Regulator Valve

WARNING: This page is about a different car, the 2004 Nissan Quest. However, it is still accessible from the selected car via links, so may be relevant.

Visually inspect the components for wear and damage.
Check the regulator valve sliding surface and the regulator spring.
Coat the regulator valve with engine oil. Check that the regulator valve falls smoothly into the valve hole by its own weight.
If damaged, replace the regulator valve set or the oil pump body as necessary.

Fig 1: Identifying Regulator Valve And Spring
Courtesy of NISSAN MOTOR CO., U.S.A.