Maintenance cost and unexpected failures can be drastically reduced in low speed diesel engines using vibro-acoustic condition monitoring. This methodology has presented as a reliable method for detection of manufacturing faults, running damages and other abnormalities in engine and its components. Continuous trending keeping deviations of monitored parameter allows also reduction of fuel consumption, optimize exhaust emissions, and increase components life time and increase safety. This paper describes a methodology for vibration monitoring and fault diagnosis based on time-windowing and frequency analysis. The effectiveness is demonstrated based on the results of two year operation on a large two stroke power plant diesel engine located in Mahon, Spain.