Spark plug fouling detection for ignition system

An engine system, comprising:

• an engine including a cylinder;
• an ignition system including an ignition coil and a control wire coupled to a spark plug of the cylinder;
• and
• a controller with computer readable instructions on non-transitory memory for:
• over a vehicle drive cycle, setting a diagnostic flag for each of a change in an adaptive knock term over a first threshold, an engine pre-ignition rate over a second threshold, an engine misfire rate over a third threshold, an exhaust catalyst degradation rate over a fourth threshold, and an exhaust oxygen sensor degradation rate over a fifth threshold;
• during a first condition when two or more diagnostic flags are set, indicating fouling of the spark plug due to fuel additive accumulation;
• and
• during a second condition when only one diagnostic flag is set, not indicating fouling of the spark plug due to fuel additive accumulation and initiating a spark plug cleaning routine.