Fixing a Misfire on an Older Car: Temporary Spark Plug Lead Insulation | MyMechanic

The Problem: Arcing Plug Leads

A spark plug lead is essentially a heavily insulated wire. Its only job is to carry a high-voltage pulse from the distributor or coil pack to the spark plug. Over time, the rubber insulation breaks down from heat and age.

When a crack develops, the electricity, following the path of least resistance, will jump from the crack to the nearest metal ground point (like the engine block or a bracket). This is called arcing. You lose the spark at the plug, and that cylinder misfires.

Symptoms of an Arcing Wire:

• A rhythmic engine misfire or stumble.
• An audible "ticking" or "snapping" sound from the engine bay that matches the misfire.
• In the dark, you might even see a tiny blue spark jumping from the wire to the engine.

Roadside Triage: Finding the Faulty Lead

Safety Warning: The ignition system carries dangerously high voltage. Never touch the plug leads with the engine running.

1. 1. Visual Inspection (Engine OFF): Carefully inspect each spark plug wire from end to end. Look for obvious cracks, chafing, or burn marks, especially where the wire runs close to hot or sharp metal parts.
2. 2. Listen for the Arc (Engine ON): If a visual inspection reveals nothing, start the engine. Listen carefully for the distinct snapping sound of an electrical arc. Try to pinpoint which wire it's coming from. This is often easier in a quiet environment.

The Emergency Insulation Repair

If you've found a crack in a wire, you can perform a temporary patch to re-insulate it and get the spark back to the plug.

What You'll Need:

• High-quality electrical tape (brands like 3M Super 33+ are excellent).
• A clean, dry cloth.

Patching Procedure (Engine OFF):

1. 1. Clean the Wire: Thoroughly clean and dry the area around the crack. The tape will not stick to a greasy or dirty wire.
2. 2. Stretch and Wrap: Starting about an inch before the damage, stretch the electrical tape slightly and wrap it tightly and smoothly around the wire. Overlap each layer by about half the width of the tape.
3. 3. Build Up Layers: Continue wrapping past the damaged area for another inch. Apply at least 3-4 tight layers to build up a good insulating barrier.
4. 4. Reroute if Necessary: If the wire was arcing because it was too close to a metal part, try to gently reroute it to create more clearance.

After the repair, start the engine. The misfire and ticking sound should be gone. This is a temporary fix to get you to a workshop for a new set of plug leads.

Frequently Asked Questions

Conclusion

For owners of cars with traditional plug leads, understanding how to spot and temporarily fix an arcing wire is a valuable roadside skill. It demonstrates how a simple failure can cause a dramatic problem, and how a simple repair can provide a solution. For all Singaporean drivers in Malaysia, whether your car is old or new, MyMechanic has the expertise to diagnose and resolve your ignition system troubles, ensuring your journey can continue safely.