Abstract: Engine conversion systems for converting an internal combustion engine from a single to a multiple-fuel engine are described. After engine conversion, an electronic control unit (ECU), can control amounts of a first fuel (for example, diesel) and amounts of a second fuel (for example, propane) that are provided to combustion chambers within the engine while operating in a multiple-fuel mode. The conversion system can include a diesel oxidation catalyst to reduce undesired exhaust emissions of the engine, and backpressure sensors for maintaining engine exhaust backpressure within a pre-conversion range of exhaust backpressures. The conversion systems can be configured for converting engines with mechanically-controlled or electronically-controlled fuel systems. The ECU can be configured to transition from operating in a multiple-fuel mode to a single-fuel mode if the ECU detects conditions that prevent the supply of predetermined amounts of the first and second fuels for detected operating conditions.