I have a 1997 Ford Explorer 4.0 L V-6 with 211,000 miles. Motor was replaced approximately 12,000 miles ago and have had no problems until this weekend.
On a roadtrip, aren't problems always discovered on a "road-trip", the engine began misfiring climbing a moderate hill. At the bottom of the hill the engine died and would turn over but not restart. On investigating, I discovered the 25 amp ignition fuse was blown. Replaced the 25 amp fuse and the engine ran for less than 1 minute before blowing the fuse again. Replaced the fuse with a 30amp, the only other one I had with me, and managed to limp the vehicle the final 40 miles to my destinaton.

I then pulled the ingition coil and discoved it was cracked on the bottom with a solid white substance oozing out of the crack. I visited the local auto wrecking yard and purchased a used ignition coil and installed it on the vehicle. The vehicle started and ran fine and performed well on a short 20 minute test drive. The following morning, I drove the vehicle 15 miles and discovered that it ran smoother and appeared to have more power than it did previously. Approximately 15 miles into this drive, the engine once again started misfiring. Pulled the vehicle to the side of the road and with the engine ildling very roughly, checked under the hood. All appeared to be well when the engine died and a "puff" of blue smoked emitted from the front of the ignition coil.

The vehicle now sits 3 hours away from home and I am looking for any advice available. After 211,000 miles, did the original iginition coil pack finally go bad? Did I purchase a bad ignition coil pack by purchasing a used one? Is there something I am unware of in-front of the ignition coil that could be shoriting it out?

Again, any advice is appreciated.

Thanks
Glenn Siron
Yakima, WA

 

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What is the voltage to the coil when the truck is running??
I presume he has checked for any stray grounds/shorts!
Is the intake manifold under the coilpack getting exceptionally hot?? Measure it..

 

Glenn, I suspect you have a plug wire that's open or nearly so. Our Explorers use a "lost spark" system in which one bank of cylinders serves as a ground circuit for the coil pack. (Each coil fires 2 cylinders, on both the exhaust and intake strokes). An open or very high resistance wire might be causing the coil to overheat.

I'd check each wire with an ohmmeter, and look for the one with high resistance or showing an open. Replace it and the coil pack, and drive it back home.

There's an outside chance the resistor is burnt open inside one of the plugs, which would give the same open condition. Bottom line is new wires and new plugs are in order (and another new coil pack).....Resistance values are about 5K ohms for a plug, and up to about 10K ohms per foot of wire.

 

Did you ever figure this issue out? I know its been a while but I have the same issue right now

 

I would agree, replace all parts with OE ignition parts. New plugs, with proper gap, new wires, and a new Motorcraft coil.

 

I don't want to replace all that to find out it wasn't even that and then the coil pack frys again because I bought an aftermarket pack and it fried within minutes of sitting there with key on, engine off. That shouldn't have to do anything with the quality of the part and not getting Motorcraft.

 

Do you understand how a coil works?

There is nothing that can fry a pack related to the other wiring. Allow me to explain exactly how a coil works.

A coil is a bunch of wires that are insulated from touching itself or other wires. It just wraps around a magnetic core, usually iron. In these vehicles one side of the coil is hooked up to a constant Hot 12V power source anytime the engine is on. The other end of each coil is connected to the ignition computer, or more specifically solid state relays or drivers within the computer. They function as an on-off switch, they are normally interrupted or open, and when they close, the coil energizes. As the coil energizes, it builds up a magnetic field. It doesn't just suddenly become magnetized. the field actually takes a certain amount of time to build, and during this building phase, the lines of magnetism are in a state known as flux (change). As these lines cross and intersect with other wires, it causes electrons within those wires to move and follow the direction of flow. This principle allows the secondary coil, which has no power source, The secondary coil instead just forms a loop.

How coils produce high voltage is simple. The primary coil has just a few windings and a heavier gauge wire. The secondary coil then has more windings, depending on how much voltage it is supposed to produce. The magnetic flux basically produces a high electrical potential in the second coil. However, current only flows through the secondary coil when the primary coil is in a state of change. If the coil is left off, the secondary coil does nothing, and when it is left on, the secondary coil does nothing. When the coil is first energized, the secondary coil generates a voltage relative to the voltage and current input, and when the coil is first shut off, the secondary coil also generates a voltage as the magnetic field collapses.

At the high voltages produces, the electricity wants to take the path of least resistance, and it will jump through gaps and insulators if it has to in order to complete its circuit. In the case of these cars, that circuit consists of two spark plugs, two ignition wires, the coil, and the engine block. The total resistance would be the combined resistance of both wires, plus the spark gap of both plugs. The block itself offers very little resistance, neither does the coil.

So if the plugs are worn, the spark gap that has to be jumped if the gap of both plugs. This means if each plug wears .010 then the coil has to jump and extra .020. This puts a lot of strain on the insulators in the various parts of the ignition system, because not only is the coil sparking on two plugs, the way waste spark systems are set up, it fires twice each revolution, once on the end of the compression stroke, and the other on the end of the exhaust stroke.

So what happens if the resistance is too high because the spark gaps are too great, or the wires have deteriorated? The current will try to find a new path that is easier. This new path could include, jumping through the insulator on the plug wires themselves, or it could jump between windings inside the coil pack. When the energy does jump through the insulators, it scorches the insulator, which makes it easier to jump through it in the future. Pretty soon the insulator has broken down, and the coil doesn't work anymore because it has internally shorted.

