 
> there. I can hear occasional detonation when I
> accelerate even when the air 
> temps are cool (60F). But the car idles fine at
> 700rpm.
> Would removing the blow by from the intake cause
> running problems? My 
> concern is whether the mixture ratio be altered
> since it'll be only fresh 
> air passing through the intake.
> Thanks,
> Will
> 1990 200q @ 206k miles.
> _______________________________________________
> quattro mailing list
> quattro at

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