The invention relates to a tank filling system having a protective cap and having a pouring spout. The protective cap has an end position wherein it partially covers the pouring spout, and wherein the protective cap has an opened position wherein it is removable from the pouring spout into a removed position. The pouring spout includes at least one valve, wherein the valve has a venting position and a closed valve position. In the venting position, the valve frees at least one passage for a fluid. In the closed valve position, the valve closes the passage. In the end position of the protective cap, the valve is in the closed valve position. During the adjustment of the protective cap from the end position into the opened position, the valve is forcibly adjusted at least temporarily into the venting position.