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I. The outlet pressure is almost equal to the inlet pressure, with no pressure reduction. This fault is characterized by the outlet and inlet pressures of the pressure regulator being nearly identical, and the outlet pressure not changing as the pressure adjustment knob is turned. The causes and troubleshooting methods are as follows. 1. Hydraulic jamming occurs due to burrs on the edges of the grooves on the main spool or in the valve body holes, or because of debris trapped in the gap between the main spool and the valve body holes; as a result, the main spool gets stuck in its maximum opening position. Because of this large opening, the oil pressure does not decrease. At this point, based on the above situations, measures such as deburring, cleaning, and restoring the accuracy of the valve holes and valve spools can be taken to resolve the issue. 2. If the fit between the main spool and the valve bore is too tight, or if the valve bore or spool gets scratched during assembly, this can cause the spool to get stuck in the fully open position; in such cases, it is possible to select an appropriate clearance. The fitting clearance for J-type pressure relief valves is generally 0.007 to 0.015 mm; the valve hole can be appropriately ground before fitting the valve core. 3. The short-damping hole of the main valve spool or the valve seat hole is blocked, resulting in the loss of the automatic adjustment function; the spring force of the main valve pushes it to its maximum opening position, causing unobstructed flow, with the inlet pressure equal to the outlet pressure. φ1.0 mm steel wire can be used, or compressed air can be used to clear the damping holes; after that, they can be cleaned and reassembled. 4. Pressure relief valve: The damping element with damping holes is pressed into the main valve spool; during use, it may come out due to insufficient interference fit. After rushing out, the pressure in the oil inlet chamber and the oil outlet chamber becomes equal (with no damping). The areas under force on the upper and lower parts of the valve spool are equal, but since there is a spring in the oil outlet chamber, the main valve spool always remains in its fully open position, ensuring that the outlet pressure equals the inlet pressure. At this point, it is necessary to remachine the damping element with a slightly larger outer diameter and reinsert it into the main spool. 5. Pressure relief valve: the oil drain hole is plugged with an oil plug at the time of manufacture. When this oil plug is used without being unscrewed, it traps oil in the upper chamber of the main spool (the spring chamber), causing the main spool to remain at its maximum opening position with no pressure reduction. The J-type tubular valve is the same. This phenomenon can also occur with J-type plate valves if the mounting plate is designed such that the L-port is not connected to the oil sump. 6. With tubular valves, it is easy to install the valve cover in the wrong direction during disassembly and repair (by 90° or 180°), which blocks the oil leakage port and prevents oil from being discharged. This leads to the same problem of oil accumulation, causing the main valve to remain at its maximum opening position without any pressure reduction. During repair, just ensure the valve cover is installed in the correct orientation. 7. For pressure relief valves, if the cover is installed in the wrong direction, it can cause the output oil port to communicate with the drain port, resulting in no pressure reduction; this aspect also needs to be taken into consideration. II. The outlet pressure is very low; even when the pressure adjustment knob is tightened, the pressure does not increase. 1. The inlet and outlet ports of the pressure regulator are connected in reverse: for plate valves, it is due to an error in the design of the mounting plate, while for cartridge valves, it is caused by incorrect connection of the pipes. The oil inlets and outlets of the J-type pressure relief valve are exactly opposite to those of the Y-type relief valve. When using it, please pay attention to the stamp marks near the oil inlet on the valve (such as Pl, P2, L, etc.), or refer to the product catalog of hydraulic components to avoid incorrect design or connection. 2. The pressure at the oil inlet is too low; after passing through the throttle opening of the pressure reducing valve, the pressure output at the oil outlet is even lower. In this case, the cause of the low pressure at the oil inlet should be identified (for example, a fault with the relief valve). 3. The load in the circuit downstream of the pressure regulator is too low, preventing pressure from being established. In this case, considering installing a throttle valve in series downstream of the pressure regulator can be a solution. 4. Poor contact and lack of sealing between the pilot valve (tapered valve) and its seat surface occur due to the accumulation of dirt ； Either the pilot cone valve is severely scratched, or the valve seat fitting hole is out of round or has gaps, resulting in poor sealing between the pilot valve spool and the valve seat hole. 5. During disassembly and repair, the cone valve was omitted or not installed in the valve seat hole. In response to this, the assembly or sealing condition of the cone valve can be checked. 6. The long damping hole on the main spool is blocked by debris; as shown in Figure 3-21, the oil in chamber P2 cannot flow into the main valve spring chamber through this long damping hole e. As a result, the feedback pressure from chamber P2 cannot reach the pilot cone valve, causing the pilot valve to lose its ability to regulate the pressure at the main valve outlet. When the damping hole is blocked, the main valve P. The chamber loses the effect of the oil pressure p3, causing the main valve to become a direct-acting spool valve with a very weak spring force (only the main valve balance spring). As a result, when the pressure at the outlet is low, it is possible to overcome the force of the balance spring and reduce the size of the throttle opening ymin in the pressure reducing valve. This allows the pressure p1 at the inlet to be significantly reduced to p2 through the throttle opening ymin, preventing the outlet pressure from rising. The long damping hole should be kept unobstructed. 7. If the pilot valve spring (pressure-regulating spring) is mistakenly installed as a soft spring, or if the spring suffers permanent deformation or breaks due to fatigue, it results in an inability to increase the P2 pressure; only a low fixed value can be achieved, and this value is far below the maximum adjustment pressure of the pressure reducing valve. 8. The pressure adjustment knob cannot be screwed in fully due to damaged threads or insufficient effective depth, which prevents the pressure from being adjusted to its maximum level. 9. The seal between the valve cover and the valve body is poor, resulting in severe oil leakage. The possible causes are the missing or damaged O-ring, loose tightening screws, as well as errors in the flatness of the valve cover’s end face; generally, it is bulged around the edges and concave in the middle. 10. The main spool gets stuck in a partially open position due to dirt, burrs, etc., resulting in low outlet pressure. It can be cleaned and deburred. III. Unstable pressure, large pressure fluctuations, and sometimes high noise. According to relevant standards, the pressure fluctuation for pressure regulators should be ±0.1 MPa, while for the JF type it is ±0.3 MPa; any deviation from these standards indicates large pressure fluctuations and unstable pressure. 1. The pressure relief valve is of the pilot-type; the pilot valve is interchangeable with the relief valve. Therefore, the reasons for large pressure fluctuations and the methods to address them can be found in the relevant sections on relief valves. 2. When a pressure regulator is used beyond its rated flow rate, oscillation of the main valve often occurs, resulting in an unstable pressure regulation. In such cases, the pressure at the outlet experiences a cycle of \"pressure increase, then decrease, again increase, then decrease.\" Therefore, it is essential to choose a pressure regulator of the appropriate model and specification. 3. A high back pressure at the oil drain port L can also cause large pressure fluctuations and unstable pressure; it is advisable for the oil drain pipe to return oil separately. 4. Spring deformation or poor stiffness (due to inadequate heat treatment) results in large pressure fluctuations; a qualified spring can be replaced. The above are purely personal opinions; Haiyou, please go ahead and add more