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When hydraulic system pressure keeps rising and falling, do not start by dismantling the pump or replacing the pressure valve. First, confirm whether the pressure fluctuation is a real change in system pressure or a measurement problem caused by the pressure gauge, pressure sensor, or signal chain. Then troubleshoot in the following order: operating conditions → hydraulic oil → suction side → pump → pressure control valve → actuators/piping → instrumentation and signal.
Common causes include unstable pump output, air entering the hydraulic system, restricted suction, abnormal relief or pressure control valves, internal leakage, oil contamination, and control-loop oscillation.
First confirm that the pressure is actually fluctuating before determining which part of the hydraulic system is causing the problem.
Pressure fluctuations can originate from the pump, valves, hydraulic oil, piping, actuators, or control system, so troubleshooting should not focus on only one component.
A faulty pressure gauge or pressure sensor can create a false pressure fluctuation.
Air entering the hydraulic system, excessive suction resistance, oil contamination, and sticking valve spools can all contribute to unstable pressure.
The most effective approach is to use reliable pressure measurement points and trend data to progressively isolate the fault instead of replacing components blindly.
Hydraulic system pressure is not expected to remain absolutely constant under all operating conditions.
Pressure naturally changes when the load, actuator movement, valve opening, or system control state changes.
The key question is:
Under essentially stable operating conditions, does the pressure still show abnormal periodic or random fluctuations?
Typical symptoms include:
The pressure gauge pointer moves rapidly up and down
Pressure periodically rises and then suddenly drops
Pressure reaches the setpoint but cannot remain stable
Pressure continuously oscillates while the actuator is operating
Pressure starts fluctuating after the equipment has been running for some time
Pressure is stable during cold startup but becomes unstable as the temperature increases
Different symptoms may point to completely different troubleshooting paths.
Therefore, in hydraulic troubleshooting, “pressure rising and falling” is a symptom, not the root cause.
Understanding the basic operating logic of a hydraulic system helps determine where to begin when abnormal pressure occurs.
A typical hydraulic system may include:
Hydraulic Reservoir → Hydraulic Pump → Pressure Control Components → Directional/Flow Control Components → Actuator → Return System → Reservoir
The hydraulic pump primarily provides flow.
System pressure, however, is related to:
Load
Flow
Flow resistance
Pressure control components
Actuator condition
One important point that is often misunderstood is:
If pump output flow is unstable, the pressure control valve is continuously adjusting, an actuator has internal leakage, or air is present in the system, these conditions may appear as pressure fluctuations.
Before troubleshooting begins, it is recommended to record several basic parameters.
First determine the equipment's normal:
Minimum pressure
Normal pressure
Maximum pressure
Pressure setpoint
Without a normal operating reference, it is difficult to determine whether the current pressure is actually abnormal or simply the result of normal operating conditions.
Avoid simply describing the condition as:
“The pressure is unstable.”
Instead, record specific values:
Set pressure: X MPa
Actual pressure: X–X MPa
For example:
The normal pressure setpoint is approximately 10 MPa, while the actual pressure periodically fluctuates between 8.5 and 10.5 MPa.
This type of data provides much more diagnostic value than simply saying “the pressure keeps rising and falling.”
Observe whether the pressure fluctuation is:
Rapid and high-frequency
Slow and gradual
Periodic
Random
Present only during a specific operating stage
Different fluctuation patterns may indicate different fault paths.
Record whether the problem occurs:
During cold startup?
After the equipment has been running for some time?
Does it become worse as the oil temperature increases?
If pressure is stable when the system is cold but begins fluctuating after operation, pay particular attention to oil temperature, viscosity changes, internal leakage, and valve condition.
For example:
Stable during standby
Pressure fluctuates when the hydraulic cylinder extends
Normal when the cylinder retracts
Fluctuates under high load
Stable under low load
This information can help determine whether the problem is related to a specific hydraulic circuit or actuator.
Hydraulic pressure problems should generally be investigated according to the following principle:
Rather than:
Pressure abnormal → Replace the pump immediately
A recommended troubleshooting sequence is:
First confirm that the pressure gauge, pressure sensor, or pressure transmitter is operating reliably.
Check the load, pump speed, valve status, and actuator movement.
Check the oil level, oil condition, filters, and suction piping.
Determine whether the pump has wear, insufficient suction, cavitation, or unstable output.
Pay particular attention to relief valves, reducing valves, unloading valves, and other pressure control components.
Check for external leakage, internal leakage, and abnormal actuator conditions.
Use continuous pressure data to determine the fluctuation period, amplitude, and operating conditions under which the fluctuation occurs.
This approach—starting with simple, external, and easily verifiable factors before moving toward core components—can reduce unnecessary disassembly and component replacement.
