Hydraulic Excavator Troubleshooting: 10 Common Problems, Causes, and Diagnostic Test Sequence

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Hydraulic Excavator Troubleshooting: 10 Common Problems, Causes, and Testing Sequence

The hydraulic system is the main system that operates the boom, arm, bucket, swing motor, travel motor, and various attachments on an excavator. Power from the engine is transmitted through the main pump and converted into pressurized oil flow, which is then directed by the control valve to the actuator.

In modern excavators, hydraulic performance is not determined solely by the condition of the pump. The system involves the engine, main pump, pilot system, main control valve, relief valve, sensors, controller, solenoids, hoses, hydraulic cylinders, motors, filters, cooler, and oil quality.

Therefore, symptoms such as slow movement do not automatically mean that the main pump is damaged. The causes may include incorrect engine speed, a clogged filter, low pilot pressure, pump control problems, a leaking main relief valve, excessively hot oil, internal cylinder leakage, or an unsuitable hydraulic load.

Volvo Construction Equipment explains that excavators use pressurized fluid to generate force while operating the boom, arm, attachments, swing, and travel functions. In newer systems, hydraulic commands can also be controlled electrohydraulically through electrical signals sent to the main control valve.

This article discusses ten hydraulic excavator problems commonly found in the field, along with the sequence of inspections and tests.

Important note: Pressure, flow, temperature, cycle time, pilot pressure, case drain, engine speed, and internal leakage limits differ for each model and serial number. Always use the shop manual, Testing and Adjusting Manual, and hydraulic schematic for the unit being inspected.

Safety Before Inspecting the Hydraulic System
The hydraulic system can retain pressure even after the engine has been shut down. Accumulators, load-holding valves, check valves, cylinders, and attachment loads can maintain pressure within the system.

Before carrying out an inspection:

• Park the unit on level and safe ground.
• Lower all attachments to the ground.
• Shut down the engine according to the correct procedure.
• Move the control levers to release residual pressure according to OEM instructions.
• Apply lockout and tagout if required.
• Use pressure gauges, hoses, fittings, and quick couplers with the appropriate pressure ratings.
• Do not stand beneath an attachment that is supported only by hydraulic pressure.
• Do not use your hands to search for leaks.
• Caterpillar recommends using paper or cardboard to locate hydraulic leaks because a jet from a small hole can penetrate the skin. HSE also warns that a hydraulic injection injury may appear minor on the surface but can cause serious tissue damage, require major surgery, or even result in amputation.

If pressurized oil enters the skin, the condition must be treated as a medical emergency. 

Basic Principles of Hydraulic Excavator Diagnosis

Before replacing any components, understand the following four principles.

1. Flow determines movement speed

Slow boom, arm, bucket, swing, and travel functions are generally related to insufficient flow.

Possible causes include:

• Low engine speed.
• Pump displacement does not reach the target.
• Incorrect pump control pressure.
• Suction restriction.
• Internal leakage.
• Incorrect oil viscosity.
• Valve does not open fully.

2. Pressure determines the ability to resist load

An excavator that moves fast enough without a load but cannot lift properly may be unable to build or maintain pressure under load.

Possible causes include:

• Main relief pressure is too low or the valve is leaking.
• Worn main pump.
• Internal cylinder leakage.
• Leaking control valve.
• Faulty load-holding valve.
• Incorrect engine or pump power control.

3. Temperature affects oil viscosity

Oil that is too thick under cold conditions can increase pressure loss and interfere with pump filling. Conversely, oil that becomes too thin when hot increases internal leakage in pumps and valves and reduces lubrication film thickness.

Parker technical documentation explains that high viscosity can increase pressure loss, worsen suction filling, and increase the risk of cavitation. Viscosity that is too low increases leakage through pump and valve clearances while reducing lubrication film thickness

A pressure test is not the same as a flow test

4. A main pump may reach relief pressure while still producing low flow.

Therefore:

A pressure test assesses the system’s ability to build pressure.
A flow test assesses the volume of flow produced.
A case drain test assesses indications of internal leakage in the pump or motor.
A cycle time test assesses the final performance of attachment movement.
A cylinder drift test assesses the actuator’s ability to hold a load.
Data That Must Be Collected Before Testing
Inspection data Result
Unit model
Serial number
Main pump model
Hour meter
Hydraulic oil type
Oil operating hours
Filter operating hours
Oil temperature
Actual engine speed
Work mode
Active fault code
Logged fault code
Operator complaint
Most recent work performed on the unit
Check whether the problem occurs:

• While the oil is still cold.
• After the unit has operated for several hours.
• During only one movement.
• Across all hydraulic functions.
• During single operation.
• Only during combined operation.
• At engine idle.
• At engine high idle.
• When the attachment is under load.

