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

William Jahn willjahn975 at gmail.com
Wed Apr 3 15:29:06 PDT 2019

 Jim , in 2009 I removed the vacuum can and I could see the red rubber was
torn around the metal center that connects it to the arm . I just have it
there so the cans spring keeps the advance plate from moving and so there
is not a large opening in the distributor . I did try what you suggested
yet nothing changed , it still ran better with the sensor unplugged. I
didn't know the can was bad until 2009 yet I always had that sensor plugged
in . It was only when I raised the engine RPM to check the charging system
output that I noticed the issue I have, if it's an issue I can't say. The
only thing concerning is I don't want it to run lean and in turn run hot
and I have no way to tell if it's lean or rich .

 For some reason today my gmail  I use on windows 10 does not work when it
did yesterday have no idea why. If it's not one thing it's another.


On Wed, Apr 3, 2019 at 3:11 PM Jim Adney <jadney at vwtype3.org> wrote:

> 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
> > 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
> device.
> >  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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