[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 

Sorry if this has already been asked and answered.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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