This study was conducted to determine the effects of the fouling process on the piezoelectric injectors in a transparent common-rail diesel engine. Piezoelectric injectors are characterized by a ceramic actuator that can dilate or retract when it receives a pulse of current. The piezo element controls a valve, which creates an imbalance in the pressure that is exerted at each end of the needle, causing the needle rising or closing. Two same model injectors were tested; one was new and the other one was fouled on a vehicle. The aim of the experimental investigation was to evaluate the performance of a new and a fouled piezoelectric injector in terms of injection and flame evolution. It was evaluated how the nozzle carbon deposits affect the injection quantity and combustion. The experimental apparatus was a single-cylinder research engine equipped with a Euro 5 multi-cylinder head. A second-generation common rail injection system and 6-hole piezoelectric injectors were used too. The engine was optically accessible; a sapphire window was set in the bottom of an elongated piston. The engine was fuelled with commercial diesel fuel and it ran at 1500 rpm in continuous mode during the tests. Pressure signal in the engine was detected by a quartz pressure transducer that replaced the glow plug of the real engine. Injection rate was measured for both injectors. Optical techniques based on 2D-digital imaging were used to detect the injection phase and combustion evolution. A high speed camera captured the images from the combustion chamber reflected by a 45° inclined mirror set in the elongated piston. Steady-state measurements of nitrogen oxide and opacity were carried out in the raw exhaust by commercial instruments. It was found that piezoelectric injectors had a shorter injection delay than more traditional solenoid injectors whether they were new or fouled. The new injector guaranteed a good jet penetration that allowed a more efficient combustion than the fouled one. The main consequence was that the new injector produced lower exhaust emissions.