Abstract: Most of the stainless steel oil coolers were found to leak during 100% encapsulation test after manufacturing. The quantity, distribution, shape, and compositional variation of the cracks of the failed products were analyzed by appearance and cross-sectional metallographic analysis. The influence of the TIG welding on the temperature field and residual stress of the cap and shell was simulated. The root cause for the cracks were studied by comprehensive analysis, and the improvement effect was verified by cross-sectional metallographic analysis. The studies show that the root cause for the leakage of the stainless oil coolers is the diffusion of the Cu brazing seams because of high temperature during TIG welding, resulting in intergranular cracks of the cap and the plates. The structure improving measurements to increase the thickness of the cap and the shell were proposed aiming at root cause.