Abstract: To reveal the reasons behind the cooling system failure of a specific model caused by the pipeline elongation and the replacement of gear pumps, two types of external spur gear pumps were selected as the research objects. The operational characteristics of the external spur gear pumps and the fluid dynamics within the pump body were studied. Numerical fluid simulations on the gear pump’s operation were conducted using pump fluid dynamics calculation software. The study examined the flow conditions and cavitation at the gear meshing area, as well as the characteristics of inlet and outlet pressure and flow rate. The results showed that there were high-pressure and low-pressure zones in the fluid domain at the gear meshing area of the external spur gear pump. The formation of these zones was related to the meshing and separation of the gear teeth. Simultaneously, the pressure values of these two fluid regions were also determined by the gear module of the gear pump. When a larger gear pump was installed, there was a notable decrease in the minimum pressure at the gear meshing area, accompanied by significant shock fluctuations. Due to the one-way sealing of the gear shaft, air was drawn in, leading to the occurrence of air resistance and causing the failure of the cooling system. This problem could be more effectively addressed by employing a bidirectional seal. The findings of this study will shed light on the design and improvement of cooling systems.