Spark Plug: Description and Operation

Resistor spark plugs are used in all engines and have resistance values of 6,000 to 20,000 ohms when checked with at least a 1000 volt tester.

Faulty or fouled spark plugs may perform well at idle speed, but frequently fail at higher engine speeds. Faulty plugs can be identified in a number of ways: poor fuel economy, power loss, decrease in engine speed, hard starting and, in general, poor engine performance.

Spark plugs also malfunction because of carbon fouling, excessive electrode air gap, or a broken insulator. SEE Powertrain Management/Ignition System/Testing and Inspection/Procedures, for spark plug diagnosis.