Engine miss, weak spark, now what?


Hello. I have a 1997 Ford F150 with a 4.6l v8 with about 80,000 miles. It recently began to run rough and the check engine light came on. Autozone pulled the codes or me which was just the P0301 code which indicates a misfire on cylinder #1. So I bought a spark tester that you basically just touch to each wire and it flashes if spark is present. It flashes when touched on the #1 spark plug wire, but very dimly. It flashes a lot brighter on all of the other spark plug wires. I tried replacing both the wire and the plug, no difference. Does this mean the coil pack (I think that’s what it is called) is bad? Also I have two coil packs, one on the right and one on the left, are replacements specific from one side to the other?




A V8 has 4 coil packs. Two wires run off each. The fault may be with the coil.

If in doubt as to which coil to replace or at least have tested, follow the #1 cylinder plug wire.


Thanks RoadRunner. I saw in the repair manual where it indicated that the #1 and #6 wires were paired together in some way, so maybe there is some sort of cover over the two coil packs that makes it look like one coil pack with 4 plug wires coming out of it. If the coil pack was bad would both the #1 and #6 show weak spark? I only see a weak spark on #1.


Your truck only has 2 (two) coil packs. Each coil has 4 (four) terminals and fire 4 plugs 2 on each bank. The information from the earlier poster is incorrect.


Roadrunner is not wrong, its just how you define coil pack. What you see is a coil pack assembly with 4 terminals.


Now you and Roadrunner are both wrong! The truck has 2 coil packs only. They are the same right to left. Each coil has 4 terminals on it. They do not come apart. I have provided apicture to support my statement.


My Lincoln Mark with the 4.6 has 2 coil packs; each with 4 terminals.
Any one terminal bad on either coil pack means the entire coil pack must be replaced.
Mine look like the posted pic.




The quad-coil packs are identical. Swap the coil pacts. IF the weak spark moves, then, the fault is in the coil pack. If not, it’s time for mr. digital voltmeter and mr. wiring diagrams, or mr. repair manual instruction.


Thanks for all of the replies, especially dartman and OK for the pic and link. I’ll probably just go ahead and get a new one and hope that takes care of the miss.



Just for reference, a slight misfire due to a spark plug or plug wire can knock out a coil over time.

It’s like a line of dominos. Push one and they all start falling.
Mifiring plug damages plug wire (of which there have been some problems with 4.6 wires anyway) and this in turn can knock out a coil terminal and on down the line may even get the ignition module.


Thanks for the additional info, I had no idea thats what happens when a misfire starts. I’ll definitely replace the spark plug and wire for that cylinder when I replace the coil pack. I’ll tacklet the rest of the plugs/wires once the truck is running right again. Hopefully, the coil pack is as far back as I’ll have to go.


Don’t replace just one plug, regardless of running condition. If you’re going to do one, do all. It’s just like brakes and tires, you never do one, always two, or four, in axle sets. You don’t need one new plug and seven old plugs in this engine. You’re asking for issues.

And same goes for wires. All, not one. Always. It may even serve well to do both coil packs. The coil pack setup on my engine is solid. It’s three coil packs as one unit. They can’t be seperated.

ok445: I didn’t realize that bad plugs can foul wires and coils. But once I saw you say it, I thought about it, and it makes perfect sense. So now I’m wondering, can bad coil packs foul wires and plugs? Do the dominoes tip in both directions?

Appreciate it!




OK4450 is exactly right. When a plug fouls the spark will seak the easiest route to ground. Usually through the plug wire. As the plug wire breaks down the coil will start arcing to itself. This internal arcing will not only burn up the coil but also the ignition module. To answer your question about the reverse effect it is also true to a point. A failing coil pack (one that doesn’t provide spark) will obviously cause the plug to foul do to no source of ignition for the fuel/air mixture. Replacing the coil alone will not fix the plug. The fouled plug may attempt to clean itself but it will never function properly. The wire should not be affected by a failing coil. That is my thought but I would like to hear OK’s response on the matter as well.


I ask because the three coil packs I mentioned on my car, were failing when I bought the car. I just drove the car anyway because I had no choice, but anytime I passed half-throttle the engine would misfire like it was posessed. I actually got a fairly nice whiplash from this once.

It took me some time to track the problem to the ignition coils. First I had to rectify a bad O2 sensor, a bad cam sensor, a bad cam sensor magnet, a worn timing set, worn out plugs, and worn out wires, oh yeah, and a broken wire running to the knock sensor. After all that was fixed and the problem was not elimiated, I replaced the coil packs, and the problem went away immediately and has not come back.

But I can still noitce some misfiring and hesitation under heavy throttle, generally past 3000RPM. And the engine doesn’t idle as smoothly as I wish it would, it’s definitely not firing on all cylinders, at least not consistently. I ran those brand new plugs and wires on the bad coils for about 5,000 miles, maybe a little longer.

Seeing as they’re coming up on 20,000 miles old (in under one year too. Dayum lotta driving!), I’ll probably be replacing them soon anyway. But when I pull them and compare them to the color spark diagnosis, I’ll wonder if I’ll see anything unusual as result of running them with the bad coils.



Actually, there is nothing I can add to Dartman’s comment here. It’s all word for word right on the money.

When a plug misfires even one time that spark is going to try to go somewhere and if it can’t jump the plug gap then it will try to burn it’s way through something else.


This is a major disadvantage of coil packs over the old style coil. The old style coils were oil filled so if they arced internally, the oil repaired itself. I’ve seen a lot of old hot rods back in my youth that used epoxy coils, but one misfire with those and the coil started degrading quickly. Epoxy doesn’t repair itself.


There are two coils inside that assembly, I see that as two coil packs in one assembly. Its just how you see things.


The coils are bidirectional (can’t think of a better term here) in that they create a + and - spark, one at each end. It sends a spark to each of the two cylinders at the same time, but one is called a wasted spark. It occurs at the top of the exhaust cycle where there is nothing to ignite. If the coil has shorted at one end, not a complete short but a turn to turn short or worse, a layer to layer short, it reduced the number of turns at that end of the coil, therefore a weaker spark.


The coils on my old Harleys are oil, or tar, filled and still working at 60+ years of age.
The plug wires on those are anchored in the oil/tar/whatever and locked in place with packing and packing nuts.

When removing or installing plug wires the coil must be powered up for at least 5 minutes. The coil warms, the tar softens inside, and the old wires can be removed or installed easily as the case may be.
The originals seldom fail but some of the aftermarket ones that were not oil/tar filled were prone to failure as you say.