Changing Spark Plugs and Checking Spark Plug Condition
Spark plug replacement procedure with gap specification and torque, plus a diagnostic guide for reading plug condition to identify engine problems.
READING SPARK PLUG CONDITION
Pick what the removed plug looks like to read what the cylinder is telling you.
Changing Spark Plugs
Tools
- Spark-plug socket
- Ratchet
- Torque wrench
- Engine oil
Procedure
- Check the gap on the new spark plugs. Most Bosch plugs come pre-gapped, but the gap should be verified anyway. The gap should be 0.7–0.8 mm (0.028–0.031 in).
- Coat the threads of the new spark plugs with a light coating of engine oil.
- Disconnect the first spark plug wire. Replace one spark plug at a time to avoid getting the plug wires in the wrong location.
- Using the spark-plug socket and ratchet, remove the first spark plug.
- Install the new spark plug and torque to 25–30 N·m22 ft·lb (18–22 ft-lb).
- Reconnect the spark plug wire, making sure the connector is fully seated on the spark plug.
- Repeat steps 2 through 6 for the remaining spark plugs.
Reading Spark Plug Condition
A used spark plug is a window into how the cylinder it came from is running. Remove the plugs and compare each one against the conditions below. Keep track of which cylinder each plug came from — a single odd plug points to a problem in that one cylinder, while the same appearance on every plug points to a fault affecting the whole engine.
Normal
Brown to grayish-brown in color with only a small amount of electrode wear. The plug is doing its job and the mixture and ignition are correct.
Dry black (carbon) deposits
Dry, black, sooty deposits. The mixture is too rich or the spark is too weak. This condition can cause a miss or hesitation.
Oily deposits
Wet, oily deposits. Oil is reaching the combustion chamber past leaking valve seals or a worn piston oil ring. This condition can also cause a miss or hesitation.
Worn electrodes
Brown to grayish-brown in color but with obvious electrode wear. The spark plugs are simply worn out and should be replaced.
White insulator, eroded electrode
The electrode is excessively worn with no deposits and the insulator is white, as if the plug were new. The engine mixture is too lean, or the spark plug heat range is too hot for the engine.
Heavy tannish deposits
Heavy tan-colored deposits. These come from oil leaking past worn valve guide seals into the cylinder, or from a buildup of gasoline additives.