[T3] '72 FI won't idle down

[Jim Adney]

On 10 Jun 2026 at 22:47, David wrote:

> The PCV valve is THE regulator of how much crankcase air that gets sucked
> into the IAD. The PCV valve has a fixed diameter orifice, and what air gets
> through it - all goes through to the IAD. 

I think this is where your reasoning gets off track.

The "PCV" valve in the underside of the breather box isn't much more than a 
check valve. While it certainly has some sort of limit to how much air it can 
pass, I think that's much more than would commonly be expected. Most of 
the time, that valve is fully open and quite willing to pass much more air than 
it's getting.

I think it's designed that way to allow for wear and tear on an older engine, 
that might have more blowby and other leaks.

I just read your post on the actual size of the hole in the IAD cover. I accept 
your measurement over my old and failed memory. I think I've made that 
particular mistake before and been corrected before. Thanks! So there's NO 
restriction anywhere in the PCV system, other than the size of the leaks that 
allow outside air to leak in somewhere.  

So unexpected air leaks anywhere before the valve wll increase the PCV 
bypass air into the IAD, leaks after the PCV valve will add to that air, and 
both will consequently increase the idle speed.

Whether the throttle valve is closing completely is still a question. Wear in 
the throttle throat might be allowing extra air to get around it, but only if you 
have a later replacement TVS, which has the internal stop that makes 
adjustment easy but will keep the throttle valve open.

That can be easy to test: Just loosen the two screws that hold the TVS 
adjustment, maybe after carefully noting its position. If the idle drops when 
you do this, the throat and butterfly have worn and have been letting extra 
air in. If so, you'll need to readjust the TVS position, with the butterfly in its 
new closed position. If the idle didn't change, just put the TVS back to where 
you found it.  

If you want my faq on adjusting the TVS, email me.

One more thing: I'm puzzled by your mention of what you observe when you 
shut down your warmed engine and then start it up again after 30 minutes. 
That didn't sound quite right to me. I would not expect much cooling to 
happen in 30 minutes.

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