This study focuses on investigation of the method of identifying injector faults in a JCB 444T2 diesel engine using acoustic emission (AE) technique. Different kinds of injector faults were seeded in the four-cylinder, four-stroke, and turbo-engine. Then, faulty injectors are tested to evaluate AE based injection fault detection. The AE signals recorded from the tests were processed in the angular, frequency and joint angular-frequency domain. The results from joint angular-frequency analysis have shown that AE can clearly monitor the changes in the combustion process due to its high signal to noise ratio, where other vibro-acoustic sources have little influence. Using features in the AE signal, faults of injector can be identified during the operation of the engine.