An internal combustion engine comprises an engine controller (ECU) governing fuel supply to the combustion engine by a fuelling map, which adapts the amount of fuel supplied to the engine responsive to engine speed, engine load and a volumetric efficiency of the engine. A variable intake system with at least one valve changes an effective tuning length of said variable intake system wherein degradation of said variable intake system can be diagnosed by comparing a calculated pressure variation of an inlet pressure of said variable intake system with a predetermined value for the pressure variation. The engine controller (ECU) adapts the fuelling map responsive to the diagnosis of degradation of the at least one valve of the variable intake system.