Possible for two ignition coils to go bad at the same time?

Maintenance/Repairs
1

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!

Terry

2

It’s possible.

It’s like replacing a headlight. One headlight goes out, and after replacing it the other goes out a day or a week later.

Tester

3

@cartinkerer, here’s something to consider:

I’m somewhat familiar with those engines, as our fleet’s trucks use the same engine.

Anyways, I noticed that there are early and later versions of those ignition coils. The early ones have a green “label” on top. The later ones have a black “label.” Any chance the coils that failed still had the green “label” on top?

Are you also changing the plugs at the appropriate intervals? I would think running an engine with old worn out plugs will make the coils work that much harder.

4

@db4690, ok, so do tell. Yes, both coils have black print on green labels. The label reads EE03A, F7TU-12A366-BA, Motorcraft. These could conceivably be the original coils that came with the car… not sure. I’m the second owner. I change the platinum plugs at least every 100k miles (yes, I should really do that closer to 50-80k).

5

@cartinkerer

Here’s the part number of the new Motorcraft coil. It’s different than yours, probably newer.

http://www.rockauto.com/catalog/moreinfo.php?pk=511569&cc=1305144

6

@db4690 The link you gave actually links to the exact Motorcraft replacement part I picked up today and installed at cylinder 6.

Another difference in all the new coils I’ve installed, including today’s, is that the end that plugs onto the spark plug is a full wound coil that winds up wrapping around the plug contact. All the coils I’ve removed (including today and the other one that appears to be bad) simply have a flat piece of metal as a contact that simply pushes up against the plug contact. I’ve always wondered about the quality of a flat “pancake” contact just bumping the plug vs. something that actually wraps around the plug contact and makes a much greater surface area connection.

So, my plan right now is to go ahead and replace the second coil tomorrow and see where I go from there. I can’t see a reason that replacing the other coil won’t solve the problem, but I actually wouldn’t mind having a spare coil (or two) in case it turns out to be something else.

Thanks again for your input.

7

@cartinkerer I was glad to be of help.

If you get misfires again, don’t wait too long. It could take its toll on your catalytic converter.

And I’m glad to know that you bought a factory coil.

8

I think it’s a mistake to leave plugs in for a 100k miles; platinum or not.

Aged spark plugs and/or moisture is usually what kills coils.

9

@db4690 Thanks. Yes, so far I’ve driven it about 8 miles with a misfire (MIL flashed for less than a block) and will probably go to a parts store about a mile away tomorrow so it’ll be a total of about 10 miles. Regarding the cats, I periodically have a P0420 and P0430 (cat below efficiency) but I plan on getting rid of the car this year or next so I’ll just clear those as they come. I see a cat code once or twice a month. (I drive the car around 3000 to 4000 miles per month). Can’t justify replacing the cats if I may dump the car in several months.

@ok4450, Yeah, I know… I’ve been too bad about changing the plugs. I’ll actually be doing it again this coming week. I’ve heard stories about age welding the plugs in and you then can actually damage the engine when removing old plugs. I AM still good about oil changes (at least once a month). My old '98 Cadillac Deville had over 750k miles on it when I finally donated it, and I attribute that mileage to oil changes.

10

@cartinkerer actually, those engines have a tendency to blow the plugs out, along with the threads and the coil.

11

Just because something’s unlikely doesn’t mean it’s impossible. But it feels that way to the person experiencing it.

12

@cartinkerer, My point about the plugs being in place is not necessarily about their freezing in the head although that is a distinct possibility.

It’s that aged plugs may be misfiring and this is tough on the coil or coils as the case may be. There may be no noticeable running symptoms to the driver, no CEL on, and no codes present while it’s doing this.

Plug gaps should be inspected and adjusted as necessary as excessive plug gaps can have the same effect on the coil as misfiring. Essentially, it 's working the coi(s) harder than they should be.

To edit a bit, you might consider a compression check also based on the high miles of the engine. Weak compression can lead to misfires and right on down the line.

13

Actually it is very common for other coils to be bad when one starts misfiring. Think about it the other coils are just as old and have been subjected to all the same stuff. I have an '04 T’bird with COP’s. One went bad at about 22k miles. Then at about 50k another went, at 52K another and at 55K another. At that point I’d had enough and replaced all of them. Been fine, several years later and now at 67K miles. I figure I’ll have all the “boots” changed at about 80K miles, because I think it is the rubber insulated boots that really go bad, and then take out the coil. I’m going to test my theory by replacing the boots preemptively every 40K miles.

If you are thinking of changing out the plugs soon, get a new set of the boots from NAPA and change them out too at the same time. A set of boots is about $15.