Distributors started disappearing from engines back in the late 1980s and early 1990s as the auto makers began to phase in distributorless ignition systems (DIS) followed by coil-on-plug (COP) ignition systems. Everybody said distributors and eventually spark plug wire sets were going away.
Well, here it is more than 20 years later and auto parts stores still carry a hefty inventory of distributors, caps, rotors and spark plug wire sets. But in addition to these parts, store shelves are also stocked with DIS coil packs and COP coils. There are more SKUs than ever and the product mix is constantly changing.

One of the challenges posed by COP ignition systems is that every coil configuration is different. Though the same coil is used for each cylinder on an engine, the coils that fit one engine usually don’t fit anything else. Consequently, little consolidation of part numbers and applications is possible — thus the huge increase in coil SKUs for late-model vehicle applications.

It is true that many of the ignition parts for older engines with distributors are slowly going away, but not as fast as new parts are being added for late-model vehicles with DIS and COP ignition systems. We interviewed several major aftermarket ignition parts suppliers for this article and were told that there are literally hundreds of new ignition SKUs for DIS and COP coils. What’s more, most of these parts are fairly expensive compared to traditional ignition parts. Individual COP coils typically sell for $50 to $100 or more each and some DIS coil pack assemblies can be quite pricey ($250 to $350 or more) — which means more dollars invested in your inventory.

All this is creating shelf space nightmares for retail stores that try to maintain an inventory of current ignition parts that provides broad coverage. If a customer calls or walks into your store, asks for a particular coil and is told you don’t stock it and that it may take several days to get it from your warehouse, chances are he’ll keep looking until he finds another store that has it or one that can get it same day.

We also asked the ignition parts suppliers if there are any late-model vehicle applications that are experiencing unusually high coil failure rates. One that was mentioned was the COP coils for Ford 5.4L V8 engines. These are found on 1997-2005 Ford F150 and F250 pickups, 1998 to 2006 Ford Crown Victorias, 1999-2003 Ford Mustangs and also the 6.8L V10 engines in various Ford pickups. The problem with these coils is that the plugs are deeply recessed and the plug wells tend to trap water if the vehicle is driven through deep puddles. The water gets down into the wells, wicks up the spring that engages the top of the spark plug and shorts out the coil. The engine then develops a misfire, the Check Engine Light comes on and the aftermarket sells another $60 replacement coil.
Chrysler has also had some high COP coil failure rates on its 3.5L V6 engine which is used in numerous Dodge and Chrysler models (300M, Intrepid, Pacifica, Dodge Magnum, Concorde, etc.). One manufacturer attributed the problem to excessive heat. The replacement coils on these engines typically sell for around $70.

COIL DIAGNOSIS
A bad coil on a COP ignition system will cause one cylinder to misfire. The OBD II misfire monitor will pickup the problem and set a P030X code that corresponds to the cylinder that is misfiring (where X represents the number of the cylinder that is misfiring).

It’s not as simple to check for spark on a COP ignition system as it is on an application with plug wires because it involves some extra steps. If the COP coil can be easily removed without having to remove a lot of other stuff that might be on top of it, the COP coil can be unbolted from the engine, pulled out and a spark plug or spark tester can be attached to the end (the plug or tester must be grounded). If cranking the engine fails to produce a spark, the coil is considered bad.
The coil’s primary and secondary resistance can also be tested with an ohmmeter. Primary resistance in a COP coil is typically 0.5 to 3 ohms, and secondary resistance is typically thousands of ohms. A coil that reads open, shorted or out of specs is considered bad and needs to be replaced. The same goes for any coil that has visible cracks.

Ford released a technical service bulletin (TSB 04-16-1) that said approximately half of all COP coils that are returned under warranty have nothing wrong with them. The problem is misfire misdiagnosis, not coil failure. The bulletin said rough running engines that set misfire codes can have multiple causes. All too often technicians jump to conclusions and replace the COP coil on a misfiring cylinder before thoroughly checking out other components such as the spark plug, fuel injector and compression.

