EMI = Electromagnetic Interference
EMI from spark plug wires can cause erroneous signals to be sent to engine management systems and other on-board electronic devices used on both racing and street vehicles in the same manner as RFI (radio frequency interference) can cause unwanted signals to be heard on a radio receiver.
Engine drive-ability problems ranging from intermittent missing to a dramatic loss of power can result when engine management computers receive signals from sensors that have been altered by EMI radiating from spark plug wires. This problem is most noticeable on modern street vehicles used for commuting where virtually every function of the vehicle’s drive train is managed by a computer.
For many reasons, the effect of EMI on engine management computers is never predictable and, often, improper diagnosis for a drive-ability problem results in huge repair costs for a consumer because engine management sensors and other parts are replaced when all that was needed was a set of ignition wires capable of suppressing EMI (usually replaced as the last resort). As production vehicles age, engine management sensors, connectors and wiring deteriorate (and corrode) and become more susceptible to EMI radiated from spark plug wires not able to adequately suppress EMI. The problem is often exacerbated by replacing the original ignition system with a high-output system.