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

William Jahn willjahn975 at gmail.com
Thu Apr 4 10:58:39 PDT 2019

What I said about the change in advance while using a timing light and then
pulling the cans vacuum line off was only to demonstrate that with a
working can and the throttle was open enough to apply vacuum to the can you
would see a change in the timing mark position. I only brought this up
because I was trying to figure out IF a working can might solve the
unsteady rpm climb from 950 to 2,100 rpm no load condition I am having .
The vacuum can I have which is the only one I have does in no way work, I
can see the diaphram is torn in fact I can blow into the line and air comes
out the arm side. I worked on cars and trucks for over 50 years and there
are all sorts of vacuum cans that work heaters and A/C systems , they are
either working or not. I used to use an A/C vacuum pump to locate vacuum
leaks on Fords since there were so many cans and lines and valves. I
understand how the D-jet works yet have no way to know if my system is
running rich or lean or why the IAD temp sender makes such a difference.


On Thu, Apr 4, 2019 at 8:25 AM Jim Adney <jadney at vwtype3.org> wrote:

> On 3 Apr 2019 at 17:10, Jim Adney wrote:
> > 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 just got confirmation that a richer mixture burns SLOWER.
> So, at least in the range where the engine is reasonably running, making
> the
> mixture richer won't compensate for a lack of advance. If anything, that
> would make the timing worse.
> BTW, at one point William said that his vac can was broken because he
> could see the broken diaphram, but he later said that he was sure that he
> could demonstrate a bit of vacuum advance by a slight change in advance a
> bit above idle when the vac adv hose was pulled off.
> These 2 observations are contradictory. There's no such thing as a weak
> vacuum advance. Check it by sucking on it with a small hose. If the arm
> pulls
> in and the can holds vacuum, it's good. If the can holds vacuum but the
> arm
> doesn't pull in, the can is good but the points plate is stuck.
> If you can't hold vacuum in the can, the diaphram is broken and you need a
> new can.
> The cans are either good or bad. There's nothing in-between.
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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