This symptom pattern helps narrow down the source of the problem before measuring instruments are installed.

Quick Hydraulic Excavator Diagnosis Table

Main Symptom	Initial Inspection	Further Testing
All movements are slow	Engine speed, oil level, filter, pilot pressure	Pump pressure, flow, control pressure
Weak after warming up	Oil temperature and viscosity	Hot pressure, flow, case drain
Unable to lift properly	Main pressure and relief setting	Cylinder isolation and leakage test
Oil overheats quickly	Cooler, fan, oil level, and restriction	Internal leakage and relief operation
Noisy main pump	Oil level, suction hose, and aeration	Inlet vacuum and case pressure
Cylinder drifts downward	External leakage and control position	Drift test and cylinder isolation
Oil leakage	Hose, fitting, seal, and routing	Pressure test after repair
Jerky movement	Pilot pressure, air, and contamination	Solenoid current and valve inspection
Low pressure	Engine speed, pilot pressure, and pump command	Relief valve, flow, and leakage test
High case drain	Oil temperature and drain restriction	Pump efficiency and contamination check

1. Slow Hydraulic Excavator Movement

Slow hydraulic movement can occur across all functions or only in one specific function.

A. All hydraulic functions are slow

If the boom, arm, bucket, swing, and travel functions are all slow, inspect the components shared by all functions.

Possible causes:

• Engine speed is too low.
• The engine is derated or experiencing low power.
• The hydraulic oil level is low.
• The oil is too viscous.
• The hydraulic filter is clogged.
• The suction strainer is clogged.
• Air is entering the suction line.
• Pilot pressure is low.
• Pump displacement does not reach its maximum value.
• Pump control pressure is incorrect.
• The main pump has internal leakage.
• The main relief valve is leaking to the tank.
• Power-shift or pump torque control is incorrect.
• The controller is limiting the pump command.

Danfoss lists excessive inlet vacuum as a cause of low output flow. A restriction in the case-drain line can also increase case pressure and cause the system to operate slowly.

B. Only one function is slow

If only a particular boom, arm, bucket, swing, or travel function is slow, focus the inspection on that function’s circuit.

Possible causes:

• The pilot valve does not open fully.
• The pilot hose is clogged or leaking.
• The proportional solenoid is faulty.
• The main control valve spool is stuck.
• The orifice is clogged.
• The port relief valve is leaking.
• The check valve is stuck.
• The cylinder has internal leakage.
• The hydraulic motor has internal leakage.
• The hose is pinched or internally damaged.
• Pump flow is not distributed correctly during combined operation.
• Inspection sequence for slow movement

Confirm which function is slow.

• Check the fault codes.
• Check the work mode and power mode.
• Measure the actual engine speed.
• Check the hydraulic oil level.
• Check the oil temperature and condition.
• Check the filter restriction indicator.
• Inspect the main pump suction hose.
• Check for foam or aeration in the tank.
• Measure the pilot pressure.
• Perform a cycle time test.
• Measure the main pump pressure.
• Check the pump control pressure.
• Perform a pump flow test if the pressure is normal but the cycle time remains slow.
• Inspect the cylinder or motor associated with the affected function.
• Compare the results with OEM specifications.
• Result interpretation
• Pressure is normal, but all functions are slow

Possible causes:

• Low pump flow.
• Low engine speed.
• Pump displacement is not reaching full capacity.
• Suction restriction.
• Excessive internal leakage.
• Only one function is slow

Possible causes:

• The pilot circuit for that function.
• The main control valve spool.
• The port relief valve.
• The cylinder or motor.
• The hoses and piping.
• Single operation is normal, but combined operation is slow

Check:

• Pump flow sharing.
• Pressure compensator.
• Load-sensing signal.
• Flow-control valve.
• Pump merging or dividing circuit.
• Engine-pump power matching.

Komatsu explains that work modes are designed to match engine speed, pump flow, and system pressure to the application. Therefore, an incorrect mode or pump-control command can affect hydraulic performance even when the mechanical components are not necessarily damaged.

2. Hydraulics Are Strong When Cold but Weak After Warming Up

This condition often indicates increased internal leakage as oil viscosity decreases.

When the oil is cold, it is still thick enough to help seal the clearances between components. As the temperature increases, the oil becomes thinner, allowing greater leakage past the pistons, valve plate, cylinder block, control valve spools, relief valves, or cylinder seals.

