Spark plug fouling detection for ignition system

A method for an engine comprising:

• boosting intake air delivered to the engine with a turbocharger;
• and
• for each combustion event of a cylinder of the engine, direct injecting fuel to the engine and outputting a dwell command on a control wire of a spark plug ignition coil;
• estimating an ignition current switching time taken for a current of the control wire to drop below a pre-determined value;
• counting a fraction of combustion cycles of the cylinder over a given vehicle drive cycle that have the ignition current switching time longer than a threshold duration;
• and indicating fouling of the cylinder's spark plug due to fuel additive accumulation on the spark plug based on the fraction being larger than a threshold percentage.