Re: 75 twinspark questions (long)

[David Masters <david@xxxxxxxxxxxxxxxx>]

Rich (very correctly) replies to Jonathan:
Jonathan wrote:

>Now that I've done almost all that I can think of to get my 75 to run
>well, its time to ask for more help. The car seems to misfire when the
>throttle is wide open at low revs. From idle it takes more than 3
>seconds to get to 4000 revs. Does this seem normal?
>The misfiring goes away above 4000 revs, or by backing off the throttle.

Your car is exhibitiing symptoms that can be caused by two problems:

1)  Lean mixture.  In an injected engine, it's very difficult to tell when
the mixture is lean.  In a carbureted engine, a lean mixture usually
results in a rough or stumbling idle.  In an injected engine, this is
uaually <not> the case.  There is usually just a reduction in idle RPM.  If
the car has an idle control servo, lean mixture may be undetectable.
[
]  DJM:  you can still read the voltage on the O2 sensor at idle to see
 if it is lean.  On the AFM "looser is leaner" on the screw.  The idle
 control will keep the idle spot on 850-900, as you say.

Make absolutely sure that there are no leaks in the intake plumbing:  no
cracks, no loose connections--especially in the plumbing between the air
flow meter and the throttle body.  The typical symptom of lean mixture in
an injected engine is stumbling under load, just as you describe.  If your
car has L-Jetronic, the mixture can be adjusted at the air flow meter.  A
failure of the meter itself, like a sticking or stiff flap, will cause
excessively lean mixture during transients and result in stumbling.  If
your car has Motronic, the air flow meter is much less suspect.

Also check the O2 sensor.  The situation you describe is dynamic, and so
the injection system should be running open-loop.  However, since the O2
sensor has a great deal of effect on mixture, it's worth checking.
[]
DJM: Ensure that at steady running on the road you are getting
oscillation of the voltage on the O2 sensor.

Don't forget to take a spark plug reading.  Very white deposits on the
center electrode and insulator are indications of lean mixture.
[]
DJM: So simple and so true!

2)  Loss of spark at high pressure.  As the manifold pressure in the engine
rises, there is more resistance to arcing between the spark plug
electrodes.  If your ignition system is already compromised in some way,
it's possible to have sufficient spark at low manifold pressure (closed
throttle at low RPM, steady throttle at high RPM) but then have the spark
interrupted at high manifold pressure (open throttle at low RPM).  This
situation can be caused by too much total gap in the ignition system.  Make
sure the spark plugs are gapped properly--not open too far.  Make sure that
the center electrode of the distributor caps isn't worn out and no longer
making contact with the rotor.  (I have seen this happen--more than
once--and result in exactly the symptoms you describe.)  Make sure the
rotors aren't badly eroded, increasing the gap between their tips and the
distributor cap electrodes.  Make sure the distributor cap electrodes
themselves aren't badly eroded.  If the caps and rotors have been on the
car for more than 50,000 miles, consider replacing them.  I have also seen
Hall Effect sensors go bad, providing a weak signal to the ignition module.
 I don't know if your 75 uses one, but if it does, check it.  I've fixed a
number of VW products by simply replacing the distributor rotor, which
carries four Hall Effect strips.  Last, and least likely, check the
ignition wires.  I have yet to see a set really fail, but it's certainly a
possibility.  In your case, I would suspect the wires running from the
coils to the distributors the most.
[]
DJM:  All true.  YMMV but in my experience the spark wires are very good.
The rotor buttons don't come off, the whole shaft and button is changed. I 
could only get aftermarket ones from Facet.  Works fine.  The guy said no 
one changes them unless they break, but mine was 10 years old.  Simple to 
change.  For distributor caps I have used Bosch, but there are probably 
other aftermarket ones around.


[]  DJM:  That wire on the coils is for the tacho.  Is there a chance that 
you have connected the feed loom to the wrong coils?  They each use a 
different pin to get a signal from the ECU.  While my dizzy on the head was 
broken, I could only get a good idle and steady running.  Acceleration was 
poor.  All I had was spark that was timed for the back plug.  The timing 
for the front plug is different, I presume.  The coil wires of each dizzy 
go to front and rear plugs respectively. Apparently the later twin sparks 
do it differently?

Both dizzy caps and rotors seem worn, but I couldnt pull the rotor off
to see better. How do these come off? Any tips for cheap alternatives a
la 635csi dizzy cap fits 164 and is cheaper?
[]
DJM:  How about the AFM, have you checked that it moves freely, and have 
you back probed it (Pins 4 and 5 from memory) with the ignition on, to 
ensure linear output.

[]  Keep Diagnosing!!

Cheers,
david
Newcastle AUSTRALIA.

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