Mechanical water pump face seals from heavy duty diesel on-highway engines which were reported to leak were analyzed and were compared to nonleaking seals. The siliconized graphite seal face pairs did not exhibit any significant wear of the faces. Coolant leakage was traced to deposit film build-up, which resulted in separation of the seal faces.

Analysis of the films by wavelength dispersive spectroscopy revealed elements from the coolant and coolant additives (Si, C, O, S, P, K, Na, B, Mo), corrosion metals from the engine (Fe, Cu, Cr) and hard water elements and contaminants (Ca, Mg, Zn, Al, Mn, Cl). Calcium from hard water and iron from corrosion are believed to accelerate the deposit formation.