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Failure mode - Cummins 5.9L Diesel (Dodge Ram HD)
12v overflow valve weak (hard start / low power)
Reliability tier: verified - verify before ordering, see /disclosure/
Symptoms
- long crank when warm, low power and smoke under load, erratic high-idle
- symptoms drift with temperature - a tired spring in the overflow valve bleeds rail pressure off
Compare with the other causes of the same sign on this engine: hard start, long crank or no-start (4 causes) · loss of power, limp mode or derate (4 causes) · black smoke or smoke under load
Root cause
The P7100 overflow valve holds return-line pressure; a weak spring leaks pressure back to the tank and the pump loses metering head. A $ consumable that mimics expensive pump problems.
Diagnosis
- gauge the P7100 fuel pressure (should hold spec at cranking and idle)
- swap-test the overflow valve before any pump work
Fixes
replace the overflow valve (and the return-line banjo washers while in there) ()
Prevalence
common