Fleece Performance Coolant Bypass for 2003-2007 Manual TransmissionIncreased Coolant Flow, Decreased Coolant Temperatures The rear cylinders on the 5. 9L Cummins suffer from a lack of coolant flow, and therefore, experience the highest coolant temperatures. This can cause excessive pressure to build in the cooling system, especially the rear of the block and blow out freeze plugs externally, or the smaller diameter plugs under the valve cover, leading to a failure. Our coolant bypass kit adds a secondary thermostat,