A Gnawing Problem

When is a problem not a problem? When it doesn’t affect the vehicle’s reliability, drivability or convenience … but that doesn’t mean that we can ignore trouble when it’s brewing. There’s an old saying that if you live long enough, you’ll see just about everything and this issue is a classic: Chewed up plug wires. Not “chewed up” in the slang sense of damaged, but literally gnawed on by small animals! Canadian critters like porcupines and bears have been rumored to chew on tires occasionally, and birds’ nests in air cleaners as well as spiders in throttle bodies are seen occasionally … but how often do they go after the ignition secondary? The following pictures show how a small animal damaged the plug wires on a Ford V-6, both at the coil pack and along the wire length.

Perhaps as surprising as the extent of the damage is the lack of degradation of the ignition’s performance. This was caught at an oil change; the car ran well and stored no codes as a result of the shredded insulation. SSGM tested the internal resistance of the damaged wires and compared them to new replacements and found no electrical issues with the chewed wires. We then used a laboratory high-voltage insulation tester and ran 3.5kV across the wires with the same result. What did we learn? That the outer jackets of modern plug wires are not the most important part of the electrical properties of the cable, at least in a healthy engine, when tested at typical running coil voltages. The actual running coil voltage is key here. The voltage developed by the coil is the minimum necessary to jump the spark plug gap and is rarely close to the maximum possible voltage that the coil can deliver. Why?

The two main reasons are:

1 Clean, properly gapped plugs, especially modern rare metal types like platinum, require lower voltages to spark, all other cylinder conditions being equal. Harder electrode and ground strap materials also wear much more slowly, keeping the gap closer to the correct spec longer. In

addition, modern computer engine control allows the spark to more completely burn the fuel mixture, reducing the fouling deposits that can provide a leakage path for spark current.

2 In older engines with distributors, the laws of physics make the magnetic saturation time of ignition coils a factor at high rpm. The shorter relative saturation time between firing events, the lower the peak voltage available to drive the firing current. Weak high rpm spark was one reason why early electronic ignition systems were often rejected for racing or high performance street applications.

What does this mean? From a service perspective, it means that wire service shouldn’t wait until the engine misfires or shows drivability problems or trouble codes … if the outer jacket is damaged, replace the set preventively. Service writers and techs will have to convince motorists to accept that this is necessary service even though the vehicle is running well. It’s a good idea to keep a couple of damaged wires around as a demonstration tool, and offer to show the owner the old wires when they pick up their vehicle. And if the owner has a sensitive personality, remember that squirrels are a nuisance, but they’re cute … if you tell them that their car has rats they’ll likely be shopping for a new vehicle.