• Common symptoms
• Cycle time is normal at the beginning of operation.
• Movement becomes slower after one or two hours.
• The unit cannot lift properly after warming up.
• Hydraulic oil temperature increases.
• Main pump pressure drops when hot.
• Case drain increases as the oil becomes hot.
• Cylinder drift increases as the temperature rises.
• Pump noise changes after extended operation.

Possible causes

• Worn main pump.
• Worn valve plate or cylinder block.
• Internal cylinder leakage.
• Internal leakage in the main control valve.
• The main relief valve leaks when hot.
• The port relief valve does not seal properly.
• Oil viscosity is too low.
• The oil is contaminated with fuel or another fluid.
• The hydraulic oil does not meet specifications.
• The oil cooler is not working properly.
• The seals or O-rings lose their sealing ability when hot.

Fluid quality, cleanliness, temperature, and viscosity affect the efficiency and service life of the hydraulic system. Bosch Rexroth states that hydraulic fluid transmits power, lubricates components, reduces friction, prevents corrosion, and removes heat.

Hot-oil performance test procedure

Stage one: testing while cold
Record:

• Hydraulic oil temperature.
• Engine speed.
• Main pump pressure.
• Pilot pressure.
• Boom cycle time.
• Arm cycle time.
• Bucket cycle time.
• Swing speed.
• Travel speed.
• Cylinder drift.
• Case drain, if permitted by the procedure.

Stage two: warm up the system

Operate the unit through a safe work cycle in accordance with OEM procedures until the hydraulic oil reaches the specified test temperature.

Do not increase the temperature by continuously holding a function at relief longer than the limit specified in the manual.

Stage three: repeat the tests

Repeat all measurements using:

• The same engine speed.
• The same work mode.
• The same attachment position.
• The same load condition.
• The same measuring points.

Stage four: compare the results

Parameter Cold          Hot Change
Main pressure
Pump flow
Boom cycle time
Arm cycle time
Bucket cycle time
Case drain
Cylinder drift

3. Excavator Cannot Lift Properly

An excavator may appear to operate normally without a load but fail to lift an attachment, material, or a specific load.

This problem does not always originate from the main pump. Lifting capability is also affected by the main relief valve, port relief valve, cylinder, control valve, engine power, pump control, boom position, working radius, and the unit’s lifting capacity.

Possible causes

The load exceeds the lifting capacity.
Low engine power.
Engine speed does not meet specifications.

• Low pump pressure.
• The main relief setting is too low.
• The main relief valve is leaking.
• The port relief valve is leaking.
• Pump displacement does not reach the target.
• Incorrect power-shift pressure.
• Internal leakage in the boom cylinder.
• The control valve spool is leaking.
• The load-check valve does not seal properly.
• The counterbalance valve or holding valve is faulty.
• The hydraulic oil is too hot.
• The system is derated.

Diagnostic sequence for weak lifting

• Make sure the load is within the unit’s lifting chart.
• Check the unit position and lifting radius.
• Confirm the symptoms in the boom, arm, and bucket functions.
• Check the fault codes and derate condition.
• Measure the engine speed while the function is under load.
• Check the hydraulic oil temperature.
• Measure the main pump pressure.
• Compare the boom function pressure with the pressure of other functions.
• Inspect the main relief valve and port relief valve.
• Perform a cylinder drift test.
• Perform a cylinder isolation test.
• Inspect the main control valve if the cylinder is normal.
• Perform pump flow and case-drain tests if the required pressure or flow cannot be reached.

Distinguishing between the pump, relief valve, control valve, and cylinder

All functions are weak

Check:

• Main pump.
• Main relief valve.
• Pump control.
• Engine power.
• Pilot pressure.
• System derate.
• Only the boom is weak

Check:

• Boom cylinder.
• Boom spool.
• Boom port relief valve.
• Load-check valve.
• Boom regeneration circuit.
• Holding valve, if equipped.
• The attachment cannot hold the load

Check:

• Cylinder piston seal.
• Main control valve.
• Load-check valve.
• Counterbalance valve or holding valve.
• External leakage.

Some excavators use a lifting mode that increases hydraulic pressure for lifting operations within the system limits. This indicates that lifting capability is related to pressure control and machine configuration, not only to the size of the main pump.

4. Hydraulic Oil Overheats Quickly

Heat in the hydraulic system comes from energy that is not converted into useful work.

When pressure drop, internal leakage, a relief valve that continuously opens, flow restriction, or inefficient component operation occurs, some of the power is converted into heat.

Parker explains that the cooler must dissipate energy that is not used by the system. Maintaining the temperature within the appropriate range helps preserve viscosity, efficiency, oil life, and component life.

