Quad-4 (1228707 ECM) Rough Idle Problem

[Will Remaklus <remaklus@attglobal.net>]

Greetings,

For the past month or so I have been working on trying to fix a rough
idle problem on a 1991 W-41 Quad-4 engine.  In the early 90s I replaced
the stock Quad-4 in my 1988 with a 190 HP W-41.  This involved major
changes to the wiring harness, as there are significant wire routing
differences between the two years.  At that time I also installed a
4 into 1 header with the O2 sensor on cylinder #2.

To make a long story short, getting the car to pass the North Carolina
emissions test has been problematic for the past five years.  I
generally fail multiple times before getting the car to pass and
there does not appear to be a good reason why it eventually does
pass.

Currently I have HC > 1000 along with what sounds like an idle
miss.  So far I have had the fuel injectors blueprinted, and
replaced the ignition system (coils, housing, DIS module, etc.)
and ECM with spares that I have accumulated over the years.
These changes gave no noticeable improvement to the problem.
Here are some other facts relating to the problem:

  * Leakdown test shows all cylinders equal.
  * Compression is roughly 200 in all cylinders.
  * Replaced O2 sensor with a heated unit positioned at the
    collector.

To further diagnose the problem I wired up an ALDL to RS-232 level
converter and collected data while driving, idling, etc.  Since I
have never seen the 1228707 datastream documented, my decoding of
the data may be suspect.  In any case, I think I have correctly
identified most of the non-bitmapped fields.  Here is what I have
observed:

  * Idle RPM has variations of +/- 75 RPM or so.
  * Advance at idle is 11.5 - 21.5 degrees, varying in an
    apparently erratic manner.

Since the 2X crank signal is only twice per crank revolution, I added
a 24 slot wheel with an IR LED/Phototransistor pickup to the crank.  The

additional detail shows that engine RPM is varying with a period of four

crank revolutions.

Any idea what is going on?  More importantly, can someone please explain

why the ECM is varying the advance at idle by 10 degrees?

Will Remaklus
Raleigh, NC

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