The rear cylinders on the 5.9L Cummins suffer from a lack of coolant flow and because of this experience the highest coolant temperatures. This can cause excessive pressure to build in the cooling system, especially the rear of the block. This can blow out freeze plugs externally, or the smaller diameter plugs under the valve cover, leading to a failure. Fleece’s coolant bypass kit adds a secondary thermostat that works based on temperature instead of excessive pressure. This solves the problem before pressure becomes an issue by ensuring consistent temperatures across all six cylinders.