• Symptoms of hydraulic overheating
• The hydraulic oil temperature warning is active.
• The unit becomes weaker after warming up.
• Movement becomes slower.
• Seals or hoses fail quickly.
• The oil changes color or smells burnt.
• The main pump becomes noisier.
• The hydraulic tank becomes very hot.
• The engine fan operates continuously.
• The controller reduces pump flow.

Some hydraulic excavators use a control system that reduces oil flow when the hydraulic or engine temperature becomes too high in order to protect components.

Possible causes

• Cooling problems
• The hydraulic oil cooler is covered with dust or mud.
• The inside of the cooler is clogged.
• The cooling fan is not working.
• Fan speed is low.
• The fan reverse function is not working.
• The seals around the cooler are damaged, allowing air to pass through gaps.
• The cooler bypass valve is stuck.
• The ambient temperature is very high.

Oil problems

• The hydraulic oil level is low.
• The oil does not match the required viscosity grade.
• The oil is oxidized.
• The oil is mixed with another type of oil.
• The oil is contaminated with water.
• The oil contains air.
• Filter restriction is too high.

Internal leakage

• The main pump is worn.
• The hydraulic motor is worn.
• The cylinder has internal leakage.
• The main control valve is leaking.
• The relief valve is leaking or continuously opening.
• Pump standby pressure is too high.

Operation

• The attachment is held at relief for too long.
• The workload exceeds the unit’s capacity.
• The operator repeatedly performs stall operations.
• The work mode is incorrect.
• The additional attachment does not match the unit’s flow and pressure specifications.

Hydraulic overheating diagnostic sequence

• Record the temperature when the warning appears.
• Make sure the sensor reading is accurate.
• Check the oil level and condition.
• Check the specifications of the oil being used.
• Visually inspect the cooler.
• Clean the cooler fins according to the correct procedure.
• Check the fan speed and airflow direction.
• Inspect the fan motor or fan control.
• Inspect the cooler bypass.
• Check the filter differential pressure.
• Check for return restriction.
• Measure standby pressure.
• Check whether the relief valve remains active.
• Perform cylinder drift and leakage tests.
• Perform main pump flow and case-drain tests.
• Review the oil analysis results.

The Cat hydraulic oil cooler is designed to transfer heat from the oil so that the system temperature remains controlled. Cooler cleanliness and proper airflow are important parts of an overheating inspection.

5. Noisy Hydraulic Main Pump

Abnormal main pump noises may include:

• Humming.
• Growling.
• A gravel-like sound.
• Knocking.
• Whining.
• A harsh sound that changes with RPM.
• A sound that appears only when a function is operated.

Not every pump noise indicates internal damage. The main pump may produce noise due to cavitation, aeration, inlet restriction, loose mounting, a damaged coupling, high case pressure, or pressure pulsation.

A. Cavitation

Cavitation occurs when the pressure at the pump inlet is too low, causing vapor cavities to form and then collapse in a higher-pressure area.

Possible causes:

• Low hydraulic oil level.
• Clogged suction strainer.
• Kinked suction hose.
• Internal damage to the suction hose.
• Oil is too viscous.
• Clogged hydraulic tank vent.
• Pump speed is too high.
• The suction line is too small.
• Oil temperature is too low.

Bosch Rexroth and Danfoss warn that improper inlet conditions can cause cavitation, low output flow, noise, and component damage.

B. Aeration

Aeration occurs when air enters and mixes with the oil.

• Possible causes:
• Loose suction-line clamp.
• Cracked hose.
• Damaged O-ring.
• Oil level is too low.
• Return oil creates a vortex.
• Oil returns to the tank above the oil level.
• The system has not been bled after repairs.
• The pump shaft seal draws in air.

Signs of aeration:

• Foamy oil.
• The oil appears cloudy.
• Jerky movement.
• Noisy pump.
• Unstable hydraulic response.

C. Mechanical damage

• Worn pump bearing.
• Damaged piston or slipper.
• Worn valve plate.
• Damaged cylinder block.
• Damaged pump coupling.
• Loose mounting bolts.
• Misalignment.
• Metal contamination.

Inspection sequence for a noisy main pump

• Determine when the noise occurs.
• Check the oil level.
• Check for foam in the hydraulic tank.
• Inspect the suction hose and clamps.
• Inspect the tank breather.
• Inspect the suction strainer.
• Check oil viscosity and temperature.
• Measure inlet pressure or vacuum.
• Measure pump case pressure.
• Check for case-drain restriction.
• Measure case-drain flow.
• Inspect the filter and magnetic plug.
• Perform an oil analysis.
• Inspect the pump coupling and mounting.
• Stop the unit if debris or severe knocking is found.

