[T3] '72 FI won't idle down

[Jim Adney]

Hi Dave,

Sounds like Gemini found one of my old posts claiming that there is a 
restrictive port in some of the IAD covers. I just went downstairs to check, to 
make sure I wasn't just depending on old, undependable, memory. I have a 
pile of IADs that I looked thru. Here's what I found:

All the IADs have a 311 133 005 cast into the bottom side. This is NOT the 
IAD part number; it's the internal VW # for the casting that the IAD was 
machined from. All the IADs have a letter suffix. Once machined, and the 
cover added, the part # changes, depending on what the finished part is 
intended to fit. Over the years, the casting had to change, as options were 
added, but the castings were always made so that they could be machined 
into the current version or any previous version.

This was common practice at VW, and probably elsewhere. If you think 
about it, it makes sense. VW had to keep track of their inventory, including 
all the raw and "raw" material that would eventually become parts.

For your '72, I believe your engine should be in the range U5000001 to 
U5057000. The IAD for engines in this range that were mated to manual 
trannys is a 311 133 005 Q (or maybe O, it's hard to read.)

If your engine # is different, send it to me and I'll try to figure out what you 
need, but I don't think there will be any important difference. It's clear that 
you have the right IAD, whatever it is.

Now, to the nitty gritty. For '68-71 FI engines there is NO PVC pipe and NO 
hole in the cover. The '72-3 IADs are all easily distuinglished by the 
presense of that odd PVC pipe. The 2 that I opened had the large 8 mm 
(5/16" = 0.312") opening, as you descirbed. All the others, I think I have 5 
with the PVC pipe, are equally easy to blow thru that pipe. 

'72 is the ONLY year that Type 3s used the dual advance/retard vacuum 
can, so ONLY those 2 IADs (one for MT, one for AT) have the vacuum ports 
for both. I have one '72 AT IAD, but no '72 MT IAD. The AT IADs all have a 
special, spring loaded lever arm for the throttle cable, so that's easily to see, 
and it's not possible to switch that arm over without major work. So you have 
the right IAD.

Looking at the parts list, I was surprised that there's no separate part # for 
the cover. The cover and body of all the IADs were sold together, as a single 
assembly. While your cover might have been swapped, I'm quite sure there 
are only 2 covers: with and without the PVC pipe.

Oops, I JUST found a third cover in the microfilm. It was the one used in 
'68-9 for the optional cold start valve. That one must have had a tiny orifice, 
but it did not have the PVC pipe. Instead, it just had a small tube, at the rear 
of the cover, for the undersize fuel hose to clamp on to. I've never seen one 
of those, and you don't want one.

I'm sure you have the right parts for your car.

There's one thing that bothers me about the PVC system. It seems like 
there's no limit on the amount of air that can be let in from the air cleaner to 
the vent ports on the '72-3 heads. I'd think that some restriction there would 
be helpful. The PCV system only needs a small amount of outside air to help 
carry out blowby gasses and water vapor.

What happens to your idle speed if you close off both of those hoses?

Other cars that i have had from that era had a flame arrestor in the breather 
hose, presumably to keep a backfire from igniting combustable gasses in the 
crankcase, causing an explosion. The arrestor I've seen seemed to be a 
tangle of copper wire, that would have imposed a small amount of flow 
restriction. I've looked for such a thing in my engines, parts book, and 
microfilm and haven't found any sign of them.

Finally, I should add that I used to have a FI MT '72 with all the equipment 
that your engine has. Over the years, I removed some of the extra stuff that 
imposed limits on the vacuum advance and on decel, but I never had the 
problem you're describing, and I never messed with the PCV system; it 
remained stock.

Currently, I have a '73 with the same PCV system. It's still stock, except that 
I had to replace the 2 red breather hoses to the heads, simply because they 
had been abused and crushed. Other than that, that engine has never been 
touched. That car also has none of the symptoms you have.

My advice remains to look for leaks before the PCV valve: breather box, 
breather stand, valve covers, AAR gasket etc. I keep trying to imagine what 
kind of air leak would be okay 30 minutes after a shutdown, but then, slowly 
or quickly, open up. So far, I've come up blank.

I look forward to seeing you and everyone else at the Invasion. We can talk 
more there.

Jim

On 11 Jun 2026 at 23:11, David wrote:

> Today I had several nice long conversations with Gemini, and I learned a few things that clarified to me what the 3mm restriction is that talked about in these reply´s. I know Gemini isn´t always correct, so correct me if needed.
> 
> Gemini said that only the ´72 & ´73 ( newly added PCV system to existing engines ) IAD front cover plates have now been repurposed with a very small restriction designed into them that regulates the flow of air being sucked into the IAD through the ´72 & ´73 only PCV valve. This same front cover was used in pre-`72 models for a different purpose (non PCV engines), but in order to use this front cover this time to regulate air flow from the PCV - they needed to neck down the flow of air being pulled through the wide open PCV valve. This is where MY ´72 system has a problem because my engine actually has a pre-´72 cover plate with the .312" "orifice": so it is trying to suck way too muh airthrough the PCV valve! No wonder I have never been able to get this engine to idle down correctly.
> 
> And here I thought I had every correct ´72 component on my ´72 engine. It turns out the ´72 & 73´ IAD has the same part number on the casting, but VW changed the front cover plates starting in ´72, even there is no part number stated on the cover plates..... They simply used the correct cover plate for the PCV equipped cars. In my case - someone changed the IAD cover plate & I did not know the difference.
> 
> So this is where this 3mm orifice restriction applies. I´m going to figure out the best approach to getting this restriction into my engine..... probably along the lines of what Keith did. I do not want to destroy my PCV valve because they are unobtanium, so I´ll add the restriction elsewhere.
> 
> Interesting thing is Gemini said (several times) that the restriction orifice was 4.5mm in diameter, so if there is anyone out there that has a ´72 or ´73 end cover plate in their possession that can measure this orifice: that would be awesome information to have. The catch here is the cover plate would need to off the engine and separate from the IAD in order to measure this little orifice..... But I thought I´d ask 
> 
> Dave Pallo
> 
> ´72 Square ~ Elwood
> 
> Fairport, New York
> 
>  
> 
>  
> 
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-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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