An experimental investigation of the use of an engine coolant exchange system for prewarming diesel engines before cold starting is discussed. This coolant exchange system involves connecting the coolant system of a fully warmed-up and running engine (e.g., a spark ignition engine) to that of the cold diesel to be started using hydraulic hoses with quick connect fittings and an auxiliary pump. The investigation was performed using a 4,3 liter V6 indirect injection diesel engine since this represents a difficult case for cold starting. The starting characteristics using the coolant exchange technique are compared to those using the production glow plug system, which includes a fuel heater and afterglow. It is shown that the coolant exchange system allows this engine to be started down to −26 °C, much colder than the −13°C limit for the production glow plug system. It is believed that the coolant exchange process will allow starting at even lower temperatures if the problem of fuel waxing encountered at −26 °C is overcome. It is also shown that the coolant exchange process results in lower opacity of the diesel exhaust during the cold start transient. Also, based upon indirect evidence, it appears that the exhausthydrocarbon emissions may be improved, the wear rate may be decreased significantly, and the torque loss normal for diesel acceleration during the cold transient may be decreased significantly. Thus, the coolant exchange system addresses all of the problems normally associated with diesel cold start operation. Advantages of the coolant exchange system in comparison to other starting aids are discussed.