6. Hydraulic Cylinder Drifts Downward

Cylinder drift is movement of the piston rod without an operator command or the cylinder’s inability to maintain the attachment position.

Drift does not always mean that the cylinder piston seal is damaged. Oil can also leak through the main control valve, port relief valve, load-check valve, holding valve, hose, fitting, or external seal.

Parker troubleshooting documentation recommends not immediately assuming that the source of drift is the cylinder. If no internal cylinder leakage is found, other components in the circuit must be inspected.

Possible causes

• Internal leakage through the piston seal.
• Worn cylinder barrel.
• Cracked piston.
• Leaking main control valve spool.
• The port relief valve does not seal properly.
• Leaking load-check valve.
• Leaking counterbalance valve.
• Faulty holding valve.
• Leaking hose or fitting.
• The control lever does not return to neutral.
• Pilot pressure is still moving the spool.
• The solenoid valve does not return to the neutral position.

Cylinder drift test procedure

• Warm the hydraulic oil to the specified test temperature.
• Park the unit on level ground.
• Clean the rod and cylinder area.
• Position the attachment according to the manual procedure.
• Shut down or keep the engine running according to the OEM method.
• Mark the piston rod position.
• Measure the change in position over a specified period.
• Record the temperature, time, position, and load.
• Compare the result with the OEM limit.

Isolation test procedure

The purpose of an isolation test is to distinguish leakage in the cylinder from leakage in the valve.

The general procedure is:

• Position the attachment safely.
• Release residual pressure.
• Install blocking or support according to the procedure.
• Isolate the cylinder ports using approved equipment.
• Repeat the drift test.
• Compare the results before and after the cylinder is isolated.
• Interpretation

Drift stops after the cylinder is isolated:
The source of the leakage is likely in the control valve or in the circuit upstream of the cylinder.

Drift continues after the cylinder has been isolated:
The leakage is likely in the piston seal, barrel, piston, or another cylinder component.

The isolation method must follow the unit manual because closing the ports of a cylinder that is still supporting a load can trap extremely high pressure

7. Hydraulic Oil Leakage

Hydraulic leakage can be divided into two types:

• External leakage: oil escapes and is visible from the outside.
• Internal leakage: oil passes through internal clearances or seals without being visible from the outside.

Sources of external leakage

• Cracked hose.
• Hose rubbing against another surface.
• Stretched hose.
• Damaged crimping.
• Loose fitting.
• Damaged O-ring.
• Damaged flange seal.
• Leaking cylinder rod seal.
• Leaking pump shaft seal.
• Leaking control valve cover.
• Cracked cooler or tank.
• Fatigued piping.

Parker explains that rod seal leakage is usually related to a worn or damaged seal, but rod damage, contamination, and alignment must also be inspected.

Checklist for finding the source of a leak

• Park the unit in a clean area.
• Lower the attachment.
• Release residual pressure.
• Clean the wet area.
• Inspect the hose routing.
• Inspect points of contact and abrasion.
• Inspect the clamps and supports.
• Inspect the fittings and flanges.
• Inspect the cylinder rod.
• Operate the unit from a safe distance.
• Use cardboard to detect a pinhole leak.
• Mark the source of the leak.
• Replace components according to specifications.
• Clean up spilled oil.
• Perform a pressure test after the repair.

Mistakes to avoid

• Overtightening fittings.
• Using sealant on connections that do not require it.
• Reusing old O-rings.
• Combining incompatible hoses and couplings.
• Welding or patching a pressure hose.
• Installing a hose while it is twisted.
• Allowing a hose to rub against the structure.
• Searching for leaks with your hands.

Contamination and incorrect hose installation can accelerate hose assembly damage. Hoses and couplings must be selected as a complete system with the appropriate pressure rating and compatibility.

8. Jerky or Intermittent Attachment Movement

Jerking is movement that is not smooth, surges, pauses momentarily, or changes speed even though the joystick position remains relatively constant.

Possible causes

• Fluid and supply
• Low hydraulic oil level.
• Air entering the system.
• Oil viscosity is too high.
• Clogged filter.
• Suction restriction.
• Contaminated hydraulic oil.
• Unstable pump flow.

Pilot and electronic control

• Fluctuating pilot pressure.
• Stuck pilot valve.
• Clogged pilot filter.
• Unstable joystick sensor.
• Faulty proportional solenoid.
• Loose harness or connector.
• Unstable voltage supply.
• Unstable controller command.

