Nowadays, due to the implementation of strict legislations of carbon emissions because of climate crisis, alternative energy sources have gained great interest, thus the implementation of drop–in fuels such as biodiesel and HVO (Hydrotreated Vegetable Oil) in diesel engines has increased through the years, with the aim of completely  replacing fossil-based fuels. However, the usage of the aforementioned fuels can lead to significant problems due to their chemical composition which in turn can result even in the failure of the engine. These problems can be caused because of the formation of IDIDs (Internal Diesel Injector Deposits) which can be formed because of contamination in the fuel system. Moreover, there are plenty of contaminants but in the hereby thesis project contamination of the fuel system by coolant will be studied comprehensively. The main objective of the project is understanding the behavior of coolant in a fuel blend by investigating the solubility of coolant in B7 and its capability of forming deposits through a thermal test unit called TDT (Thermal Deposit Test). The following step was to apply a variety of instrumental techniques such as FTIR – ATR, SEM - EDS and Pyrolysis GC – MS. Thus, the aforementioned was utilized to understand the deposits formation mechanism by analyzing their morphology, structure and chemical composition. The investigation results in, that greater concentration and stirring rate produce more deposits, additionally the deposits started to form at about 150  °C and increased while the temperature increases. Furthermore, the key component of the deposits is sodium carboxylates and their structure looks stringy and porous which is something that has not been observed before. Last but not least further addition of oxidized fuel and engine oil to the test sample with coolant in order to reproduce similar conditions as in a diesel engine is a proposal for  further research