This paper introduces a new approach based on the van der Pol oscillator for detecting weak signals in powertrain fault diagnosis. In contrast to the detection technique relying on the Duffing oscillator, this proposed approach focuses on identifying weak signals by examining state transitions from non-periodic to periodic states. Numerical simulations demonstrate that the van der Pol oscillator-focused approach offers significant advantages over the Duffing oscillator-based one, particularly regarding its enhanced sensitivity to weak signals under strong noise disturbance. Extensive investigations into the proposed approach further confirm the effectiveness of the van der Pol oscillator in weak signal detection.