Field diagnosis
Engine bogs down or stalls when digging: match the pump to the engine
When the engine pulls down, the pump is usually asking for more torque than the engine can deliver. These checks separate an engine fault from a pump control fault.
Check these in order
| Check | What it looks like | What it tells you |
|---|---|---|
| Total power / torque control setting | On a tandem pump each regulator is set to absorb a defined share of engine torque. A setting above what the engine can supply overloads it before any relief opens. | Compare the setting with the engine rating - this is the first number to verify. |
| Regulator spool movement | A sticking torque-control or negative-flow spool keeps displacement high for a moment too long, so the engine stumbles every time load rises. | Watch the regulator stroke with a gauge on the control port while loading the machine. |
| Standby pressure and negative-flow orifice | A blocked orifice or a restricted negative-flow line makes the pump read demand higher than it really is. | Check the orifice, the line and standby pressure against the specification sheet. |
| Engine air and fuel side | A restricted air filter, a tired turbo or low fuel delivery reduces torque with no hydraulic symptom at all. | Compare rpm at no load and at full load before condemning the pump. |
| Oil temperature | Thin overheated oil leaks internally, so more engine speed is needed for the same flow and the engine lugs. | Measure temperature after 30 minutes of duty; oil above specification is a fault in its own right. |
| Return line back pressure | A clogged return filter or an undersized return line loads the engine and heats the oil continuously. | Gauge return pressure and renew the filter if it is above specification. |
Units and parts this usually leads to
K3V112DTP
20–25 t hydraulic excavator - 112 mL/r (per pump)
K5V140DTP
27–33 t hydraulic excavator - 140 mL/r (per pump)
K5V200DTH
40–45 t hydraulic excavator - 200 mL/r (per pump)
Field notes on this complaint
The notes our own team works to on this symptom. Each one is a principle you can check on the machine, a step from the case-drain and cleanliness procedure we follow, or a figure transcribed from our catalogue - not advice copied from a video or a forum. Anything we could not check against a catalogue, a manufacturer document or a working machine is left out.
- Relief and regulator before engine. How much torque the pump asks from the engine is decided by the relief and the regulator setting. Check those before pulling the engine apart. (circuit principle, checkable on the machine)
- Measure at the pump outlet. A reading taken at the wrong port says nothing about the main relief. Measure at the pump outlet with the machine held at relief. (our own case-drain and cleanliness procedure)
- Before blaming hydraulics: the manual starts at the fuel filter The engine literature we hold puts fuel first. The Zaxis operator manual notes that the fuel filter can clog earlier than normal when the fuel is contaminated, which shows up as reduced engine power or black smoke; the Isuzu / Hitachi workshop routine checks fuel quality and fuel-filter restriction before anything else. So when a machine loses power under load - especially with black smoke - start on the engine side, then work back to the pump. (manufacturer service or operator manual we hold (title on request))
Common questions
Can stalling be fixed by turning the main relief down?
Lowering the main relief changes the pressure at which oil is dumped; it does not set the torque the pump takes from the engine in normal digging. Torque control is what matches the pump to the engine, so change settings only to the machine specification.
It only stalls in one function - is that the pump?
No. A single-function stall points at that circuit: relief setting, cylinder seal leakage or a sticking spool. A pump torque problem shows on every function at the same time.
Do you supply regulator parts?
Yes - regulator assemblies, spools and springs are catalogue part lines. Send a photo of the regulator and the pump nameplate so the variant is confirmed before dispatch.
Why it happens
The mechanism behind this symptom, from our technical articles: reading the principle first usually shortens the checklist.
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