Failure mode - Cummins 6.7L ISB Diesel (Ram HD)

Head gasket failure (tuned / overheated trucks)

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

coolant loss, combustion gas in the cooling system, pressurised degas bottle
white smoke, sweet smell; overheating events in history
misfire on startup after sitting (fluid into a cylinder)

Compare with the other causes of the same sign on this engine: white smoke from the exhaust (3 causes) · losing coolant (3 causes) · overheating or temperature creeping up (3 causes) · misfire, rough idle, hesitation or surging (5 causes)

Root cause

RARE at stock power - the 6.7 holds its gasket as delivered (per an aftermarket maker's guide, stock trucks seldom lift heads). Failures cluster on higher-tune trucks and on overheats (fan clutch, coolant loss neglected). The 6.7 uses a single-torque head bolt design; sustained high cylinder pressure is the driver, not age.

Diagnosis

block-test the coolant for combustion gas
monitor cooling-system pressure rise vs RPM (combustion leak signature)
compression / cylinder leak-down if a specific hole is suspected

Fixes

fix the overheat cause first (fan clutch, coolant leak, DPF derate ignored) or the new gasket dies too ()

head gasket replacement; performance head studs are the standard choice on rebuilt/tuned engines ()

Prevalence

rare