Details matter

About two months ago the Super Stalker start behaving badly.  The engine was misfiring constantly under heavy load.  The ECU was showing a “3X reference circuit” diagnostic trouble code.  A quick review of L67 information showed this was normally attributed to a failed crankshaft position sensor (CPS).   As this engine has about 80K miles on it I didn’t think a failed CPS was that far fetched so I replaced it.  This is not a really difficult job, but does take some time, especially when I need to squeeze mechanical work around family commitments.  After I replaced the sensor, the engine ran perfectly and I went on a few long drives, total of about two hours running time, then the problem came back with a vengeance. I couldn’t even get out of the garage.

The ECU wasn’t logging any codes so I assumed this  was a new problem somewhere else in the ignition system.  I went through the ignition system with a fine toothed comb, but could find no problems.  After SEVERAL hours of trouble shooting spread across six calendar weeks the ECU finally threw the 3X reference circuit DTC again.   I couldn’t really imagine that a brand new sensor had gone bad already and the wiring harness checked out fine to the Ignition  Control Module (ICM) checked out.  I realized the reason the DTC had not been thrown earlier is that it takes 30 seconds at RPM for the ECU to latch the code, and I hadn’t run the engine that long while I was trouble shooting.

Dennis sent me a schematic of the ECU harnessing.  This showed the 3X reference circuit being carried on a line from the ICM to the ECU.  I had not previously checked around the ECU since this really seemed like an ignition problem.  On examining the ECU I noticed that the connectors to the ECU were not firmly seated.  I reseated the connectors and the problem went away.  At that point I remembered that I had gone into the scuttle while I was replacing the CPS to take care of some other minor improvements.  I had removed the ECU when I did that.  So replacing the CPS probably did not actually fix the problem, reseating the connectors on the ECU fixed it and I was fooled by coincidence.

The reason the ECU connectors were coming out is that I had never put in the M6×45 retaining screws that would hold the connectors to the ECU.  They are in there now, and I have had several trouble free hours, and the engine is running far better than before.  I think that some behaviors I had been ignoring, like poor return to idle, had actually been caused by the loosening connectors.  Details count…