On some electric excavators, the hydraulic oil cooler is so efficient that in cold weather the oil can be chilled below the optimum range. Thick oil increases pump suction and can cause cavitation, but there's a more subtle risk: internal leaks that only appear when the oil is cold.
Cold, high?viscosity oil doesn't easily flow through small clearances. When the machine is started with cold oil, the main pump may struggle to fill, and the pilot circuit can be starved. If there's any internal wear in the main control valve-say, a spool with a slightly scored bore-the cold oil may not leak past it easily, so the spool moves sluggishly or sticks. The machine may seem sluggish or slow to respond. As the oil warms up and thins, the same clearance now leaks more easily, and the spool starts to move normally-so the problem "goes away" as the machine warms.
If you only ever test the machine hot, you'll never see this. The trick is to test the hydraulic response from a cold start, with oil below 20°C if possible. If a function is slow or sticky cold but fine hot, suspect a valve with internal wear that's being "sealed" by thick oil. Replacing the valve is one fix; another is to install a hydraulic oil heater or a thermostat that limits cooler flow until the oil is up to temperature.
