Plugging Away At Ignition Misfires, Larry Carley, Undershood Service, September 2000

Although vehicle designers are developing engines without spark plug wires (see our feature story on COP ignition systems beginning on page 20) the elimination of plug wires on distributorless ignition systems is not going to occur overnight. Technicians in the future are still going to see plenty of plug wires on customer vehicles. And that means techs still need to understand how to diagnose ignition problems caused by spark plug wires.

Ignition misfire can produce a variety of driveability symptoms including hard starting, rough idle, hesitation, a drop in fuel economy and a big jump in hydrocarbon emissions. Diagnosing the underlying cause is never easy because there are so many different possibilities. It could be worn or fouled spark plugs, bad plug wires, a weak coil, dirty injectors, low fuel pressure, an air leak, etc. But since our focus is on the plug wires, let’s start there.

First, visually inspect the wires for obvious signs of damage (burned or cracked insulation, chaffing, contact with the exhaust manifold, loose plug boots or terminals, etc.). If the wires are ancient and show obvious signs of deterioration or damage, you’ve probably found the problem. Replace the entire wire set.

If the wires pass a quick visual inspection, start the engine, then look and listen for arcing while the engine idles. A snapping or cracking noise would tell you secondary voltage is finding a shortcut to ground. Observing the engine in the dark may help you see where the voltage is leaking. Any fireworks that are visible along the length of the cables or at the plug boots, coil tower or distributor terminals would tell you one or more wires need replaced.

Still don’t see anything wrong? Hook up your ignition scope and observe the secondary firing patterns on your screen.

A bad plug wire with excessive internal resistance usually causes an intermittent misfire rather than a steady miss because the voltage required to fire the plugs changes with engine speed and load. It is lowest at idle and under light load, but rises sharply as the throttle opens or the load increases. Thus the engine may run fine most of the time, but misfire or stumble during acceleration.

An open plug wire or spark plug will cause the firing voltage for that cylinder to spike to the coil’s maximum output. If you find the firing voltage is high in a cylinder, turn the engine off and measure the plug wire’s resistance end to end with an ohm meter. Refer to the manufacturer’s specifications, but as a rule total resistance should not exceed about 8,000 ohms per foot. If resistance is too high, replace the wire. And if the wire is okay? Replace the spark plug.

A shorted ignition cable or grounded spark plug will cause a big drop in the firing voltage. Rubbing a grounded probe along the length of each plug wire while the engine is idling may help you find any weak spots in the insulation. Otherwise, remove and inspect the spark plug in the low cylinder. If the plug isn’t fouled or does not have a cracked insulator, replace the plug wire.

When one cylinder in the superimposed display has a firing line higher than the rest and a shorter spark duration, high secondary resistance is indicated. High secondary resistance may be caused by bad plug wires or worn spark plugs, but also a lean fuel condition.

To further isolate the cause, the KV demand for the affected cylinder should be compared to the other cylinders. If the required firing voltage is 20% or more higher than the rest, the problem is either too wide a plug gap or a lean fuel condition. But if the firing voltage is roughly the same as the other cylinders, the likely cause is high resistance in the plug wire or spark plug.

Replacing Plug Wires
Any plug wire that is found to be "bad" (damaged insulation or terminals, or excessive resistance) obviously needs replaced. But should you replace only the bad wires or the entire set?

Most experts say the entire wire set should probably be replaced for preventative maintenance if one or more wires are bad and the wires have more than 50,000 miles on them. Wires deteriorate with age, so if one or two wires have already failed, the others may be marginal at best. Replacing the entire set can help assure trouble-free driving and save your customer the hassle and expense of a return visit.

On the other hand, if the wires don’t have many miles on them and only one wire has failed (possibly due to mishandling, physical damage or burning), there’s no need to replace the entire set - unless your customer wants to upgrade his ignition wiring to a higher level of performance.

Replacement Tips
• Handle cables with care during installation. Do not jerk, force, twist or bend sharply.

• Start with the longest or shortest wire first, and change one wire at a time to avoid mixing up the firing order.

• Listen for the "click." Make sure the plug boots are all the way on the plugs, and the end terminals are locked into the distributor cap or coil towers (DIS ignition systems).

• Follow the original cable routing to avoid crossfire problems. Cables for cylinders that fire consecutively should not be routed parallel or be in close proximity to one another. Keep them separated by several inches or cross them.

• All cables should be supported by wire looms, and kept away from exhaust manifolds and sharp edges.