[T3] '73 Idle Test

William Jahn willjahn975 at gmail.com
Fri Aug 23 17:29:38 PDT 2019

Thank you Jim.

 I have the proper intake gaskets I bought a set from you in May had the
proper ones before as well with the plastic spacer thin paper gaskets.

 I changed all the vacuum lines and runner short hoses lined up the runners
to the IAD  so they fit perfect . Changed the spark plugs. I also knew the
electric AAR was not closing so I took it all apart and cleaned out the
surface rust in the rotating valves bore and adjusted the spring so it
would close sooner and lightly lubed the valve now it closes. It also
appears with the air flow through the AAR prevents the AAR from heating up
fast enough. Test was apply 12 volts and time it off car yet on a steel
vice to simulate the heat draw from the case. 7 minutes it's closed @ 80F
on the car air flow it can take 20 minutes.

 All I know is this.
1)  start the car starts right up yet the idle is 850 RPM after 2 minutes
the idle rises to 1200 RPM . This 2 minute time frame. If I start the car
and have the intake temp sensor plugged in I didn't check the RPM yet it
runs fine once the 2 minutes pass the idle drops 200 RPM and the erratic
miss fire begins , unplug air temp and the idle comes back up the 200 RPM
and the miss ends in a split second.

2) I imagine the 1200 RPM idle has to advance the timing a bit haven't
checked yet might compound the problem.

3) I checked the resistance of TS1 air temp from all the charts I have
found all agree that TS1 is in spec. I drove the car a few days ago I knew
@ 85* F TS2 head temp always reads above 2400 ohm on that short drive I
noticed it didn't idle as smooth. The very next day I checked TS2 again
same temp 85* F read 800 ohms lower . I know TS2 drops from 2272 ohm @85*F
to 77.5 ohm in 2 minutes don't know what the actual head temp is.
4) It seems this TS2 in my car has an issue , it is tight and it does read
the same ohm on the head and case and sensor case. The ohm meter reads
correct .
 It seems to run rich at startup and I can feel it and may be why the RPM
is low until it drops resistance no idea why the 1200 RPM. From all the
charts and graphs TS2 @ 85*F should be closer to 1600 Ohm and be consistent
I did get a replacement since Bosch now longer offers these Uro parts 311
906 041A. I have not installed it yet it does read the proper ohm @ temp. I
have no idea if this will change a thing I can't imagine how unless it does
not bottom out so fast. I know people ballast these I have had no luck
doing so at least with the old ones which are 0 280 130 003 I have a 012
and it read over 3k ohm @85F.

 I never had this issue before , none of it. And still have no solution.
All the work changing lines and vacuum hose , gaskets injector seals didn't
change a thing. All cylinders drop the same RPM hot and cold pulling plug
wires or injector plugs. Can't find one issue with the engine wiring , have
the proper ECU, MPS and TPS the TPS is adjusted proper and all aspects
works without fail. I can adjust the idle below spec to almost stall out.
Even sealed the metal IAD cover with a new gasket.

 All I have not done is clean the PCV and replace the hoses from oil bath
to heads.


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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