Essential Guide to Spark Plug Wires: Maximizing Performance

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Purpose

Spark Plug Wires, also called “ignition wires” or “high tension leads,” are used in vehicles with gasoline powered internal combustion engines to conduct high voltage electrical current from the vehicle’s distributor or ignition coils to the individual spark plugs.

Location

Spark plug wires run from the individual coils or coil pack located at or near the top of the engine, down the side of the engine to the spark plugs. In older vehicles, they run from the distributor, also usually located at or near the top of the engine, to the spark plugs.

Some modern vehicles employ a “coil on plug” (COP) ignition system in which the coil sits directly on the spark plug. These vehicles do not use a spark plug wire; instead, a rubber boot connects the coil to the spark plug.

Some vehicles, including certain Lexus and Toyota models, use a COP system for one cylinder bank, with spark plug wires leading from those individual coils to the spark plugs for the cylinders in the opposite cylinder bank.

Many Chrysler “hemi” engines, including the 5.7, 6.1 and 6.4 liter V-8 engines used in both automotive and light truck applications, employ a variation of this system. These vehicles employ two spark plugs for each cylinder. One plug is fired directly from a coil on the plug, while the other is fired through a spark plug wire by a coil on the opposite cylinder bank, with both plugs firing on compression and exhaust strokes to minimize emissions.

Operation

A spark plug wire is constructed with a core, typically nylon or aramid fiber impregnated with carbon. This core conducts electricity, but has a designed electrical resistance to minimize rapidly fluctuating voltage in the spark plug wires from interfering with radio reception and distorting electronic signals to and from the vehicle’s computers. (Electrical “resistance” measures how much the material impedes the free flow of electricity.)

The core is coated by a “suppression layer” of latex or silicone with high electrical resistance designed to further contain any electrical interference. Some spark plug wires, termed “spiral design” or “magnetic inductance” wires, wrap a winding of very thin wire around this suppression layer to create another electrically resistant layer around the core. (Spiral design wires are used as original equipment in many Honda, Toyota, Nissan, Infiniti and Lexus models.)

The next layer is heat resistant synthetic rubber that further insulates the electrical current traveling through the core. This layer is wrapped with braided fiberglass for strength, which is then covered with a silicone outer covering. The spark plug end of the wire is covered with a “boot” that houses the electrical connector that attaches to the spark plug terminal and provides a tight seal around the spark plug’s ceramic insulator.

When the high voltage accumulated in the coil is released, electrical current travels through the spark plug wire to the spark plug. At the spark plug, this current encounters an electrical resistance caused by the air gap between the spark plug electrodes.

The voltage in the spark plug increases until it is sufficient to overcome this resistance, which also briefly builds voltage in the spark plug wire. The current then crosses the spark plug gap, creating the spark that ignites the air/fuel mixture in the cylinder and discharging any remaining voltage accumulated in the spark plug wire.

Drivability Symptoms

All 1996 and later automobiles and light trucks are equipped with Second generation On-Board Diagnostics (OBD2). OBD2 vehicles employ “generic” Diagnostic trouble Codes (DTCs) that can be retrieved with a Code Reader or Scan Tool. On these vehicles, DTC P0300 indicates a random misfire and DTCs P0301 through P0312 indicate misfires in cylinders one through twelve, respectively.

These misfire codes can be set by defective spark plug wires, but also have other possible causes, including fouled, worn, or defective spark plugs or a defective coil or coil pack. These are Type A codes that illuminate the instrument panel Malfunction indicator Light (MIL) on the first occurrence. If the misfire is so severe that it could damage the catalytic converter, the MIL will flash during the misfire.

In vehicles manufactured before 1996, misfiring has the same potential causes. However, the MIL will not illuminate on these vehicles because the pre-OBD2 diagnostic systems are limited to detecting electrical circuit faults. Deteriorated spark plug wires also may contribute to hard starting, poor fuel economy, loss of engine power, rough idle, and engine hesitation or stumbling. These symptoms may be more pronounced in wet weather.

Inspection, Test, and Diagnosis

Visually inspect the spark plug wires, making sure each wire is fully seated on its spark plug and checking for cracks or heat damage in the covering. Most spark plug wire failures, however, are internal. Over time, the repeated cycling of high voltage through the wires burns away some of the carbon molecules in the core, increasing resistance and decreasing the capacity of the wire to transmit high voltage to the spark plug.