Main control valve

Stuck spool.

• Contamination on the spool.
• Damaged return spring.
• Clogged orifice.
• Stuck pressure compensator.
• The load-check valve is not functioning.
• Faulty regeneration circuit.

Actuator

• Damaged cylinder barrel.
• Damaged piston seal.
• Bent rod.
• Faulty cushion valve.
• Internal damage to the hydraulic motor.

Diagnostic sequence for jerky movement

• Identify the function that is jerking.
• Check whether the problem occurs when the system is cold or hot.
• Check the hydraulic oil level.
• Check for foam and aeration.
• Check for filter restriction.
• Check the fault codes.
• Monitor the joystick command.
• Monitor the proportional solenoid current.
• Measure the pilot pressure.
• Compare the pilot pressure with that of a normally operating function.
• Inspect spool movement.
• Inspect the valve for contamination.
• Perform a cylinder or motor leakage test.
• Inspect the main pump control if all functions are jerking.

Oil cleanliness is extremely important because contamination can interfere with components that have small clearances, including valves and pump controls. Bosch Rexroth identifies hydraulic fluid quality and cleanliness as important factors for operational safety, efficiency, and system service life.

9. Low Hydraulic Pressure

A pressure test must be performed systematically. Do not immediately turn the main relief adjustment screw without first confirming the condition of the engine, pilot circuit, pump control, hoses, gauge, and measurement point.

Possible causes of low pressure

• Low engine speed.
• Insufficient engine power.
• Low pilot pressure.
• The main pump does not reach the required displacement.
• Incorrect pump control pressure.
• Low main relief setting.
• Leaking main relief valve.
• Leaking port relief valve.
• High internal pump leakage.
• The main control valve leaks to the tank.
• Internal leakage in the cylinder or motor.
• Suction restriction.
• The hydraulic oil is too hot.
• The pressure sensor is inaccurate.
• The controller limits the pump command.

Danfoss lists the pressure-adjusting valve, pilot valve, pump, cylinder, and valve leakage as several possible causes of low or unstable working pressure.

Pressure test equipment

• Pressure gauge with the appropriate rating.
• Test hose.
• Quick coupling.
• Thermometer or data monitor.
• Tachometer or diagnostic tool.
• Hydraulic schematic.
• Testing and Adjusting Manual.
• Appropriate personal protective equipment.

Pressure test sequence

• Confirm the pressure test specifications.
• Make sure the gauge is in good condition.
• Check the pressure ratings of the hose and fittings.
• Install the gauge at the correct test port.
• Warm up the hydraulic oil.
• Set the work mode according to the manual.
• Make sure the engine speed meets specifications.
• Record the standby pressure.
• Operate the function being tested.
• Record the pressure before relief operation.
• Record the relief pressure.
• Do not hold the function at relief longer than the OEM limit.
• Compare one pump with the other if the system uses tandem pumps.
• Check the pilot and control pressures.
• Continue with a flow or leakage test if the pressure does not meet specifications.

How to interpret pressure test results

Standby pressure is too high
Possible causes:

• The pump does not destroke.
• The load-sensing signal is faulty.
• The control spool does not return to neutral.
• The pump regulator is stuck.
• The orifice is clogged.
• Standby pressure is normal, but relief pressure is low

Possible causes:

• The main relief setting is too low.
• The main relief valve is leaking.
• Internal pump leakage.
• Engine speed drops under load.
• The pump does not reach full displacement.
• Pressure is low in only one function

Possible causes:

• Port relief valve.
• Control spool.
• Load-check valve.
• Cylinder or motor.
• Hose circuit.
• Pressure is reached, but movement is slow

Possible causes:

• Low pump flow.
• Restriction.
• Internal leakage.
• Low pump displacement.
• Low engine speed.

Increasing relief pressure without OEM approval and specifications may damage hoses, pumps, valves, cylinders, the structure, and other components. OSHA states that modifications to achieve higher pressure must be approved by a competent engineer or the equipment manufacturer.

10. High Main Pump Case Drain

    The main pump has internal clearances that allow a small amount of oil to lubricate and cool the components inside the pump housing. This oil is returned through the case-drain line.

A certain amount of case drain is normal. However, an increase in case-drain flow may indicate increased internal leakage.

Danfoss explains that the case-drain line also removes hot fluid from the pump or motor. The line must be installed correctly and must not have excessive restriction.