The Ford bulletin recommends doing a power balance test to isolate the weak cylinder, then checking compression to rule out mechanical issues such as a bent or burned valve, weak or broken valve spring or leaky head gasket).

Next, the fuel injector should be checked to make sure it is working properly (battery voltage to the injector’s positive terminal when the key is on and a pulsing ground to the injector negative terminal when the engine is running). A leakdown test can be done using a scan tool to see if all the injectors are flowing equally.

If the coil is delivering a good spark and the plug is firing, but the cylinder is weak (misfiring), the fault may be a dirty or weak fuel injector. If cleaning the injector does not restore normal power in the cylinder, the injector needs to be replaced.

If the coil is not firing the spark plug or only produces a very weak spark, the fault may be the coil itself or the coil’s primary voltage supply. Swapping the original coil for a known good coil, or switching two adjacent coils to see if the misfire moves from one cylinder to another, are both techniques that can be used to identify a bad coil. Coil primary and secondary resistance should also be tested with an ohmmeter, and if the coil passes, the problem is likely in the coil primary voltage supply circuit. There may be a bad connection, or a fault in the ignition module or PCM coil driver circuit depending on how the ignition system is configured.

PLUG WIRE SETS
If distributors are slowly disappearing, the demand for spark plug wire sets is also declining. Though many DIS ignition systems have plug wires (as do General Motors’ coil-near-plug ignition systems), COP ignition systems are appearing on more and more engines and are eliminating the need for plug wires altogether by placing individual coils directly over each spark plug.

On older engines, plug wires are often a source of trouble. Over time, plug wires can be damaged by heat and vibration causing the engine to misfire. In addition to misfiring, bad plug wires can cause hard starting (particularly during wet weather), poor fuel economy, rough idle, hesitation when accelerating and increased hydrocarbon (HC) emissions. On 1995 and newer vehicles with OBD II Onboard Diagnostics, misfiring will usually set a fault code and turn on the Check Engine light.

Plug wires should always be inspected if any of these symptoms are present and when the spark plugs are changed. If wires show any obvious damage such as burned or cracked insulation, chaffing, loose plug boots or terminals, the wires should be replaced. Also, if visible arcing is present new wires are needed. Wires should also be replaced if their resistance measured end to end with an ohmmeter exceeds OEM specifications. As a rule, if more than one plug wire has excessive resistance, the entire set should be replaced.

Many technicians recommend replacing the plug wires when the spark plugs are changed to reduce the risk of potential ignition problems down the road. That’s still a good idea, but impossible on engines with COP ignition systems because there are no plug wires. So until engines with plug wires disappear completely (which won’t happen for many years to come), parts stores will have to stock wire sets for many engines — in addition to all of the new COP coil SKUs that are crowding their shelves.

Wire sets tend to be rather bulky and take up a sizable amount of shelf space. Even so, they are not going away fast enough to open up sufficient shelf space for all the new COP coils and other parts. That means jobbers have to carefully manage their inventories, keep on top of their local markets and stock only those ignition parts that are moving off the shelf.

SPARK PLUGS
Spark plugs are another product category that has undergone a lot of change in recent years. Most late-model engines are now factory-equipped with long-life platinum or iridium spark plugs. When these metals are used in the electrodes, they greatly reduce electrode erosion and wear. This allows the plugs to last three to four times longer than standard spark plugs.

The downside of longer plug life is that the aftermarket now sells fewer replacement spark plugs. It also means parts stores now have to stock a wide assortment of long-life plugs in addition to standard plugs — and for each brand of plug they sell (ACDelco, Motorcraft, Champion, Bosch, Denso and NGK). That means more part numbers and more shelf space.

The plus side of carrying an expanded spark plug inventory is that it creates a upgrade selling opportunity when a customer who drives an older vehicle with standard plugs needs new plugs. Platinum and iridium plugs cost more than regular spark plugs, but are well worth the difference to many motorists when you consider the plugs may not have to replaced again on a high-mileage engine. Long-life plugs are also a plus if an engine has plugs that are difficult to replace (as is the case on many FWD vehicles with transverse mounted engines). With long life plugs, most motorists only need to replace the plugs once.