A removed mechanical seal that smells burnt has overheated at the faces. The smell comes from the elastomer, not the metal. Heat is generated by friction between the rotating and stationary faces once the lubricating fluid film is lost. The three usual causes are dry running, flush system failure, and installation error. A blue or rainbow tint on the carbon face signals oxidation. Diagnosis starts with the faces, then picks the elastomer by the temperature the faces actually reach, not the nameplate figure.