Often the path of least resistance is jumping through the coil casing and grounding out on the block.

So thats a quick rundown on how a coil works, and how a coil fails.

So if you are blowing up coil packs, in means the electrical resistance it is trying to overcome is too great, or they are cheap coil packs. Using all OEM parts helps ensure proper operation. Using proper spark plugs with a correctly set gap prevents the coil from being strained, and using good OEM grade wires helps ensure that there is enough current flow and the superior insulators used prevents the wires from shorting against the block. And the Motorcraft coil packs themselves are exceptional quality. I have used aftermarket coils, including the performance coils, and the quality is inferior. In fact, performance coils often highlight any weaknesses in the ignition system, and very often that weakness is the coil itself. Most aftermarket coils are budget price competitive units, that use inferior insulators, and thinner wire. And often the aftermarket coils do not fit properly, and the wires won't clip on correctly due to sloppy molding. I have found if you want the best most reliable operation, use OEM or better parts.

There is nothing in the electrical system that can fry a coil pack from the primary side. Again, the coil is designed to handle full 12V+ current. It will not draw more current than it can handle. The ignition drivers are just on-off switches.

On most of these vehicles, the ignition functions are handled by the EDIS module, some later years they are handled by the engine computer directly. The EDIS module is still a computer, its just not the main engine computer, instead it is a dedicated computer for running the ignition system. It is rare that the EDIS modules fail, even if they did, they should not hurt the coil. If in doubt, the module can be tested at most auto parts stores. This is not a part you would replace lightly, last I priced one out they were around $200. They are a part you would probably want to replace with either a Motorcraft or Standard brand.

 

Thank you so much for the reply. I reviewed some generic diagrams of coil packs while I read the info and now I fully understand how the system works. Its good to know that its very unlikely that something is wrong on the primary side. Those spark plug wires are about 6-7 years old. The plugs aren't that old however (2 years) and are OEM. I could see it being the plug wires since they're a little older. I've never had an issue with the coil pack. It's the original on there.

Why would it cause it to blow the fuse at #19? Is it because that current has no where to go and so it finds a weak point at the fuse?

 

I replaced the coil pack and wires this time. And same thing happened. The car misfired and the pack fried within less than a couple minutes. I wouldn't think that the coil pack would fry within only a couple minutes because of a low quality part. It has be something else.

The new wires I got replaced the wires that I purchased from Autozone 10 years ago. And the wires weren't even the problem.

What do you guys think?

 

These are my quotes of these I have done that I posted in another forum. What do you guys think?

"I put in brand new wires and another new coil (under warranty). I checked the gaps on the plugs. They're all good. I replaced those plugs not too long ago so they are in good shape.

It did the same thing. Fired up good for about 2 minutes and started to misfire. I haven't pulled off the coil pack but I could smell that it had melted."

"When I said I checked the plugs I only checked the plugs associated with the blown coil (3 an 4). But yesterday I checked all of them. #1 was covered in soot and had oil on the threads. 1, 2, 4, and 5 both had some oil on the threads. And after checking the gaps they were in specs except for #4 . However half were at .52 and some at .56.

I suspect the oil maybe from a leaky valve cover gasket however."

"So I hooked everything back up again to see if I could isolate the issue and repeat fuse #19 being blown again. Before, everytime I replaced fuse #19 and then turn the key on, it would blow within a minute.

I placed the original blown coil with plug wires attached. I put the old plugs back in as well. I disconnected the following: fuse #19 , PCM diode, PCM relay, coil pack connector, and radio suppressor.

I replaced each item in the same order as listed above with key on. First the #19 fuse, then the PCM diode, relay, etc. After eventually having everything connected, fuse #19 was still intact. I can't repeat the blowing of fuse #19 . Now this is frustrating. The only I noticed was that the battery was bit lower than spec. It was at 12.2V. Maybe there wasn't enough voltage when performing the test.

I started the car with the original blown coil pack. It was misfiring as expected but still no blown fuse #19 ."

 

You mention plug #4 was not in spec. What was it gapped at?

Also, just so we make sure we're all talking the same thing, let's confirm the cylinder numbering is correct since I don't know your background and not all manufacturers number their cylinders the same way Ford does. On your Explorer, the cylinder nearest the front of the vehicle and on the passenger side is cylinder #1. Behind that is #2, and next to the firewall is #3. Going back up to the front of the vehicle on the driver's side is cylinder #4, middle on driver's side is #5, and next to the firewall is #6.

I also checked the power distribution for Fuse 19 (25A mini fuse in the interior fuse panel) according to the factory service manual. This fuse provides Start/Run power to the PCM power diode (which then powers the PCM power relay), the radio noise capacitor (to remove ignition noise from the radio), and the coil primaries. It sounds like either you had a short circuit in one of the coil primaries (may be heat related) or the capacitor shorted momentarily and is now an open circuit. Have you tested the noise capacitor? While the primary function is to reduce ignition noise, it might also reduce switching noise for the coils. I don't know that there would ever be enough noise to cause a coil to fail in the way you describe, but worth checking.

One other question - which 4.0L engine does your Explorer have, the OHV or SOHC?

-Rod