Product description
T3 Exhaust Manifold for 5.9 Cummins 2003–2007 | 2-Piece Expansion Design | 3rd Gen | CARB Exempt
The Diesel Power Source® T3 3rd gen exhaust manifold for 2003–2007 Dodge Ram 2500 and 3500 trucks with the 5.9L common rail Cummins engine is a direct bolt-on replacement that flows nearly 30% more than the factory manifold and lowers rear cylinder EGTs by up to 200°F. This 2-piece expansion joint design is cast from High Silicon Molybdenum (HSMD) ductile iron, CFD designed and flow-tested, and includes drilled and tapped ports for pyrometer and boost pressure fittings. It replaces OEM part numbers 5135788AB, 5135788AC, and 4931223.
This manifold is CARB exempt with an Executive Order (EO) number, making it legal for installation and street use in California. The factory heat shield is removed during installation as it is no longer necessary with the DPS manifold design.
Why the Stock 3rd Gen Manifold Fails
The factory 2003–2007 5.9 Cummins exhaust manifold is a single-piece casting that shrinks significantly over time from repeated thermal cycling. This shrinkage causes manifold cracking, gasket failures, and exhaust port misalignment where the manifold ports shift out of alignment with the cylinder head ports. Once this happens, exhaust leaks form, turbo efficiency drops, EGTs climb, and the rear cylinders run dramatically hotter than the center cylinders. Replacing a cracked stock manifold with another single-piece casting simply restarts the same failure cycle.
The DPS 2-piece expansion joint design solves this by allowing the manifold to expand and contract with heat instead of fighting it. The expansion joint absorbs thermal stress, eliminating the warping, cracking, and port misalignment that destroy factory and single-piece aftermarket manifolds.
The Rear Cylinder Problem — And Why Your Pyrometer Isn’t Telling You the Whole Story
One of the most damaging and least-understood failure modes on the 2003–2007 5.9L common rail Cummins is rear cylinder overheating caused directly by the factory manifold’s restricted exhaust flow. The stock manifold is so restrictive on the rear two to three cylinders that those cylinders run approximately 200°F hotter than the front cylinders under load. That heat differential is severe enough to cause pistons to seize in the cylinder bore — an engine-destroying failure that has happened to owners who never saw a dangerous number on their EGT gauge.
The reason is straightforward: most trucks run a single pyrometer probe positioned near the turbo inlet, which reflects an averaged temperature across all six cylinders. That average masks what is actually happening at the back of the engine. Owners have lost rear pistons and scored cylinder walls without ever triggering an EGT alarm, because the gauge was not measuring the cylinders that were actually overheating.
The DPS manifold solves this at the source. CFD testing of our balanced runner design showed rear cylinder temperatures within approximately 2°F of front cylinder temperatures. In real-world testing — accounting for coolant flow variation and intake air temperature differences across the engine — front and rear cylinders stayed within approximately 50°F of each other under load (at 1500 degrees F). That is the difference between a truck that destroys its rear pistons and one that runs reliably for hundreds of thousands of miles.
Why Runner Size Matters
Some competitor manifolds advertise oversized runners or large tube designs. While this sounds appealing, runners that are too large actually slow exhaust gas velocity. The exhaust then has to re-accelerate as it bottlenecks through the turbo flange, causing a loss in horsepower and turbo spool-up. DPS has tested competitor manifolds with oversized runners that decreased horsepower by 40 HP on a 400 HP truck.
The DPS manifold uses CFD-tested runner geometry that maintains exhaust velocity from the ports through the collector while flowing approximately 30% more volume than stock. The runners are balanced so that end-port EGTs are much closer to center-port EGTs, reducing the hot spots that cause manifold cracking, gasket failures, and piston damage.
**DOES NOT INCLUDE MANIFOLD HARDWARE/GASKETS**
