So how does the EGR system actually work on a DD15? The DD15 uses a three-piece exhaust manifold, and the EGR passage is built directly into the center section of that manifold.When the EGR valve opens, a portion of exhaust gas is diverted through a hot pipe and into the EGR cooler. Inside the cooler, exhaust gas flows through internal tubes surrounded by engine coolant. The gas enters extremely hot and exits at a lower temperature before being routed back into the intake.It’s still hot — just c