Aug.26,2026
A symptom-to-part field guide for equipment managers, workshop owners and fleet operators sourcing hydraulic components for XCMG, SANY, ZOOMLION and other brands.
In this article:
When an excavator loses hydraulic power, the temptation is to condemn the whole system. In practice, most failures trace back to a small set of components — and knowing which one saves you downtime and a wrong-order return. This guide maps the visible symptoms to the parts that actually fix them, then explains how to source those parts with correct fitment.
A hydraulic system uses oil as the working medium. The hydraulic pump converts engine mechanical energy into hydraulic energy and sends it through the circuit. Control valves (including the main control valve, relief valves and pilot valves) direct flow and set pressure. Cylinders and hydraulic motors convert that pressure back into motion — lift, dig, swing, travel. Filters and coolers keep the oil clean and cool. Every symptom below maps to one or more of these components.
| Symptom | Likely failed / worn part | First check |
|---|---|---|
| Whole machine has no action | Suction filter, oil level, pump coupling, main pump, main relief valve, suction hose | Oil level, suction filter, hose integrity |
| All actions slow / no power | Main pump, pump regulator, main relief valve, pilot filter | Relief & pilot pressure test |
| One function weak (boom/arm/bucket) | That circuit's spool valve, cylinder, seal kit | Cylinder sink test, spool inspection |
| Cylinder drifts / leaks at rod | Seal kit, rod, cylinder tube | Seal replacement or full cylinder |
| Travel or swing weak on one side | Central swivel joint, travel/swing motor, related valve | Swivel seal, motor pressure |
| Oil overheats | Hydraulic oil cooler, fan belt, oil grade, filter element | Cleaner, belt tension, oil spec |
| Pump noisy / hot shell | Internal pump wear (plunger, valve plate), suction filter, oil condition | Oil sample, suction filter, pump output |
If every function fails together, the fault is in the parts shared by all circuits — before the valves:
Because these parts sit on the common supply side, fixing them restores all functions at once.
When only the boom (or arm, or bucket) is slow and weak, the pump and main relief are fine — the problem is local to that circuit. Check, in order:
Hydraulic oil should stay in the 30–60 °C range and never exceed 70 °C. Overheating accelerates seal ageing and oil breakdown. Causes and the parts that fix them:
Rule of thumb from field guidance: about 70% of hydraulic problems are oil-related, and roughly 90% of those come from solid contamination. Controlling cleanliness (target NAS ≤ 8) and changing filters on schedule prevents most breakdowns before they start.
The pump is the most expensive component, so confirm it before replacing. Typical failure modes:
Measure before you buy. Replacing a relief valve that costs a fraction of a pump often resolves "low pressure" complaints.
Correct fitment is the single biggest cause of returns in international parts buying. XZHM verifies every order against machine brand, model and serial number before shipment, and can identify a part from a nameplate photo or the old component. Useful internal links:
Send XZHM your machine brand, model and serial number, or a photo of the nameplate and the old part. Our technical team confirms fitment before shipment and quotes genuine or vetted aftermarket options for XCMG, SANY, ZOOMLION, Lonking and Volvo. Email xzhmparts@gmail.com or reach us via our about page.
Whole-machine hydraulic failure usually points to the common supply side: low hydraulic oil level, a clogged suction filter, a broken pump-to-engine coupling, a failed main pump, a stuck main relief valve, or a burst suction line. Check oil level and the suction filter first — they are the cheapest parts to replace.
If only the boom (or another single function) is weak, the fault is in that circuit: the spool valve for that function, the cylinder, or its seals. A cylinder that sinks under load or leaks oil from the rod indicates internal leakage — replace the seal kit or the cylinder.
Overheating is driven by wrong oil viscosity, a clogged hydraulic oil cooler, a slipping fan belt, low oil level, or contamination causing internal wear. Keep oil in the 30–60 °C range (below 70 °C), clean the cooler and replace the oil and filters on schedule.
Industry guidance puts roughly 70% of hydraulic system problems in the oil, and about 90% of those trace back to solid contamination. Controlling oil cleanliness (target NAS ≤ 8) and replacing filter elements on time prevents most failures.
The highest-frequency replacements are hydraulic filter elements (suction and return), seals and cylinder rebuild kits, relief and pilot valves, couplings, and — at end of life — the hydraulic pump and main control valve.
For a scored rod or badly worn tube, a complete cylinder (such as a loader lift cylinder) is usually faster and more reliable than a field rebuild. For minor internal leakage, a seal kit restores performance. Match the part number to the exact machine model and year.
Provide the machine brand, model and serial number, or a photo of the nameplate and the old part. XZHM verifies fitment against model and serial before shipment to avoid wrong orders — email xzhmparts@gmail.com or share your details via the contact page.
XZHM is an authorized dealer for XCMG, SANY and Lonking and supplies Zoomlion, Volvo and other brands. Browse the excavator and loader parts categories or send your model/serial/photo to xzhmparts@gmail.com for a matched quote.
Published by XZHM (Xuzhou HM Machinery) — authorized dealer and global supplier of XCMG, SANY, ZOOMLION, Lonking and Volvo heavy equipment parts and used cranes. Explore our parts catalog or read more industry news.