Possible causes of high case drain

• Worn pistons and cylinder block.
• Worn valve plate.
• Damaged slipper or swash plate.
• Damaged pump bearing.
• Pump scoring.
• The oil is too hot.
• Oil viscosity is too low.
• Abrasive contamination.
• Cavitation.
• Aeration.
• Overpressure.
• The pump has operated beyond its service-life limit.
• High case pressure does not necessarily mean high flow

Case pressure can increase because of:

• A pinched case-drain hose.
• A hose that is too small.
• A clogged case-drain filter.
• A clogged fitting.
• Incorrect return-line routing.
• The drain line is connected to a pressurized area of the tank.
• The oil is too viscous.

A restriction in the case-drain line can interfere with response and damage the shaft seal even when the pump’s internal condition is not necessarily the main cause.

Case-drain test preparation

Prepare:

• A flow meter or calibrated container.
• A stopwatch.
• A pressure gauge for case pressure.
• Temperature monitoring equipment.
• A test hose with the appropriate rating.
• Clean plugs and caps.
• A recording form.
• The OEM manual.
• Case-drain measurement sequence
• Make sure the procedure is appropriate for the pump model.
• Clean the test-port area.
• Inspect the routing of the case-drain hose.
• Check for restriction before testing.
• Install the measuring equipment according to the procedure.
• Make sure the pump housing remains filled with oil.
• Start the engine under safe conditions.
• Warm the oil to the specified test temperature.
• Record case drain at low idle.
• Record it at high idle if required.
• Record it while the pump is loaded according to the procedure.
• Measure case pressure at the same time if required.
• Compare the first and second pumps.
• Compare the results with the OEM limits.
• Inspect the oil filter and review the oil analysis results if the readings are high.

Result Interpretation

High case drain under all conditions
The pump may have experienced severe internal wear.

Case drain increases sharply as pressure is raised
Internal leakage may increase when the pump is placed under load.

Case pressure is high, but flow remains normal
Check for restriction in the drain line.

One pump is high, while the other is normal
Focus the inspection on the pump showing the high reading, but still check the load circuit and control pressure for each pump.

High case drain accompanied by metal debris
Stop operation and carry out further inspection. Operating the pump may spread contamination to the main control valve, cylinders, motors, and other components.

Hydraulic Excavator Troubleshooting Flowchart

OPERATOR COMPLAINT
↓
Confirm the function, temperature, load, and time of occurrence
↓
Check the service history and most recent work performed
↓
Read the fault codes and actual parameters
↓
Inspect the oil level, leaks, hoses, filters, and cooler
↓
Does the problem occur in all functions?
├── YES
│ ↓
│ Check engine speed, pilot pressure,
│ pump control, main relief valve, and main pump
│
└── NO
↓
Check the pilot circuit, control spool,
port relief valve, hoses, and related actuator
↓
Measure the hydraulic oil temperature
↓
Perform a cycle time test
↓
Perform a pressure test
↓
Is the pressure normal, but movement slow?
├── YES → Perform a flow test and internal leakage test
└── NO → Check the relief valve, pump control, and pump condition
↓
Perform a cylinder drift test, motor leakage test,
or case-drain test according to the symptoms
↓
Compare the results with OEM specifications
↓
Determine the root cause
↓
Carry out repairs, flushing if required, and a final test
↓
Document the results

Checklist Pemeriksaan Awal Hydraulic Excavator

Item pemeriksaan	Normal	Abnormal	Keterangan
Hydraulic oil level	☐	☐
Warna dan bau oli	☐	☐
Oli berbusa	☐	☐
External leakage	☐	☐
Suction hose	☐	☐
Tank breather	☐	☐
Hydraulic filter	☐	☐
Hydraulic cooler	☐	☐
Cooling fan	☐	☐
Hose dan piping	☐	☐
Cylinder rod	☐	☐
Main pump noise	☐	☐
Fault code	☐	☐
Engine speed	☐	☐
Hydraulic temperature	☐	☐

Pressure-Test Recording Sheet

Parameter	Low idle	High idle	Loaded	Spesifikasi
Engine speed
Pilot pressure
Pump 1 pressure
Pump 2 pressure
Main relief pressure
Boom pressure
Arm pressure
Bucket pressure
Swing pressure
Travel pressure
Power-shift pressure
Hydraulic temperature

Cycle-Time Test Sheet

Fungsi	Waktu aktual	Spesifikasi	Status
Boom raise
Boom lower
Arm in
Arm out
Bucket curl
Bucket dump
Swing 3 putaran
Travel jarak tertentu

