Combustion reactions can only be exothermic. In chemical reactions, old bonds are broken to form new ones. Depending on the energy difference between broken and formed bonds, the total reaction energy is either positive or negative.
Combustion is a rapid oxidation process, or, in plain language, burning. An exothermic reaction is a physical or chemical reaction that releases energy in the form of heat. It involves breaking higher-energy bonds and forming lower-energy bonds. An endothermic reaction is a process in which heat energy is absorbed from the surroundings by the reactants. It involves breaking the lower-energy bonds and forming higher-energy bonds.