BMW 850i Spark Plug Wire Replacement Cost

Spark plug wires are conduits for the transfer of high voltage electricity from the ignition coil to the spark plugs. Most vehicles typically have four to eight spark plug wires connected to the ignition distributor cap.

The distributor rotor receives the high voltage electricity from the ignition coil wire, and sends it through the distributor through an electrode on the tip of the rotor. As the rotor spins, electrical power is transferred from the rotor to individual electrodes inside the distributor cap. Once that high voltage reaches the distributor cap, the corresponding spark plug wire will carry that high voltage to the spark plug terminal. This action is timed for the moment the cylinder needs the spark plug to fire, and the sequence creates a combustion event inside the engine.

A failing ignition wire set will create misfires in the engine, and lead to hesitation, extreme power loss, engine stalling, poor fuel mileage, and illumination of the check engine light. The on-board diagnostics (OBD) trouble codes stored in the vehicle's computer will reflect engine misfires, catalyst inefficiency, and O2 sensor related errors. Under the hood there may be audible arching, that sounds like a rubber band snapping, coming from the ignition coil, distributor, or spark plug ends of any of the spark plug wires. Lastly, the ignition distributor cap and rotor may look burned, wet, corroded, or covered in a powder residue, and the spark plug wires may be cracked or degraded.

Ignition system components cause many problems when they fail, normally making the vehicle unpleasant or impossible to drive. Also, the amount of unburned fuel going into the catalyst could cause the catalyst to fail, requiring replacement. With severe ignition problems, the vehicle should be towed to a repair facility. Get this fixed by a RepairPal Certified Shop