Abstract: A system and method for starting a large diesel engine using at least one hydraulic motor includes an auxiliary power unit having a hydraulic pump for driving a hydraulic motor coupled to the diesel engine. The hydraulic pump may automatically pump oil from the diesel engine into a pressure reservoir until a pressure set point is exceeded. When the pressure set point is exceeded, a relief/check valve positioned between the pressure reservoir and the hydraulic pump may open to divert some lubrication oil through a heater and back into the diesel engine. To start the diesel engine, a solenoid-controlled valve, positioned between the pressure reservoir and a hydraulic motor coupled with the diesel engine, may be opened to release pressurized oil to the hydraulic motor. After energizing the hydraulic motor, the oil may be diverted back to a sump of the diesel engine.