[T3] '72 FI won't idle down

Tue Jun 9 22:46:14 PDT 2026

Lots of good data from Dave, and lots of good comments from others.

Maybe I can add some more.

There are lots of possible leaks into the intake that can cause this. Offhand, 
here's a list, likely incomplete.

Intake air runners not well aligned with the IAD, causing leaks at the large 
rubber hoses. Hose clamps there are a common and misguided cure. Don't 
do that. I've written about how to align the air runners, but ask if ....

Wrong intake gaskets used between the air runners and the heads. The 
steel gaskets that come in all the gasket sets are for Beetles; they will leak 
on some FI Type 3s. The correct gaskets are ~5 mm thick and made from 
brown phenolic with paper backing. They can be reused if scraped clean and 
fresh paper glued on.

On '72-3 cars, any leak into the crankcase will eventually get to the PCV 
breater, so make sure your valve cover gaskets are good and the mating 
head surfaces are smooth. That surface often gets crud on it. Scrape it 
clean. The usual black rubberized cork gaskets work fine as long as they are 
still soft enough that you can dent them with a thumbnail.

The 2 hoses from the valve boxes to the air cleaner need to be at least 
somewhat tight. Leaks there go directly into the case and then to the IAD.

Later replacement throttle valve switches come with an internal stop that 
makes adjustment simple. This stop can hold the throttle valve open if the 
switch is set too tight.

The AAR is commonly blamed for this, and it's true that they often stick. Test 
yours by putting your thumb over the AAR inlet hose. When the engine is 
cold this should slow down the idle speed; when the engine is hot it should 
not have any effect. If yours is sticking, I can fix it.

Dave said:

> If it's a warm engine - and then I shut it off for say 30 minutes - it will
> start right up and idle correctly for about 20 or 30 seconds, and then it will
> increase itself back up to 1300 within 1 minute or so.

This sounds maybe normal. Your AAR is slightly sticky, which is common, so 
it stays in the warm "closed" position even as the engine cools down. Engine 
vibration shakes it back to the cooler, open state.

The fact that this seems to be getting worse over long time, and that it seems 
like much of the extra air is coming from the crankcase suggests that you 
look there for leaks: valve covers, breather stand, breather box, PCV 
mounting, plastic caps on top of the breather box, AAR gasket, distributor 
O-ring, even case bolts and crank seal, but those seem really unlikely. I'd 
concentrate on the first 5 in that last sentence.

> When warm - I can put my finger inside the throat of the IAD and totally
> plug the inlet "port" for the idle speed screw. This 100% plugging will
> result in a 850 or so idle speed (with the idle speed screw still all the
> way in and throttle plate still closed).

This seems to contradict everything I've been believing, up to this point. This 
seems to point away from the PCV system and just to the idle screw not 
being able to close far enough. So color me puzzled.

The '72-3 crankcase ventilation tries to keep the crankcase at a partial 
vacuum. That means that air will be trying to get in any possible route. Most 
of those routes are insignificant, but it sounds like you have something that's 
leaking more than usual and loosening over timel.  

Bottom line: None of this will have any effect on how well the engine will run 
off-idle.

Jim

On 9 Jun 2026 at 22:33, David wrote:

> It has been about 25k miles since I restored Elwood, and I have always had
> to restrict the IAD inlet to get it to idle down to 850rpm to set the
> timing. ( yup: '72 with dual diaphragm vacuum canister w/ ports plugged).
> After every tune-up spec is set: the resulting idle speed seems to be a
> little higher than the previous last year -  to where I can't bring it down
> to spec, so I live with it. As of today the idle is around 1300 and I've had
> enough, so I'm trying to get to the bottom of it. All my FI components are
> the correct '72 versions.
> 
>  
> 
> The idle screw is just an air bleeder screw, but it is ALL the way down to
> be shutoff. Vacuum canister is new, and it properly does the job of bringing
> the idle down about 80rpm at idle when plugged in. Fuel pressure @ 30.
> Plugging the warm aux air regulator results in only a minor drop of about
> 30rpm, so that's working. I have sprayed SO much carb cleaner around any &
> all intake gaskets & seals & injector areas that I must have the cleanest
> engine out there, but zero change in rpm to find any air leaks. I even made
> a bent carb cleaner nozzle hose to spray on the underside and backside of
> the IAD. Cold start valve has never been hooked up, but no rpm change when
> sprayed with carb cleaner. Rubber air intake tubes test good for no air
> leaks as well. I have the '72 decel valve installed & working, but plugging
> it for a test showed no issues. Readjusting the throttle switch made no idle
> difference. I substituted a known good spare pressure sensor, but made no
> difference at idle.
> 
>  
> 
> If it's a warm engine - and then I shut it off for say 30 minutes - it will
> start right up and idle correctly for about 20 or 30 seconds, and then it
> will increase itself back up to 1300 within 1 minute or so.
> 
> When warm - I can put my finger inside the throat of the IAD and totally
> plug the inlet "port" for the idle speed screw. This 100% plugging will
> result in a 850 or so idle speed (with the idle speed screw still all the
> way in and throttle plate still closed).
> 
>  
> 
> IIRC - the 2 FI heat sensors have very little to do with idle speed, but
> please correct me if I'm wrong about that.
> 
> I do have a backup brain I could substitute, but I'd like to hear any
> feedback before I go that route.
> 
> Sorry for the long post. I'm trying to give all info.
> 
> Dave Pallo
> 
> '72 Square ~ Elwood
> 
> Fairport, New York
> 
>  
> 
>  
> 
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL: <http://lists.vwtype3.org/pipermail/type3-vwtype3.org/attachments/20260609/1c01404f/attachment.htm>
> _______________________________________________
> VWType3.Org mailing list - type3 at vwtype3.org
> To unsubscribe or change subscription options, visit:
> http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org
> If you need more help, contact: gregm at vwtype3.org
> 

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