[T3] '73 Idle Test

William Jahn willjahn975 at gmail.com
Fri Aug 23 18:21:15 PDT 2019

There is so much on my mind trying to figure this out. The original MPS
that came in my car was screwed together and looked old it had a slotted
screw under the black cap. It did work for a good long time VW# 311 906
051E bosch # 0280 100 106. In the car now Bosch # 0 280 100 116 no VW # .
They all look identical other than the screws in the old one. The one
before was VW 311 906 051D Bosch # 0 280 100 101. As I said replacing the D
with the 116 bosch made a little difference no one would notice other than

 I am certainly open for ideas . I used to run the fuel pressure @ 28 PSI
for a long time yet was told it should be 30 so that's where it is didn't
make any difference. I did try shutting down the pump used a switch with
fuse # 7 out across the fuse terminals to turn on/off the pump yet I didn't
really feel any change it didn't run for a minute though say 30 seconds it
used run for at least a minute maybe even longer since I used to have a
kill switch and would forget to turn it on I could get out of a parallel
street parking spot and move a bit before it quit. Yet back then I didn't
need a primp switch it would start after sitting a week just by turning the
key. It went on like this until 2014.

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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