[T3] '73 Idle Test

William Jahn willjahn975 at gmail.com
Sun Aug 25 19:40:02 PDT 2019

Jim :
I don't mind saying I'm a bit confused in the difference how your 73 acts
and mine. In mid 90's to early 2000 when driving to work it used to get in
the low 60's and I never experienced what you do, before or after
rebuilding the engine. It didn't matter what the ambient temps were it
always started and ran the same.

 You mention fuel pressure I used to always set mine @ 28 PSI and can't
recall when I raised it to 30, maybe 6 years ago. I've read about engine
wear changing the vacuum signature thus changing the way the MPS reacts.
I've read all sorts of things and read any info I can find on D-Jet and how
it operates.

 The one thing I can't do is monitor the TS2 while the engines running. I
did monitor TS1 and it appears to have no issues engine running vibration
included it reads what all the charts and graphs state and is never open or
internally shorted to ground. Couldn't find any wiring issues from each
component to the ECU plug and no short to ground there either.

 I wonder if it's at all possible for the TS2 to offer ohm readings yet be
off enough as mine is at the same temp reading 800 ohms lower and this I
could feel and if it's also possible for it to have a faulty middle range
where it's not open yet much lower than would be desired so it just leans
out at one point and stays there. You can test this yet not if any sort of
vibration affects it or if just one test in heated water or oil will always
be the same and with repeatable results.

On Thu, Aug 22, 2019 at 7:14 AM Jim Adney <jadney at vwtype3.org> wrote:

> I promised William that I would try to duplicate his idle results on my
> '73.
> This is a California AT '73 that had, at one time, EGR (Exhaust Gas
> Recirculation.) Sorry it took me so long to get around to doing this test.
> It was about 66 F here this morning when I did this. I pulled out the
> intake air
> temp sensor and then started the car. I then inserted and removed the temp
> sensor plug to see if I could hear any difference in the idle speed. No
> change in idle speed detected.
> I then let the car warm up a bit, and used my 3-way fuel pump switch to
> turn
> the pump off and on.  When this car is cold, it tends to run too rich and
> that
> causes some cylinders to flood, causing a rough idle. Turning the pump off
> allows the mixture to lean out as the pressure drops, and eventually all 4
> cylinders will start to fire. Turning the pump back on, usually causes
> flooding
> again, but after doing this several times, the engine will continue to run
> on all
> 4 cylinders.
> Note that this flooding problem and rough running goes away as soon as a
> load is put on the engine, so it does not cause a driving problem.
> With the engine warmed up enough to run on all 4 cylinders, but still not
> very
> warm, I repeated the temp sensor test. Again, I was unable to hear any
> change in idle speed as the temp sensor was plugged and unplugged.
> One thing that is very clear in all this is that with the engine either
> warm or
> cold, it idles better with lower fuel pressure, ie with a leaner mixture.
> I
> suspect this is intensional: With richer running the NOx emissions are
> reduced and NOx emissions were the big problem with all aircooled engines.
> By '73 they were struggling with our emission standards and running richer
> was one of the things they did to meet them.
> William, one thing you might try, would be to start your engine and notice
> that the idle is "rough." Then unplug the fuel pump relay and listen as
> the
> pressure drops and the mixture leans out. In my case, the idle smooths out
> and increases before the engine dies. With a cold engine, it's amazing how
> long a cold engine will continue to run after fuel pump shutdown: about a
> minute, so you have lots of time to listen.
> You can reach the fuel pump relay with your left hand while sitting in the
> driver's seat. With practice, you can unplug it and plug it back in before
> the
> engine dies. It's helpful to practice this before starting the engine.
> It's best to
> not pull the plug all the way off; just pull it far enough and tilt it so
> that one
> side disconnects. Then it's easy to push back on.
> From the way my car runs, I believe it is running rich at cold idle.
> William's
> results point to a lean mixture. My engine is a virgin: never been out,
> never
> been rebuilt. William's has been rebuilt, so is it possible that it has
> the wrong
> intake manifold gaskets, which would make it run lean? Have those been
> checked?
> Sorry if this has already been asked and answered.
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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