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 M6x45 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…