The location of the pressure measurement point can also affect troubleshooting results.
If a pressure sensor is installed near a localized restriction or special flow condition, the measured pressure may not represent the actual operating pressure of the entire system.
Therefore, the measurement point should be selected according to the actual hydraulic circuit.
Inspect:
Loose fittings
Sealing condition
Blocked pressure lines
Contamination at the measurement port
Significant mechanical vibration
If the pressure sensor is installed in an area with strong vibration, mechanical vibration may affect the instrument itself or its connection components.
For hydraulic systems with significant pressure pulsation, the measurement method and instrument response characteristics should also be considered according to the specific operating conditions.
Before installation, confirm:
Pressure range
Process connection
Sealing method
Media compatibility
Operating temperature
Vibration environment
Electrical connection
Do not select an excessively small pressure range simply to obtain supposedly higher “sensitivity.”
The hydraulic pump is one of the key components to investigate.
If the pump has:
Wear
Internal leakage
Insufficient suction
Cavitation
Drive abnormalities
it may produce unstable flow output, which can result in pressure fluctuations.
If the following symptoms occur at the same time:
Abnormal pump noise
Vibration
Changes in actuator speed
Periodic pressure fluctuations
the pump and its suction conditions should be investigated carefully.
Air is compressible.
If air enters the hydraulic system, it may cause unstable actuator movement and pressure signal fluctuations.
Common areas to inspect include:
Reservoir oil level
Suction-line sealing
Suction fittings
Piping connections
System bleeding/air removal
If the pump does not receive sufficient oil at its inlet, abnormal pump suction may occur, which can further affect system pressure.
Check:
Suction filter
Suction piping
Piping connections
Reservoir oil level
Hydraulic oil condition
If pump noise occurs together with pressure fluctuations, the suction side should receive particular attention.
Pressure control valves directly participate in system pressure regulation.
If a valve spool is:
Sticking
Contaminated
Worn
Adjusting abnormally
the valve may fail to maintain pressure within the expected range.
Unstable pressure control can appear as continuous pressure increases and decreases or periodic pressure oscillation.
Particles, water, and other contaminants in hydraulic oil can affect the normal operation of precision components such as pumps and valves and accelerate wear.
Contamination may also cause abnormal valve-spool movement or wear at sealing and mating surfaces, increasing internal leakage.
Therefore, hydraulic oil condition should not be overlooked when troubleshooting abnormal system pressure.
Internal leakage is different from external leakage.
External leakage can usually be identified by visible oil around the equipment, while internal leakage may not produce any obvious external oil leakage.
Potential locations include:
Hydraulic pump
Hydraulic cylinder
Hydraulic motor
Control valve
As internal leakage increases, the system may become unable to maintain the expected pressure and may also show symptoms such as slower actuator movement and increased oil temperature.
Changes in hydraulic oil temperature affect oil viscosity.
If the system operates normally when cold but becomes increasingly unstable as the oil temperature rises, consider:
Changes in oil viscosity
Changes in internal leakage
Changes in pump efficiency
Changes in valve condition
Do not simply assume that the pressure gauge is faulty.
In hydraulic systems using proportional valves, servo valves, pressure compensation, or closed-loop control, control parameters and dynamic response can also cause pressure oscillation.
For example:
Pressure increases → Control system reduces output → Pressure decreases → Control system increases output again → Pressure increases again
If this adjustment cycle continues repeatedly, noticeable periodic pressure fluctuations may occur.
Therefore, for hydraulic systems with automatic control, troubleshooting should also include control parameters and pressure trend analysis.
Pressure stability has different levels of importance across different types of equipment.
Abnormal pressure in a hydraulic power unit may result in:
Abnormal actuator movement
Longer pressure build-up time
Reduced equipment efficiency
Reduced control accuracy
Therefore, pressure monitoring is an important part of hydraulic power unit condition assessment.
Hydraulic pressure is directly related to operating load and actuator performance.
If pressure continuously fluctuates during operation, the following should be investigated together:
Load → Pump → Pressure Valve → Actuator → Measurement System
Hydraulic pressure changes in injection molding equipment may be associated with the pump, valves, actuators, and control system.
For automated equipment, continuous pressure monitoring makes it easier to identify trends than relying only on occasional manual readings from a pressure gauge.
In construction machinery, industrial equipment, and other hydraulically driven systems, if the following symptoms occur:
Slower movement
Jerky movement
Pressure fluctuation
Increased noise
pressure, temperature, and actuator status should be analyzed together to obtain a more complete diagnosis.
For a hydraulic system with unstable pressure, the pressure measurement instrument itself is also part of the diagnostic chain.
