An experimental apparatus was designed and built to conduct studies on the thermal stability and carbon deposition leading to coking in the fuel injection nozzles of small gas turbine engines. The apparatus is a simplified but controlled representation of an aircraft fuel system consisting of a preheating section and a test section. The preheating section simulates the heating of the fuel when it is used as a coolant on board an aircraft, and the test section simulates the geometry, temperature, pressure, and flow rates of the fuel injection nozzles. Proof-of-principle experiments were performed to verify the functionality of the apparatus and the repeatability of measurements. The pressure drop across the test section was used during experiments to monitor deposit buildup, and the effective reduction in the test-section diameter due to deposit blockage was calculated. The deposition rate was validated further using a carbon-burnoff apparatus. The experimental results showed that the pressure drop increased significantly with increasing testing time, as expected, and that measuring the pressure drop is an effective method of monitoring and quantifying deposit buildup.