1991 BMW M3 Distributor Cap Replacement Cost

Many older vehicles use distributor type ignitions, and the distributor cap and distributor rotor are necessary maintenance items for keeping the engine running smoothly.

The distributor rotor receives the high voltage electricity from the ignition coil, and sends 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. 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.

Ignition cap and rotor failure can be caused by failures elsewhere in the ignition system. The ignition wires and spark plugs should be thoroughly inspected. Old plugs or wires will burn out even a new cap and rotor.

Engine fluid leaks can cause damage to the ignition cap and rotor. These leaks should be repaired when the cap and rotor are replaced.

A failing ignition cap and rotor can cause hard starting, rough idling, and poor acceleration. It can also cause the Check Engine Light to illuminate. Continuing to drive with a bad cap and rotor can cause engine misfiring, which can ultimately damage the catalytic converter.

A failing ignition cap on rotor 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.

The ignition distributor cap and rotor are both maintenance items, and the replacement schedule is listed in the owner's manual. If the distributor cap and rotor wear out before the specified mileage, they must be replaced early. Some vehicles have issues with distributor leaks, spreading oil onto the distributor rotor and distributor cap, causing misfires. When this occurs, the cap and rotor should be replaced.

Diagnosing any of the symptoms mentioned, for a distributor type ignition system, will normally being by checking all ignition components for abnormal conditions that may be indicative of failure. If nothing is found, the spark plug wires, distributor cap and rotor, and spark plugs, more or less in that order, will be removed, inspected, and diagnosed. If the spark plug wires are too resistant to electricity, the spark plugs are worn or fouled, or the distributor and rotor are worn, corroded, or oil fouled, those components would be diagnosed as faulty.

If the distributor cap and rotor will be replaced, the spark plug wires will be replaced as well, unless they have been changed according to scheduled maintenance intervals, or they appear to have service life remaining. Likewise, and with the same exceptions, if the spark plug wires are being changed, the distributor and rotor will be changed. The spark plug wires will be marked for their location, and the new spark plug wires will be assembled with the spark plugs and ignition distributor cap using those markings. After the ignition distributor rotor is popped into place, the distributor cap will be installed, and the spark plug wires attached to mounting hardware.