An engine for actuating a hydraulic downhole pump having a piston vertically reciprocated in a cylinder includes a reversing valve carried with the power piston and serving, when in a first position, to direct fluid flow to force the piston upwardly and in a second position to provide bypass fluid flow to cause the piston to move downwardly. A pilot valve carried by the power piston and serving, when in a lower position, to direct fluid flow to move the reversing valve from the first to the second position. A probe is configured to move the pilot valve to the lower position when the piston reaches an upper limit of travel.