[T3] I'm a bit confused as to why unplugging this IAD temp sensor clears things up.

Jim Adney jadney at vwtype3.org
Wed Apr 3 15:10:58 PDT 2019

On 3 Apr 2019 at 11:50, William Jahn wrote:

> Jim I am using all the 73 parts except the IAD , Iam using the one off the
> 72, the only difference I could find at all was the 72 did not have the EGR
> tube /port on the top 

On the '72s, that flat spot was used to anchor a throttle positioner, which 
kept the throttle from slamming shut quickly. Just a minor emissions control 

>  My main point about asking about the vacuum advance since I am now using
> the 73 AH distributor and that vacuum can is shot ,

The 73 vacuum can should have 505 stamped on the arm. Have you 
checked it by attaching a small hose to it and sucking on that hose. It will 
either leak or not. If it doesn't leak down almost instantly, it's still good. Most 
of them are still good. There's nothing useful you can tell about them while 
standing in the driveway with a timing light.

OTOH, the can can be good, but the points plate can be sticky, so it's not as 
free to move as it should be.

> My question is since the vacuum can is doing nothing even not driving
> the car with a working can  it will still advance the timing a certain
> amount , I'm thinking this is why the rpm rise is so unstable until I
> unplug the IAD temp sensor, does this sound reasonable? 

I doubt it. I don't think we have any idea, so far, of what your vacuum can is 
actually doing while driving.

> Since the IAD sensor does richen the mix a certain % and @ 1,200 rpm no
> vacuum advance then it's mech advance only and may need more fuel to air ,
> if the can worked it would be a bit more advanced and might need less fuel
> to air mix.

Not sure about this, but I think a lean mixture burns faster than a rich one. If 
that's correct, a lean mixture would need less advance, not more. I know a 
lean mixture burns hotter. Someone who knows more about combustion may 
want to step in here. I have someone I can ask about this locally. 

You may just be noticing that your engine runs smoother with a richer 
mixture. It's possible that this is what VW did to meet the emissions 
requirements. When it gets a bit  warmer here, and I have some time, I'll try 
unplugging the temp sensor on my '73 while the engine is running. It's quite 
possible that this is normal.

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

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