[T3] '73 Idle Test
Jim Adney
jadney at vwtype3.org
Thu Aug 22 07:14:33 PDT 2019
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.
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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