A DMM with an “inductive pickup” can be used for initial testing. An inductive pickup is a clamp with leads that connect it to the DMM, black end to COM and red end to RPM. With the engine off, clamp the pickup around a spark plug wire. The DMM will have a dial allowing selection of either a conventional distributor or a distributorless ignition system.

Then start the engine. The DMM reading should approximate actual engine revolutions-per-minute (RPM) and move with engine speed as the throttle is opened and closed. Turn off the engine, connect the inductive pickup to the next spark plug wire, and repeat the process.

Readings below true engine RPM indicate the spark plug is not always firing when it should, which may indicate a deteriorated plug wire. A reading near zero indicates the plug is not firing at all. If the vehicle does not have a tachometer, assume the cylinder with highest reading at idle is indicating true RPM. (DO NOT touch the inductive pickup with the engine running. It could transmit a high voltage shock causing personal injury.)

Next, disconnect the spark plug boots and remove the spark plugs from a cool engine. To remove the spark plug wire boot from the spark plug, simultaneously pull on the boot and twist it to release the heat seal that forms over time between the silicone of the boot and the ceramic spark plug insulator.

A spark plug that is firing normally will have light brown or grey deposits. Black carbon deposits on the spark plug insulator tip and electrodes indicates the spark is weak or the plug is not always firing when it should. Gasoline on the plug means it is not firing at all. Abnormal deposits should confirm the DMM RPM readings and can be caused by defective spark plug wires, especially if limited to one or a few cylinders.

To isolate or exclude a spark plug wire as defective, check its electrical resistance with a digital multimeter (DMM). The red lead should be in the volts/ohms port, the black lead in the COM port, and the dial set to read ohms (Ω), which is the unit of measure for electrical resistance.

Disconnect the spark plug wire at both ends. Simultaneously touch the red lead probe to the electrical contacts in the coil or distributor end of the spark plug wire and the black probe to the electrical connector inside the boot. The reading is the resistance of the wire. Though resistance specifications vary with vehicle, typical resistance specifications are between 10,000 and 15,000 ohms, with spiral wire resistance specifications as low as 3,000 ohms.

If resistance is above vehicle manufacturer’s specifications, the plug wires should be replaced. (If specifications are not known, assume any wire above 20,000 ohms resistance is defective.) If resistance is below specifications, the spark plug wire is shorted to ground. Unless there is an evident reason that only one plug wire has become damaged, all plug wires should be replaced.

Reprogramming

Replacement of spark plug wires does not require reprogramming.

Repair/Replacement

Spark plug wires cannot be repaired and must be replaced. Though it is proper to upgrade to a higher quality spark plug wire, new spark plug wires should be “direct fit” replacements exactly matching the length of original equipment wires. So-called “universal” wire sets can have longer wire lengths that make proper routing of the wires difficult.

It is important that replacement spark plug wires be routed exactly as the originals, maintaining the same positioning of each wire in relation to the others to prevent “cross-firing.” Cross-firing occurs when one wire induces voltage in another wire, triggering a spark in that wire’s cylinder and creating misfires. (Take pictures of the routing of the original wires before removing the first wire.)

Starting with one cylinder, remove the old wire and install the new one exactly following the same routing. Then follow the same process for each remaining cylinder. If the vehicle is equipped with a distributor and conventional single coil, also replace the wire leading from the center of the coil to the center of the distributor cap. On many vehicles, spark plug wire “separators” – small plastic clips that attach to the wires – are used to route the wires and retain them in place.

These separators should be reused, with new wires routed through them exactly as were the old. On some vehicles, however – including Cadillacs equipped with the Northstar V-8 and without COP ignition - routing spark plug wires is sufficiently complex that the vehicle manufacturer recommends removing all wires and their retainers as one unit and assembling the replacement wire unit on the work bench using the originals as a guide.

To prevent the boots of new wires from adhering to the ceramic spark plug insulators, lightly coat the interior non-electric surfaces of the boot with dielectric grease before installation. DO NOT apply the grease to electrical contacts, as dielectric grease is not conductive.

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