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A Coil On Plug is nothing more than a transformer which steps up voltage to fire the spark plug, that is located directly over top the spark plug. The only real difference between COP and other ignition systems is that each COP coil is mounted directly atop the spark plug so the voltage goes directly to the plug electrodes without having to pass through a distributor or wires. It is a direct connection that delivers the hottest spark possible. Resistor plugs are generally used to suppress EMI. Spark plugs wires are going away for the same reason that distributors went away. Vehicle manufacturers want to reduce costs and improve ignition performance and reliability. Plug wires are an assembly line nuisance, and are often the weak link in distributor-less ignition systems. The plug wires must carry anywhere from 5,000 up to 40,000 or more volts to fire the plugs. This requires heavy insulation plus the ability to suppress electromagnetic interference (EMI). The wires must also be coated with a tough outer jacket to withstand high temperatures in the engine compartment and chemical attack.
As reliable as today's plug wires are, there is always the potential for trouble. Even the toughest insulation can burn if a wire rubs up against a hot exhaust manifold. The connection inside the spark plug boot between the wire and plug terminal can also be damaged if someone jerks on the wire to remove the boot when changing spark plugs. Plug wires can also radiate magnetic fields that may affect nearby sensor wires or other electronic circuits.
Attaching the ignition coils directly to the spark plugs eliminates the need for separate high voltage wires along with their potential for trouble. Eliminating the individual plug wires also eliminates the need for wire looms and heat shields. That is why coil-on-plug ignition systems are being used on a growing number of late model engines.
Getting rid of the plug wires not only saves money, it also improves the durability of the ignition system. No high voltage wires means no voltage leaks and no misfires due to "bad" plug wires. Using individual coils for each spark plug also means the coils have more time between each firing. Increasing the "coil saturation" time (the time the voltage to the coil is on to build up its magnetic field) increases the coil output voltage at high rpm when misfire is most apt to occur under load.
What are common symptoms associated with them failing?
COP problems can include many of the same ailments as other ignition systems such as misfiring, hard starting or a no start. Spark plugs can still be fouled by oil or fuel deposits as well as pre-ignition and detonation. So COP ignition systems are not immune to trouble.
If the crankshaft position sensor fails, the loss of the basic timing signal will prevent the system from generating a spark and the engine will not start or run. A failed driver circuit within the PCM can kill an individual coil and prevent that cylinder from firing. But with COP, an individual coil failure will only cause misfiring in one cylinder.
It is important to remember that ignition misfire can also be caused by other factors such as worn or fouled spark plugs, loose or damaged coil connectors or terminals, dirty fuel injectors, low fuel pressure, intake vacuum leaks, loss of compression in a cylinder, even a tankful of "bad" gas contaminated with water. These other possibilities should all be ruled out before a COP unit is replaced.
The most common trouble codes you may encounter with COP systems on OBD II equipped vehicles are P0300 series codes such as P0301, P0302, etc. that indicate a misfire in a particular cylinder. The important point to remember here is that a general misfire code (P0300) is probably not ignition related but is due to a vacuum leak or fuel delivery problem.
A code that indicates a misfire in a single cylinder (such as P0304), on the other hand, will usually be due to a fouled spark plug, weak coil, dirty or dead fuel injector, or loss of compression (burned vale or leaky head gasket).
If a misfire is due to a bad coil, you should find a coil code that corresponds to the same cylinder (P0351 to P0358). (Not ALWAYS the case as intermittent misfires won't always throw codes)
If a misfire is fuel related, you should also find a code that indicates an open or shorted injector in that cylinder (P0201 to P0208).
A COP engine that cranks but fails to start, in many cases, will often have a problem in the crankshaft position sensor circuit (code P0320). Loss of the camshaft position sensor signal (code P0340) may prevent the PCM from properly synchronizing the fuel injectors, but may still allow the engine to start and run in a limp-in mode.
What are some ways they can fail? / What causes them to fail?
There are a few ways that the COP setup can fail.
The casing which the coil itself is surrounded with will often crack and allow moisture to enter the coil. This is usually due to the 100s of heat cycles the housing and coil have gone through over the lifespan of the vehicle. It can also be onset to an already weakened housing by spraying water on your engine when it has just been run to full operating temperature.
The primary of the coil can also open, causing no output whatsoever. This is usually from nothing more than the many cycles the coil has been through as well as the age itself.
The primary can also become highly resistive if corrosion builds up at the primary side-either on the harness connector itself, or if the housing has been compromised and allowed the coil to be open to the harsh environment of the engine bay.
