Spark plug wires carry high voltage from the ignition coil or distributor to each spark plug on engines that do not mount a coil directly on top of every plug. Worn wires leak spark, causing misfires, rough idle, hesitation and a check engine light. Many older V8s, including the Chevy 350 and Vortec 5700, use a distributor and a full set of plug wires, while GM LS engines use a coil for each cylinder with a short plug wire to the spark plug.
Bad spark plug wire symptoms
- Misfires and a rough idle, often with codes P0300 to P0308; see engine misfire
- Hesitation when accelerating; see car jerks when accelerating
- A snapping or ticking sound, or visible sparks around the wires in the dark
- Cracked, burnt or loose boots, especially near hot exhaust manifolds or headers
- Lower fuel economy and radio interference
How to test spark plug wires
- Inspect: look for cracks, burns, chafing and boots that are loose or full of corrosion.
- Measure resistance: wires are often specified in ohms per foot; compare each wire with the specification and with the others. An open or very high reading means the wire is bad.
- Look and listen in the dark: with the engine running, arcing shows as small sparks or a snapping sound; a light mist of water can make a leak more obvious.
- Swap test: move a suspect wire to another cylinder and see whether the misfire code follows it.
How often to replace plug wires
There is no single interval. Many quality wires last 60,000 miles or more, but heat near exhaust manifolds and headers shortens their life. Replace them when they test bad or show damage, and many owners replace them together with the spark plugs; see bad spark plug symptoms.
Replacing spark plug wires
- Replace one wire at a time so the routing to each cylinder stays correct; the order matters, see firing order.
- Twist the boot to break it free and pull on the boot, never on the wire.
- Use a little dielectric grease inside the boots, and keep wires away from exhaust parts with the original looms or heat sleeves.
- Route wires so they do not run side by side for long stretches, which can cause crossfire on some engines.
Coil-on-plug engines
Engines with a coil on each plug have no wires, so a cylinder misfire there points to the coil, plug or boot instead; see bad ignition coil symptoms. If a cylinder still misfires with good ignition parts, run a compression test. Replacement Chevy 350 and LS-family engines are on our Chevy 350 engines and LS engines pages.
More guides: how often to change spark plugs.
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