I have a 1998 Lincoln Town Car with about 350,000 miles on it. This morning I sensed a slightly rough idle when I started it up and let it sit in the driveway. While idling I plugged in a scan tool and after a few minutes it showed a P0306, a misfire in cylinder 6. I know there could be a few causes of this but my past experience has been a bad ignition coil (this car has the coils on each plug).
Over the years I’ve had 3 coils go bad, all giving the same rough idle symptom and P030x, where x was the cylinder number. Replacing the coils solved the problem each time.
So this morning I assumed it would probably be the same. On my way to the auto parts store (4 miles), I monitored the computer, it kept throwing a P0306 (cylinder 6) and I kept clearing the code. About 2 blocks from the store, the car seemed to be running a bit rougher. When I pulled into the lot, I got a P0307 code (and no P0306). Hmm. Well, I bought a new coil and replaced coil 6. I then sat in the lot and idled it and after a few minutes it threw the P0307 again. I cleared it a few times and it came back.
I assumed there was about 0 chance that a second coil could go out within minutes of another (assumption from a car tinkerer, not a professional), so rather than buying another coil, I drove it home to do some further testing. On the way home, it kept throwing the P0307 and nothing about P0306.
So at home, I swapped coil 7 and 5, idled it for a few minutes, and, sure enough, it threw a P0305 (the problem had moved to cylinder 5, following the coil). This indicates to me that I had 2 bad coils?!?
So my question is whether it’s possible, or even common, to lose two coils at virtually the same time. I should also point out that before today I did not notice rough idling and there were no computer codes set. I also thought maybe it’s possibly a problem developing with an ignition module (but I’m not sure if my car even has one).
Anyway, it was late and I have not replaced the second coil. I did do a couple other tests. I measured the resistance on the primary of the new coil, as well as 5, 6, and 7 coils. They were all close to 0 ohms. The resistance of the secondary ranged between 5k and 8k depending on the coil. Out of curiosity, I also did a final swap of the original coil 6 with the “faulty” coil 7 now in 5 (follow me?) and it still throws a P0305, indicating both the old 6 and old 7 coils are, indeed bad. Another note is that both “bad” coils are about the same age (maybe 150k-200k miles on each), so they’re not newer ones.
Does this all make sense? Should I go ahead and replace the other coil and see what happens, or should I check something else? Thanks for any input!