[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
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
*******************************
More information about the type3-vwtype3.org
mailing list