2008 Peterbilt 389 - Cat C15 6NZ Swap - Low Fuel Pressure

Question

I've got an 08 389 that we swapped a 6NZ C15 into. It runs great at idle, but when I'm pulling a load it starts to stumble and the fuel pressure drops to 40psi (should be 70+).

I've replaced the primary and secondary fuel filters. I've also replaced the fuel transfer pump on the back of the front housing. I checked the fuel lines for kinks but don't see anything. It feels like it's sucking air or there's a restriction.

DIY_Denise · Answer

✅ Confirmed fix: Found the fuel pressure regulator (check valve) on the return side of the head was weak. The spring had lost tension, allowing fuel to return to the tank too easily instead of maintaining head pressure. Replaced the regulator valve (Part # 156-5775) and pressure stayed at a rock solid 85psi.

ShopBossMike · Answer

Check the standpipes in your fuel tanks. If you have an obstruction like a piece of plastic or a rag in the tank, it will starve the pump only when the flow rate increases under load.

2008 Peterbilt 389 - Cat C15 6NZ Swap - Low Fuel Pressure

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