Main-Pump Case-Drain Form

Kondisi pengujian	Pump 1	Pump 2	Spesifikasi
Hydraulic oil temperature
Low-idle case drain
High-idle case drain
Loaded case drain
Case pressure
Pump pressure
Engine speed When Must the Unit Be Stopped Immediately? Stop the unit and perform an inspection if any of the following conditions are found: The hydraulic oil temperature exceeds the limit. The oil is excessively foamy. The main pump produces severe knocking. Case drain increases drastically. Metal debris is found in the filter. A hose is bulging or its reinforcement wire is exposed. There is a pinhole leak in a pressure hose. The attachment lowers uncontrollably. The cylinder rod is bent or damaged. Main pump pressure is suddenly lost. The hydraulic oil smells burnt. The unit experiences dangerous jerking. A leak is located near the exhaust or another heat source. The attachment cannot be lowered safely. Common Mistakes During Hydraulic Troubleshooting Immediately concluding that the main pump is damaged Slow movement can result from engine speed, pilot pressure, pump control, the filter, suction restriction, the relief valve, the control valve, or the actuator. Performing only a pressure test A worn pump may still reach relief pressure but be unable to maintain flow during operation. Performing tests without recording the oil temperature Pressure, flow, case drain, and cylinder drift results can change because oil viscosity is affected by temperature. Adjusting the relief valve before finding the root cause Increasing the relief setting can temporarily mask the symptoms while also increasing the risk of damage. Concluding that the cylinder is damaged based only on drift The leakage may be in the control valve, port relief valve, load-check valve, or holding valve. Ignoring engine performance The main pump requires engine power. An engine that is derated or does not reach rated speed can cause the entire hydraulic system to become weak. Ignoring cleanliness when opening hoses Danfoss emphasizes that cleanliness is extremely important when removing pumps, filters, hoses, and piping because dirt is a major enemy of hydraulic components. Open ports and hoses must be immediately sealed with clean caps or plugs. Operating the unit after debris has been found Particles from a damaged pump can circulate and damage the control valve, motor, cylinder, and replacement pump. Hydraulic Excavator FAQ Is slow hydraulic movement always caused by the main pump? No. Low engine speed, low pilot pressure, suction restriction, a clogged filter, a leaking relief valve, pump control problems, and internal actuator leakage can produce similar symptoms. Why are the hydraulics strong when cold but weak after warming up? Internal leakage may increase as the oil becomes thinner. The main pump, control valve, relief valve, cylinder, and motor must be inspected with the oil at operating temperature. Does normal pressure mean that the main pump is still in good condition? Not necessarily. The pump may reach high pressure while producing insufficient flow. Its condition must be confirmed through cycle-time, flow, and case-drain tests. What is the difference between cavitation and aeration? Cavitation is associated with excessively low inlet pressure, which causes vapor cavities to form. Aeration occurs when outside air enters and mixes with the oil. Why is the main pump noisy when the oil is still cold? Cold oil is more viscous, which increases suction pressure loss. Check the viscosity grade, temperature, suction strainer, suction hose, and tank breather. Is cylinder drift always caused by the piston seal? No. Drift can also be caused by the main control valve, port relief valve, load-check valve, counterbalance valve, or external leakage. Why does the hydraulic oil overheat quickly? Possible causes include a dirty cooler, weak fan, continuous relief-valve operation, internal leakage, flow restriction, low oil level, incorrect viscosity, or operating practices that keep a function at relief for too long. What does high main pump case drain mean? High case drain may indicate internal pump leakage. However, the result must be compared with specifications at the specified temperature, RPM, and pressure. Case-drain restriction must also be checked. Should the pressure relief valve be adjusted immediately when pressure is low? No. First check the engine speed, pilot pressure, pump control, gauge, main pump, internal leakage, and relief-valve condition. Why is only combined operation slow? The problem may be related to pump flow sharing, the pressure compensator, the pump merge or divide circuit, the load-sensing signal, engine-pump matching, or the control valve. Conclusion Hydraulic excavator troubleshooting must begin with the symptoms, not with the component suspected of being faulty. The ten main problems that must be inspected are: Slow hydraulic movement. Hydraulics are strong when cold but weak after warming up. The excavator cannot lift properly. The hydraulic oil overheats quickly. The main pump is noisy. The hydraulic cylinder drifts downward. Hydraulic oil leakage. Jerky or intermittent attachment movement. Low hydraulic pressure. High main pump case drain. The correct diagnostic sequence is: Confirm the complaint → perform a visual inspection → check the fault codes → measure the temperature and engine speed → perform a cycle-time test → perform a pressure test → perform a flow test → perform a leakage or case-drain test → compare the results with OEM specifications. The decision to replace the main pump, cylinder, control valve, or relief valve should be made only after measurement results confirm that the component is the actual root cause.