At every tune-up/inspection, visually check the spark plug cables for burns cuts, or breaks in the insulation. Check the boots and the nipples on the distributor cap and/or coil. Replace any damaged wiring; cracked or shorted spark plug wires can short to ground, and cause a "miss" when the engine is running.
Every 50,000 miles (80,000 Km) or 60 months, the resistance of the wires should be checked with an ohmmeter. Wires with excessive resistance will cause misfiring, and may make the engine difficult to start in damp weather.
To check resistance, remove one wire from the engine, and measure the resistance from the spark plug connector to the distributor/coil pack connector. The resistance should be approximately 4600-7400 ohms. If the wire resistance is higher than 7400 ohms, the wire should be replaced. Coil wires (the wire that runs from the coil to the distributor) should have a resistance of 1200-2700 ohms.
Volkswagen uses a special type of spark plug wire connector that is unique; if the spark plug wires are in need of replacement, make sure to replace them with OEM-equivalent wires to ensure proper ignition system operation.
Removal & installation
If the wires are being replaced, remove one wire and match the length with the new wire. Install the new wire before removing the next old one. If the wires are to be reinstalled, remove them one at a time from the distributor cap before disconnecting them from the spark plugs. As each wire is removed from the distributor cap, mark the cylinder number on the boot and on the cap.
On some vehicles with the Motronic system, an ignition reference sensor is attached to the No. 4 plug wire. The ECU uses the input from this sensor when calculating the timing retard required to stop the engine knock.