Impreza WRX STI F4-2.5L DOHC Turbo (2007)
Spark Plug: Testing and Inspection
INSPECTION
Check the electrodes and inner and ceramic insulator of plugs, noting the type of deposits and the degrees of electrode erosion.
1. Normal:
Brown to grayish-tan deposits and slight electrode wear indicates correct spark plug heat range.
2. Carbon fouled:
Dry fluffy carbon deposits on insulator and electrode are mostly caused by slow speed driving in city, weak ignition, too rich fuel mixture and dirty
air cleaner.