From the Shop Floor: The Phantom Misfire Case
From the Shop Floor: The Phantom Misfire Case
I remember a 1997 GMC Jimmy with the 4.3L V6 that came into my bay with a persistent, random misfire that no one could solve. The owner had already replaced the cap, rotor, plugs, wires, and even the fuel pressure regulator. The codes were intermittent and pointed to multiple cylinders. After confirming fuel pressure and spark were mechanically sound, I hooked up my scope to the injector drivers at the PCM. Sure enough, the signal to injector #3 would drop out randomly. We installed a correctly programmed PCM, and the truck ran perfectly. It’s a classic example of how a failing computer can mimic other component failures, costing you time and money.