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" |
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