← Misfire / rough running on all engines
Symptoms · Duramax L5P (2017-present)
Duramax L5P (2017-present): misfire, rough idle, hesitation or surging
4 documented failures on this engine list this among their signs. They are ordered from the most widespread down. Each one shows the signs in its record's own words, how to tell it from the others, the usual fix and any recall or bulletin.
Not sure which engine you have, or want the recalls for your exact truck? Check your VIN: it names the engine and lists the campaigns for that model year. Whether a recall is still open on your VIN: nhtsa.gov/recalls.
aggregated
How widespread: Common
How it shows up
- gradual rough idle / drivability degradation over high mileage rather than sudden failure
Its other signs
- false knock-sensor codes from soot fouling in the EGR path are reported by shops as a secondary symptom
How to tell it apart
- inspect the EGR valve and intake for carbon at service intervals; clean rather than replace when the valve still actuates
- before chasing knock-sensor codes on a high-miler, check for EGR soot fouling the sensor circuits
Usual fix
clean or replace the EGR valve and clean intake deposits; replace the EGR cooler if coolant-side symptoms appear (see diagnosis)
Root cause, parts and sources →
verified
How widespread: Occasional
How it shows up
- after a bad-tank event: sudden rough running, metal contamination through the high-pressure circuit, six-figure repair quotes at the dealer
Its other signs
- 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
How to tell it apart
- 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
Usual fix
full fuel-system decontamination (tank, lines, pump, rails, injectors) after a contamination event; quality fuel + lubricity additive and on-schedule filters as prevention
Root cause, parts and sources →
aggregated
How widespread: Occasional
How it shows up
- extended cranking / rough cold idle on cold mornings
Its other signs
- catastrophic engine damage after a DIY glow plug R&R breaks a ceramic tip into the cylinder - the documented hazard
How to tell it apart
- test glow plug resistance/control via the GPCM before pulling plugs; avoid side-loading the ceramic tip on removal
- after any glow plug replacement, run the glow plug learn procedure per service documentation
Usual fix
replace failed glow plugs with the correct ceramic-tip parts and perform the learn procedure; replace the GPCM if control-side faults are present
Root cause, parts and sources →
aggregated
How widespread: Occasional
How it shows up
- rough idle, hard starting, poor fuel economy, excessive exhaust smoke (a parts retailer's symptom table)
- misfires traced to injector pigtail/connector faults rather than the injector body (community)
How to tell it apart
- cylinder-contribution/balance test before ordering injectors; inspect pigtails and connectors for heat damage
- on a high-miler with a contamination history, plan for the full high-pressure circuit, not just one injector
Usual fix
repair connectors/pigtails where faulty; replace injectors in the documented sets when contribution tests fail
Root cause, parts and sources →
Other symptoms on the Duramax L5P (2017-present)
Hard start, long crank or no-startLoss of power, limp mode or derateOverheating or temperature creeping upDEF, DPF and regeneration warnings
Everything on the Duramax L5P (2017-present)Its recalls and bulletins