Paccar MX-13 Cummins DEF Dosier Valve Crystallization - Low DEF Pressure

Question

My 2018 Peterbilt 579 is throwing a SPN 4334 FMI 18 code and the dash is warning of a 5mph derate if not fixed soon. I can hear the pump running during the prime cycle but the pressure never gets above 40 PSI on my DAVIE4 scan tool. There are no external leaks visibly dripping under the chassis or at the tank.

Brian O'Connor · Accepted Answer

The issue was a completely crystallized DEF Dosing Valve (Part #2115838). I pulled the injector and the nozzle was covered in white urea buildup, preventing it from sealing and building pressure. I replaced the injector, cleaned the decomp tube with hot distilled water, and performed a forced DPF regen to clear the codes. Total cost was about $550 for the part and two hours of labor.

wrenchqueen · Answer

Check the screen inside the DEF dosing valve itself. If you've been running cheap DEF or if the heated lines aren't making a good connection, it tends to crystallize right at the injector tip or the inlet port.

garagegrease · Answer

Make sure to inspect the harness connector at the dosing module for corrosion. Green crusties on the pins can cause the solenoid to partially open, throwing off the pressure readings during the purge cycle.

dieselmike88 · Answer

You might want to run a DEF quantity test using your diagnostic software. If the pump is pushing but the pressure is low, it could be a failing relief valve inside the pump module assembly, which is common on these Paccar units.

Paccar MX-13 Cummins DEF Dosier Valve Crystallization - Low DEF Pressure

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