2020 Kenworth T680 - Cummins X15 - Misfire Under Load - Code 3559

Question

My X15 is running smooth at idle and bobtailing, but once I hook to a 40k load and try to pull a grade, I get a distinct rhythmic stumble. It feels like a cylinder is dropping out. I'm getting a 3559 code which is 'Aftertreatment 1 Diesel Exhaust Fluid Dosier Unit - Voltage Below Normal'.

I'm confused because it feels like a mechanical fuel miss, but the code is for the DEF system. Could electrical noise on the datalink from the DEF pump be causing the injectors to skip a beat, or am I chasing two separate issues at once?

WrenchMonkey_J · Accepted Answer

✅ Confirmed fix: The 3559 code was a distraction caused by a chafed wiring harness rub-through on the frame rail near the transmission. The real issue was the #5 fuel injector solenoid failing internally when it got hot. Replaced the #5 injector and repaired the harness rub-point. The stutter under load is gone and the DEF code hasn't returned.

RustBeltRandy · Answer

Code 3559 on an X15 can definitely cause weird symptoms, but usually doesn't feel like a hard engine misfire. I'd verify your lift pump pressure. If the lift pump is weak, it'll starve the high pressure pump under heavy load.

2020 Kenworth T680 - Cummins X15 - Misfire Under Load - Code 3559

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