YIHUI technical notes · Updated 10 October 2026
This article explains hydraulic symptoms and contamination control directly on this page. It is a general Cat hydraulic-system reference for owners researching machines such as the 320D, not a serial-specific workshop procedure or fault-code dictionary. Possible causes below are not a diagnosis.
Hydraulic symptoms: what they can indicate
| Symptom | Possible contributors to investigate |
|---|---|
| External oil leakage | Damaged hoses or fittings, worn or incorrect seals, a scored or bent rod, or excessive system pressure. |
| Cylinder drift / attachment sinking | Seal or valve wear, cylinder damage or contaminated oil. Measuring drift helps assess the problem; the symptom alone does not identify the leaking component. |
| Slow hydraulic movements | Engine performance, low fluid level, valve faults, worn components or contaminated oil—not necessarily the main pump. |
| Unusual hydraulic noise | Low oil level, aeration, flow restriction, worn components or a relief-valve fault. Cavitation or bearing damage may require investigation. |
| High hydraulic oil temperature | Cooler problems, low oil, a restricted filter, unsuitable or contaminated oil, system wear or relief-valve problems. |
Source: Caterpillar Engine, Drive Train and Hydraulic Repair Indicators, PEGJ0042-02 (2012), printed pages 24–27 / PDF pages 13–14, published by Louisiana Cat. This is an original summary of selected facts, not a reproduction of the manual.
A safe first inspection
- Make the machine safe. Park on dry, level ground and lower implements. Use the machine’s own Operation and Maintenance Manual (OMM) for shutdown, isolation, lockout/tagout and pressure release. Hydraulic pressure can remain after the engine stops. Let hot components cool.
- Inspect accessible surfaces visually. Look for cracked, abraded or bulging hoses, leakage, damaged clamps and oil around cooler connections. Note damaged cylinder rods or dirt accumulating around seals. Read fluid sight gauges using the OMM’s required machine position.
- Protect the system during repairs. A qualified technician should cap disconnected hose ends promptly, use the correct assembly and routing, and verify the repair before returning to work.
Inspection and isolation principles: Cat hydraulic hose inspection and maintenance. No generic lever sequence is given here: the correct pressure-release method depends on the machine.
High-pressure fluid hazard: Never use bare hands to search for leaks, improvise a hose repair, fit incompatible couplings or open a pressurised connection. A tiny puncture can cause a serious injection injury. Suspected exposure needs immediate medical attention. Sources: UK HSE: high-pressure fluid injection hazards and the Cat safety guidance above.
Why a new seal or pump may not solve the problem
Particles can abrade rods, valves and seals, while accumulated deposits can make moving parts stick. Water and unsuitable fluid can also impair lubrication and fluid properties. A damaged rod can repeatedly carry contamination past a seal. Replacing a failed component without investigating contamination can leave the original failure mechanism in place.
A repair plan should identify where contamination entered and how far it spread. Cylinder assessment can include rod straightness and scoring, gland and piston sealing surfaces, barrel condition and bearing areas. Repairs need clean handling, protected ports and testing to the applicable manufacturer’s procedure. Depending on damage and contamination, the response may involve external filtration, system cleaning or component repair; these are not interchangeable solutions.
Source: Peterson: hydraulic contamination and system restoration, PDF pages 2–5. General technical principles only; this page does not adopt the paper’s generic dimensions as CAT 320D specifications.
What a meaningful repair report should contain
YIHUI recommends separating observations, test results and conclusions. A useful report identifies the machine and installed component, states the original symptom, names the applicable procedure, records test conditions and measured results, explains the confirmed cause, lists the repair and replacement parts, and records the post-repair checks. “Pump replaced” alone is not evidence that a slow-movement fault has been resolved.
Technical limit: Pressure settings, drift limits, cycle times, torque values and disassembly instructions must come from service information matching the complete machine serial number and installed arrangement. This article intentionally does not invent those values and is not labelled as a complete CAT 320D repair manual.