The most common culprit is the valve cover gasket cracking due to engine heat cycle degradation. Always apply a small dab of liquid silicone sealer (RTV) at the T-joints where the timing chain cover meets the cylinder head before installing a new rubber gasket.
The manual provides step-by-step diagnostic workflows for common mechanical and electronic faults seen on aging 1SZ-FE platforms: 1. Idle Knocking and Rattling 1szfe engine manual top
