GM 6.6 L8T (gas): 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.

Direct-injection high-pressure fuel pump / injector failures Occasional
Ignition service wear: spark plugs and coils Occasional
Intake-valve carbon buildup (direct injection, no port wash) Occasional
Valvetrain hardware failures: lifters, valves (no AFM - attribution guard) Occasional

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.

1 Direct-injection high-pressure fuel pump / injector failures

aggregated How widespread: Occasional

How it shows up

hard starts, long crank, fuel-pressure codes; rough running under load

Its other signs

owner-reported high-pressure fuel pump replacements at dealer rates (GM-Trucks HD forum)

How to tell it apart

log requested vs actual rail pressure on a suspected HPFP fault; rule out the low-pressure side first
check injector balance/contribution before replacing hardware; verify fuel quality

Usual fix

replace the failed HPFP or injector(s); keep the fuel system clean - DI pumps are unforgiving of contamination

Root cause, parts and sources →

2 Ignition service wear: spark plugs and coils

aggregated How widespread: Occasional

How it shows up

misfire codes, rough idle, hard starts as plugs/coils reach service life (routine wear, not a defect pattern)

How to tell it apart

coil-swap or plug-well inspection to localize a misfire before ordering coils

Usual fix

replace spark plugs at the service interval; replace the failed coil(s) localized by swap test

Root cause, parts and sources →

3 Intake-valve carbon buildup (direct injection, no port wash)

aggregated How widespread: Occasional

How it shows up

gradual rough idle, hesitation, efficiency loss over high mileage - the generic DI intake-deposit picture

How to tell it apart

borescope the intake valves when idle-quality complaints accumulate on a high-miler
distinguish carbon deposits from ignition and injector causes (same symptoms)

Usual fix

walnut-blast or chemical cleaning of intake valves; catch-can/oil-separator kits are the community preventive

Root cause, parts and sources →

4 Valvetrain hardware failures: lifters, valves (no AFM - attribution guard)

aggregated How widespread: Occasional

How it shows up

persistent ticking, misfire on one cylinder, low compression on a bore (lifter/valve hardware)

Its other signs

owner reports of dropped/broken valves and lifter replacements (GM-Trucks HD forum); some early-engine 'improperly torqued cylinder head' anecdotes

How to tell it apart

cylinder-contribution test and compression/leak-down to separate lifter, valve and injector causes (DI misfires overlap)
inspect the valvetrain via valve cover before ordering heads or long blocks

Usual fix

replace failed lifter/valve hardware; cylinder head service or replacement where a valve dropped

Root cause, parts and sources →

Other symptoms on the GM 6.6 L8T (gas)

Hard start, long crank or no-start

Everything on the GM 6.6 L8T (gas)Its recalls and bulletins

Order: how widespread each failure is in its record (near-universal, very common, common, occasional, rare), then the number of NHTSA owner complaints that match its keywords. Complaint counts are keyword matches, not a manual review of each complaint. A symptom listed here means the failure is documented with that sign; it is not a diagnosis of your truck. Tiers: verified, aggregated, contested, data-gap. How we tier and source facts.