Failure mode - Duramax 6.6L V8 Turbo Diesel (GM HD, L5P 2017+ generation)

Fuel contamination damage to the HP4/injector system (no CP4 on the L5P)

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

Symptoms

after a bad-tank event: sudden rough running, metal contamination through the high-pressure circuit, six-figure repair quotes at the dealer
the load-bearing negative fact: the L5P does NOT grenade like the LML's Bosch CP4 - GM dropped the CP4 after 2016; the L5P uses a Denso HP4

Compare with the other causes of the same sign on this engine: misfire, rough idle, hesitation or surging (4 causes) · metal in the fuel filter or oil

Root cause

The Denso HP4 high-pressure pump does not share the CP4's catastrophic cam/c roller failure pattern, but it is not immune to contamination: dirty fuel destroys HP4 internals and Denso injectors, and a full fuel-system overhaul (tank to injectors) is the documented outcome of a bad-tank event (owner-reported ~$17k dealer overhaul, dealership-paid). ULSD's low lubricity is the standing risk factor.

Diagnosis

on any suspected bad-tank event, scope for metal in the fuel filter and quantify rail-pressure complaints before the pump spreads debris
water-in-fuel sensor + regular fuel-filter changes are the standing defense

Fixes

full fuel-system decontamination (tank, lines, pump, rails, injectors) after a contamination event; quality fuel + lubricity additive and on-schedule filters as prevention ()

Prevalence

occasional