2012 Kenworth T660 Cummins ISX15 - Low Fuel Pressure SPN 94 FMI 18

Question

My ISX15 started stumbling when I hit an incline with a heavy load. The check engine light flashes and I get SPN 94 FMI 18 (Fuel Pressure Data Valid But Below Normal Operating Range - Moderately Severe). If I back off the throttle, the light goes away.

I've already swapped out both fuel filters. The secondary filter looked a bit dark, but not terrible. I'm afraid the gear pump might be failing or I have air getting into the system from a cracked pick-up tube in the tank. I'm also seeing some weird fluctuations on the fuel pressure gauge during idle.

FleetOwnerFiona · Accepted Answer

✅ Confirmed fix: The issue was the fuel gear pump itself. The internal bypass valve was sticking open, preventing the pump from building sufficient pressure when demand increased under load. Replaced the gear pump and the stumbling immediately stopped. Also, make sure to check the anti-drainback valve in the filter head; they sometimes crack and cause similar issues.

NoStartNoel · Answer

It might be the Compuchek fittings. If they are leaking air into the suction side, you'll get that stumbling. Try running the truck off a temporary fuel jug to see if the problem persists.

OwnerOpOlly · Answer

Check your fuel rail pressure relief valve. If it's leaking internally, it'll dump pressure back to the return line and you'll never maintain high pressure under load. You can test this by removing the return line from the valve and checking for excessive leakage while the engine is running.

2012 Kenworth T660 Cummins ISX15 - Low Fuel Pressure SPN 94 FMI 18

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