Engine misfire take 4 & 5

I previously posted about misfire problems different times in our 2002 T&C 3.8L. Those problems were solved by: replacing cracked flex-plate, replacing spark plug wires, repairing melted fuel injector wires, inspecting and re-wrapping engine harnesses and re-installing factory crank sensor (at same time so not sure which fixed it).

Now, at 199K miles and 4 months after the last fix above, my wife phoned about an engine miss while driving back from San Francisco (90 miles). She had 6 of her friends in the car at night, so they were concerned. I told her if the oil light stays off and the engine temperature is normal, to keep going, and just go slower up hills if it sputters. Also, shift to 3rd so the rpm's stay higher (lower cylinder pressure, easier to spark). I dug into it the next day.

The only code showed "misfire cyl 2". I confirmed no spark w/ a clamp on timing light (0 rpm), plus in-line spark tester (Harbor Freight). I swapped spark plugs and wires, no change. Here is the part that should help others. I measured a spark on #5, which shares a coil w/ #2. That indicated that the computer was commanding the coil to fire, so the wiring and crank sensor were fine. The only thing that could cause that was a bad coil pack (original I think). I changed it ($53 O'Reilly's) and then got a spark on all cylinders. The old coil pack measured 400 kohm from the #2 or #5 tower to its pin. The others measured > 2 Mohm, as did the new coil pack. That indicated an internal short.

A little theory, since you will never remember this trouble-shooting tip otherwise. In most/all cars w/ a coil pack, 2 cylinders that move up & down together share a coil. The computer fires both on the top stroke. One is on the compression stroke and needs firing, the other is on the exhaust stroke and doesn't. Thus the term "wasted spark system". It is the simplest distributor-less ignition control system and dates back to the 1990's. In our 6 cyl engines, the coil pack has 4 pins - 1 pin for each of 3 coils and 1 power supply pin. The 2.4L (4 cyl) has 2 coils and 3 pins. The computer shorts a coil pin to ground to charge a coil. When it halts the current, the coil fires into the 2 cylinders that share that coil. In 1960's cars, the current was stopped by points opening. In the 1970's a transistor replaced the points, which carries on today. BTW, the cam sensor is not involved in sparking. It is only used for sequential fuel injection. However, it can mess up the crank sensor since they share an 8 V power supply output from the computer.

No good deed ends so simply. The engine then ran w/ no codes, so I told my wife to give it a spin to insure it would work tomorrow to take our kid to school. She said it was still running rough. I verified the spark plug routing, and managed to knock something amiss as the engine then wouldn't start (better for trouble-shooting). The computer showed "bad injector signal cyl #3). I verified I still had spark, and the engine would run on starter fluid, but no hint of firing otherwise. I figured all the injectors must be off, like their power was getting shorted. I returned to the old "bad injector wiring" problem above. I found some exposed copper right by the harness connector that might have touched a steel wire I had secured it with (after factory plastic clips broke). I pulled the connector up high, where more accessible and strapped it to a heater hose. The engine ran great then. Someday I'll have the intake manifold off and will pull the whole fuel injector harness for a bench repair. Until then, I wrapped each degraded wire section w/ self-fusing silicone tape. I'll report back if a misfire episode #6.

Thanks for the info Bill!