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+--- Thread: 2024 x15 - Derate with just one code? (/showthread.php?tid=10765)
2024 x15 - Derate with just one code? - Hermjeji - 09-24-2026
2024 Kenworth T880 with a Cummins X15 Performance about 5,700 hours and roughly 250,000 km, currently showing active FC 4726 / SPN 1239 FMI 16 with count 1 and no other engine fault codes. The CEL first came on immediately after a PM about four weeks ago where the engine oil, fuel filter, and water separator were changed, and since then the truck has had no hard starting, rough idle, smoke, fuel knock, power loss, external fuel leak, rising oil level, or fuel smell in the oil. About two weeks after the CEL appeared, the dash began intermittently showing “Engine Derate In: 239 Minutes — Seek Service Immediately”; every time the warning disappears and the truck runs normally, but today the timer expired and the truck eventually goes into crawl/derate.
In my experience having just a fault code for fuel leakage doesn't lead to an engine derate, but that's just my experience. Wondering what trouble shooting I should do. I had this issue once before on a 2018 X15, but it was paired with hard starting and all it needed was the fuel water separator re-installed.
RE: 2024 x15 - Derate with just one code? - marek4792 - 09-24-2026
Those paccar fuel water separators with the bowl on the bottom are known to be complete garbage and often swapped for davco 382s.
RE: 2024 x15 - Derate with just one code? - Rawze - Yesterday
FC 4726 is also associated with the fuel rail bleeding off too fast.
- Do an injector leakage test on it exactly like this video shows... find out why its bleeding down...
info: https://rawze.com/forums/showthread.php?tid=8728&pid=75483#pid75483
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(related)...
Is the engine del-eted? / de-mandated?... if so then copy program from ECM and e-mail it to me to have it reviewed. - Could be a bad delete settings or 2 that is torturing the fueling system or causing too high pressures.