The hydrodynamic face seal of the main coolant pump operates under hydrodynamic lubrication. At low idle speed, the seal faces may touch, transitioning the lubrication regime from hydrodynamic to mixed. A new equation for the film thickness on the coning face with a wavy-tilt-dam (WTD) is developed, applying the average Reynolds equation with the Jacobson-Floberg-Olsson (JFO) boundary and using a statistical asperity contact model. Surface roughness and coning influence the minimum film thickness, load-carrying capacity, and friction coefficient, determining the shift from mixed to hydrodynamic lubrication. Under ΔP = 0.8 MPa and 20 r/min, the no-coning WTD face seal remains non-contact at σ = 0.1 µm, while it makes contact at σ = 0.56, 0.35, and 0.28 µm. The WTD face seals with fconing = 1.0 µm at all s are in contact. Smooth surface reduces the summit contact probability, promoting the seal face to lift off. Coning decreases the hydrodynamic effect, leading to lower hmin and higher chance of rough summit contact. A test rig is built, and the idling process is simulated 50 times, revealing a contact wear pattern at σ = 0.1 µm with fconing = 1.0 µm, while no wear pattern is observed with low coning. The 0 to 0.3 µm coning value and σ = 0.1 µm are appropriate for WTD face seals under mixed lubrication. This research provides a basis for the stable operation of hydrodynamic WTD face seals in MCP.