John Pirtle, February 15, 2001
Like every other car, the 928 requires a strong solid accurately-timed spark. Many of the ignition components are wear items and should be checked and replaced when suspect. This includes the spark plugs, plug wires, distributor caps, rotors, and coils. The transfer of the spark from one component to another erodes the contacts, and the components become more resistive to conductivity. Additionally these parts are very sensitive to moisture and it is essential that all parts fit and seal properly.
It is good practice (especially for reliability and performance) to simply replace the plugs, caps and rotors every 5 years or 50,000 miles. Wires should be replaced every 5 years or 100,000 miles.
Each rotor and distributor cap may be visually checked for excessive burning by removing the caps and inspecting the contacts. Healthy spark plug wires should not show cracks in the rubber sleeves and should seal tightly on the plugs and distributor caps.
An ohmmeter may be used to check a component's resistance to conductivity. The workshop manuals list testing values on page 28-62, which is for all 32-valve engines (from 1985 on). If the component tests with a higher resistance, it should definitely be replaced.
928 S4 Page
February 15, 2001