The coil isn't the only part that can fail, the boot itself may dry rot and crack over time, allowing arcing to occur which in turn will cause a misfire. It will be intermittent making it even harder to diagnose.
The spring inside the boot which sets over the spark plug can also become corroded, causing a poor connection and misfires.
How can i help prevent them from failing?
Preventative maintenance is likely the least used section in any repair manual, but it should be the other way around! Some simple things like keeping the engine bay clean and only cleaning the motor when its cold can make a big difference in the life of your components.
The use of dielectric grease on both the harness connections and the plug connections for each COP setup is crucial to prevent future corrosion.
Frequent check ups on your plug connections and the coil boot can help deter a breakdown on the road, by finding an issue before it becomes bad enough to prevent cylinder firing.
How can you test them to tell if they have failed or are failing?
There are a few ways to test the COP ignition system, however, two of the best ways are not commonly able to be performed by your average shade tree mechanic. These include using an oscilloscope to look at the firing signal itself, as well as testing the coil with a HIPOT or Megger. These two devices are basically a high voltage Ohmmeter used to test break over for devices with a high voltage/low current action. Since most people wont have any of this equipment, there is another method of testing that can be used.
Using a Multimeter on the resistance setting or an Ohmmeter, you can test a COP to some extent. It is not a surefire method, as the amount of voltage applied with a regular meter is not enough to find errant paths or weaknesses that may easily be found with 15-20 kV. It will however give a good representation of a decidedly bad coil.
You may measure the resistance across the primary coil by putting one lead on either side of the harness connection. This should read as a dead short, as it is simply a wound up piece of wire(coil). Should it read more than .8-1.0 ohms, you may have some corrosion in the coil. If it reads in the Mega Ohm range, the coil has likely burned/broken open completely and will certainly cause a misfire.
You can also measure from either of the primary sides of the coil(either wire of the harness connector), to the spring in the plug boot. This should read as an open, as there is no wired electrical connection between the two coils. It may read as low as 4-10k ohms.
Another method of testing to determine if the issue is in fact a coil, you can do a simple trick. Simply unplug a COP harness connector one at a time and see if the engine runs any differently. If it worsens, it is likely not that coil, move on to the next. If it has no change, it very well could be that coil, and you should test it further with a meter and inspect for cracking or damage.
Where can I get replacements?
Once you have figured out that you need to replace the coils, you have many options to consider when purchasing a new set, or individuals.
Prices range from 85-100$/coil to 85-100$/set of 8! Your local Ford dealer can supply them to you, gladly charging you 85$ each. Many aftermarket companies also offer replacements, and some are just as expensive as the dealership ones, while offering a "high performance" rated product. These coils are offered by the likes of Accel, Screamin Demon, MSD, etc. They promise power gains due to a longer duty cycle or higher voltage or both. This allows you to run a wider plug gap(.065) which will allow more combustion and thus more power. I personally have not tried any of these products and I don't ever intend to, as I feel my money is better spent on other aspects of my vehicle.
Now, for most of you out there like myself, A good compromise of price and quality is the way to go. For this, I turn to smaller corporations who deal through eBay and do not have a retail location. This saves them tons of money allowing them to pass on the discount to the consumer-US! I have bought multiple sets for 80-90$ with no issues and received a quality product. The choice is up to you, but my personal recommendation is to check out sources online before you hump it up to your dealership and fork over hard earned cash for the same product. Below is a list of common model numbers that can be used for replacements for the 4.6 motors.
Once you have attained you replacement COPs you may want to make sure you have all your ducks in a row before installation. Be sure you have checked ALL of your spark plugs first and purchased a replacement set if necessary. Make sure you have some dielectric grease on hand as well as a 1/4 drive socket set with wobble ends/flexible extensions.
You do NOT have to loosen or remove the fuel rails to change the plugs or the COPs. To remove the old COP simply unplug the harness, remove the retaining bolt holding the housing of the COP in place, then give the COP a bit of a turn/wiggle and it should pop right out. Replace the plug if you have decided to do so at this time, then prepare you new COP for installation by applying a small amount of dielectric grease to the inside of the spring and the lip of the boot to help it slide on easier and prevent it from sticking later. Align the new COP over the plug and push down firmly until it is seated onto the plug. Reinstall the bolt, connect the harness, and move on to the next. Be careful not to over tighten as it is easy to crack the housing. All of them can be done with just a ratchet and flexible extensions, but the back ones can be tricky and it is sometimes easier with a small ratcheting wrench.
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COP Sticky Test
