For proper functioning, general machines usually need lubricant oil as a cooling, cleaning, and sealing agent at points of mechanical contact. The quality of lubricant oil can deteriorate during operation owing to various causes such as high temperature, combustion products and extraneous impurities. In this study, a heavy load stopped during operation, and the oil was analyzed to check whether any impurities were added. Extraction using acetonitrile followed by reaction with BSTFA(bistrimethylsilyl trifluoroacetamide) showed that, trimethylsilylated ethylene glycol was present in the lubricant oil. To quantify the ethylene glycol in the oil, deuterium-substituted ethylene glycol, which acted as an internal standard, was added to the sample and then extracted with the solvent. Next, the extract was reacted with the derivatizing agent(BSTFA) and then analyzed with GC/MS. The detection limit of this method was found to be $0.5{mu}g/g$ and the recovery of oil containing $20,000{mu}g/g$ of ethylene glycol was measured to be 94.8%. A damaged O-ring and eroded cylinder liner were found during the overhaul, which implied the leakage of coolant containing ethylene glycol into the lubricating system. The erosion of the cylinder liner was assumed to be due to cavitation of the coolant in the cooling system.