An engine that starts cold and refuses to start hot is telling you something specific. Cold start problems are usually battery, starter or compression — all of them worse when cold. A hot start problem is the opposite: everything is warm, the battery is charged, the starter is free, and the engine still will not fire.
On 10 series Kobelco machines there is a common cause that has nothing to do with the engine at all: the hydraulic pumps are loading the engine during cranking. The starter has to spin the engine against hydraulic pressure, and it does not have the torque to do both.
Why hydraulics can stop a hot engine from starting
When the engine is off, the pumps should be unloaded — no flow, no pressure, no resistance. Two things can defeat that:
The pumps are not unloading. A stuck unloading valve, or a solenoid that does not pull the pumps off stroke, leaves pressure in the system at rest. The starter then turns the engine against that pressure.
Hot oil makes it worse. Warm hydraulic oil is thinner, and a worn pump or regulator that held position when cold can drift when hot. That is why the fault appears only after hours of work.
The result is a machine that turns over slowly or fires and dies — and a technician who replaces the battery, the starter and then the injection system, none of which were the cause.
Step 1: read the pressures during cranking
This is the check that separates a hydraulic problem from an engine problem, and it takes minutes.
| Parameter | Normal | Fault |
|---|---|---|
| C-1 / C-2 pump pressure while cranking | close to 0–2 MPa | above 5 MPa — the pump is loading the engine |
| B-series pilot pressure at idle | about 3.0 MPa | below 2.5 MPa — functions will stall or feel dead |
| G-3 engine speed while cranking | above 200 rpm | below 150 rpm — cranking speed is insufficient |
How to read it: the C-1 and C-2 values matter most during the crank, not after the engine starts. If the monitor shows several MPa while the starter is turning and the engine is not running, nothing is generating that pressure except a pump that is still on stroke.
What the numbers mean together: low cranking speed plus pressure at the pump is the classic combination — the starter is fighting hydraulics. Low cranking speed with zero pressure is a starter or battery problem after all.
Step 2: pilot pressure and the regulator
Pilot pressure is what moves the main pump regulators. If it is low, the pumps do not go where the controller tells them, and the machine can be reluctant to start as well as slow to work.
What to read: B-series pilot pressure at idle should sit around 3.0 MPa. Below 2.5 MPa, expect stalling, weak functions and hard starting.
What causes it to be low: a worn pilot pump, a blocked pilot filter, or a regulator that is not responding. Check the filter before condemning the pump — a restricted pilot filter produces the same reading and costs a fraction of the pump.
The regulator yoke: a worn or sticking yoke keeps the pump on stroke when it should be off, which is the mechanism behind back pressure at cranking. With the engine off, check that the yoke moves freely through its full travel and returns on its own.
Step 3: the unloading solenoid valves
The unloading valves are what take the pumps off stroke when no function is demanded. If they fail to open, the machine carries pressure at rest — and that pressure is what the starter has to overcome.
What to check:
a. Current at the solenoid. With the key on and no lever operated, the controller should be commanding the unload valves. Read the command current on the monitor; zero current means the controller is not asking them to open, which is a different fault.
b. Coil condition. Measure the coil resistance and compare it with its neighbour on the same bank. Open circuit means a dead coil; the same reading as its neighbour but no pressure change means the valve is stuck mechanically.
c. Pressure at neutral. With the engine idling and no function operated, system pressure should be low. If it is already high at neutral, the unload path is not working — that is the same fault that stops the machine starting hot.
Solenoid valves and pilot pumps for the 8 and 10 series: excavator electrical parts — send the model and serial and we will confirm which coil is fitted.
Telling hydraulic back pressure from an engine fault
| What you see | What it means |
|---|---|
| Starts cold, will not start hot, cranks slowly | Hydraulic back pressure — start at step 1 |
| Starts cold, will not start hot, cranks at normal speed | Fuel or injection — rail pressure and injectors |
| Will not crank properly cold or hot | Battery, starter or cables |
| Cranks, fires, dies immediately | Fuel supply or an engine protection shutdown |
The distinction in the first two rows is the cranking speed. If the starter turns the engine normally and it still will not fire, hydraulics are not the problem no matter what the pressure gauge says.
Frequently asked
Why does it start fine in the morning but not after lunch? Because the oil is hot and thin by then. A pump or regulator that holds position when cold can drift when hot, so the machine that was fine at 7am will not restart at 2pm.
Can a weak battery cause the same symptom? It can, but a weak battery is worse cold, not hot. If the machine starts first thing and not after work, the battery is not the explanation.
Is it safe to keep cranking? No. Cranking against hydraulic pressure overheats the starter and drains the battery fast, and it can damage the starter drive. Fix the pressure before you keep trying.
Does this apply to 8 series machines too? The mechanism is the same, but the monitor pages and pressure figures differ. Use the readings from your own machine as the reference.
Parts in this article
| Item | Part |
|---|---|
| Pilot pump | YN10V00014F1 |
| Pilot filter | Excavator filters |
| Unloading / proportional solenoids | Solenoid valves |
If the machine cranks at normal speed but will not fire, the fault is on the fuel side instead — see SK200-8 no start with P0183, P0192 and P0193 for the wiring case, and the stuck SCV case for the low rail pressure case.
Note
Pressure figures above are the reference values for 10 series machines. Monitor page names and pilot pressure settings differ between build years and controller versions — confirm against the workshop manual before condemning a pump or a regulator.
