Date:         Wed, 3 Nov 1999 19:35:09 -0800
Reply-To:     Mark Keller <kelphoto@ISLANDNET.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mark Keller <kelphoto@ISLANDNET.COM>
Subject:      ECU Logic for surging, bucking, and low power.
Content-Type: text/plain; charset=us-ascii; x-mac-type="54455854";
              x-mac-creator="4D4F5353"

I've watched with great interest the data points of various list members
on the subject erratic engine operation and power. My experience
follows.

For the past eight months, my 91 Vanagon would intermittently, have a
low power fault.  The constants were: cold air or rain, and the engine
would only have enough fuel supply to idle.

As a result I would turn off the key and back on.  Sometimes one or two
cycle would work sometimes ten to fifteen.  Eventually the erratic
behavior would quit. It would never fault unless it had rained.  It used
to take three days of rain, then two and now as soon as it rains the
trouble begins,

Today I took the van to a local VW shop who gave me his variation of the
problem.
He check the timing first and found that  some times the ECU would try
to correct a "RPM" or other parameter by adjusting the timing.  I.E. the
ECU logic indicated that a timing correction was needed.  The tech
demonstrated the engine in normal mode, and then the erratic problem
would be proceeded by the timing mark gradually shifting clockwise,
until I guess the ECU would "give up" and the engine would display the
low power systems.

He said that it appeared to him that my secondary air adjustment screw
was backed out a 1/4" too far.  He said that the function is to adjust
air flow to the AFM vane, which in turn determines where the wiper arm
"rests" at idle.  According to him an optimum position exist so that the
ECU can adjust parameters, like timing and etc.

He said that he only adjusted the secondary air.  The engine runs
differently but it not fixed.  The erratic problem is much less
sensitive.   I believe at this point that the adjustment increased the
ECU ability  to deal with a problem, but I still have a problem.

My question is what sensor inputs would the ECU logic try to adjust
timing as a corrective?

I believe that several faults could be considered:

    a wiring harness/ connection fault at either the sensor or the
distributor
    a faulty device either sensor or distributor.

Finally, my thought is that since the wide variety of list member
experience in the ECU controls doesn't point to any one cause, a better
system of diagnosis would be a logic flow chart.

I believe a flow chart  would help because the "confused ECU" seems to
be a kind of service engine light.  It appears that if we could readily
track what parameters the ECU is failing to control then we could
isolate the components of that parameter, i.e. wiring harness,
connector, sensor or control device.

Sincerely,

Mark Keller
91 Carart "Lazarus"