Therefore, before diagnosing a hydraulic component failure, confirm that the pressure measurement equipment is operating correctly.
A mechanical pressure gauge is suitable for quickly observing pressure changes in the field.
Its advantages include simple construction and no requirement for complex signal processing.
However, when pressure changes rapidly, it may be difficult to capture the complete fluctuation pattern by observing the gauge pointer manually.
A digital pressure gauge provides a more direct numerical pressure display and is suitable for field pressure checks and equipment maintenance.
It can be used as an auxiliary diagnostic instrument when engineers need to quickly verify the actual pressure value.
If pressure data needs to be transmitted to:
PLC
DCS
Data acquisition systems
Industrial control systems
a pressure transmitter may be considered.
Output options such as 4–20 mA and RS485 can be selected according to the requirements of the control system.
For pressure fluctuation troubleshooting, continuous pressure data acquisition is generally more useful than observing only instantaneous pressure values.
NOIKE-AH provides pressure measurement products for different hydraulic system applications, including:
NT30D Intelligent Pressure Transmitter
NT3051 Digital Pressure / Differential Pressure Transmitter
NT53D Intelligent Digital Pressure Gauge
NT800 Pressure Sensor / Switch
Digital Pressure Controller
The actual model should be selected according to:
Pressure Range + Medium + Temperature + Output Signal + Installation Interface + Accuracy Requirements + Environmental Conditions
rather than simply selecting a product based on its name or general category.
Common causes include unstable hydraulic pump output, air entering the system, restricted suction, abnormal pressure control valves, hydraulic oil contamination, internal leakage, oil temperature changes, and control-loop oscillation.
First confirm that the pressure measurement instrument is reliable and that the actual operating conditions have not changed. Then check the oil level, hydraulic oil, suction side, pump, and pressure control valves instead of immediately dismantling the hydraulic pump.
Yes. A pressure gauge itself can develop problems such as pointer sticking, mechanical damage, or measurement errors. If other system operating conditions appear normal but the pressure gauge reading is abnormal, cross-check it with a verified reliable pressure measurement instrument.
Yes. Air is compressible and can cause unstable actuator movement and pressure fluctuations when it enters a hydraulic system. Check the oil level, suction-line sealing, and system air-bleeding condition.
Yes. If a relief valve or other pressure control valve has contamination, sticking, wear, or abnormal adjustment, it may fail to regulate system pressure consistently and cause pressure fluctuations.
It can. Pump wear or internal leakage may reduce output capability or cause unstable flow, which can affect system pressure. A proper diagnosis should consider pressure, flow, noise, temperature, and actuator movement together.
Pay particular attention to oil temperature, oil viscosity, internal leakage, pump efficiency, and valve condition. Some problems become more apparent as hydraulic oil temperature increases.
No. The pump is only one possible cause. The pressure measurement instrument, hydraulic oil, suction side, pressure control valves, piping, actuators, and control system can also cause abnormal pressure.
Use a verified reliable pressure measurement device for cross-checking. If the reference instrument shows stable pressure while the original pressure sensor produces abnormal readings, further inspect the sensor, power supply, wiring, and signal chain.
It depends on the application. A pressure gauge or digital pressure gauge can be suitable for quick field observation. If pressure needs to be continuously transmitted to a PLC, DCS, or data acquisition system, a pressure transmitter is generally more suitable.
When hydraulic system pressure keeps rising and falling, the worst approach is to replace components based only on experience or guesswork.
A more reliable troubleshooting logic is:
The entire diagnostic process can also be summarized with the following questions:
Is the measurement instrument working correctly?
↓
Has the actual load changed?
↓
Are the hydraulic oil and suction conditions normal?
↓
Is the pump output stable?
↓
Is the pressure valve regulating correctly?
↓
Is there internal or external leakage?
↓
Is the control loop oscillating?
Pressure fluctuation itself is only a symptom.
Effective troubleshooting requires engineers to analyze the relationship between pressure, flow, temperature, actuator movement, and overall equipment status to progressively narrow down the possible fault location.
For industrial hydraulic systems that require long-term reliable operation, establishing reliable pressure monitoring and trend recording is generally more valuable than relying only on manual observation after a failure has already occurred.
Don't rush to replace the hydraulic pump or pressure valve.
If you are troubleshooting hydraulic system pressure fluctuations, you can provide the following information:
Normal operating pressure:
Pressure fluctuation range:
Hydraulic oil type:
Operating temperature:
Hydraulic pump type:
System flow rate:
Equipment type:
Pressure measurement method:
Current pressure gauge / pressure sensor model:
Operating condition when pressure fluctuation occurs:
NOIKE-AH can help evaluate a suitable pressure measurement and monitoring solution based on your actual application requirements.
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