[T3] Two Tests

Jim Adney jadney at vwtype3.org
Sun Sep 22 15:37:06 PDT 2019

On 20 Sep 2019 at 14:33, William Jahn wrote:

>  Today I looked at the PCV . it's pretty gunked up. On this site
> https://members.rennlist.com/pbanders/PCV.htm
>  It explains the PCV operation and from what I read it seems most of the
> time it stays close to closed .

Our '72-3 PCV system hardly needs any explanation. It's terrribly simple.

Filtered air gets sucked from the air cleaner into the heads and down the 
pushrod tubes.

>From there, it mixes with any blowby in the crankcase and then gets sucked 
out thru the breather box and into the IAD. Variations in the amount of air 
admitted would alter the idle speed.

The PCV valve opens under manifold vacuum, so it will be most open at idle, 
when the manifold vacuum is at its max.

One thing you could do would be to plug the pipe that leads to the front of 
the IAD. If that stops your odd symptoms, it means that there's something 
irregular going on in your crankcase ventilation system. If something is badly 
gunked up, it could be preventing the free flow of air, and only letting air thru 
in "burps" which might make the idle uneven.

I've never seen this happen, but it sounds possible, and it's easy to check.

I'm not recommending plugging that pipe as a solution; that would be bad 
and will lead to different problems. But it's worthwhile as a short test, just to 
see if the PCV system is part of the problem or not.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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