A valve mechanism supports the working process of an engine cylinder, and a camshaft is a key component required to open and close a valve. When a camshaft loosens, the balance of the engine disrupts. In the meanwhile, the generalised force at its centre of gravity (CG) alters. This study proposed a novel technique to detect camshaft loosening based on recognising the generalised force at the CG of the engine. We conducted Hanning windowed interpolation of discrete spectra to extract the precise phase and amplitude by utilising the acceleration signals at the engine cylinder and mounts and cylinder head. We then accurately computed the generalised force at the CG. Finally, we accurately extracted the camshaft loosening features by analysing the main harmonic orders for the generalised force. As indicated by simulations, our method can be used to effectively detect combustion engine faults involving camshaft loosening.