Home » Duramax LLY » Duramax LLY – ENGINE DIAGNOSIS – Low Power, Poor Acceleration

Duramax LLY – ENGINE DIAGNOSIS – Low Power, Poor Acceleration


1. Vehicle overloaded.

2. Low grade diesel fuel.

3. Sludge or wax buildup in fuel filter (low fuel supply).

4. Air in fuel supply system.

5. Fuel supply pump failed.

6. Mistimed injection pump.

7. Restriction in air intake system.

8. Vacuum pump failure.

9. Engine shut-off solenoid has intermittent open or short.

10. Turbocharger, related wiring, or wastegate actuator fault may have occurred. May also be caused by exhaust gasket failure.

11. Injection pump, or injector failure.

12.Engine problem:
– Timing chain and gears worn.
– Cam and injection pump gears mistimed.
– Piston, ring, or valve wear.
– Worn cam lobes.
– Damaged piston(s).


1. Lighten vehicle load.

2. Drain tank and refuel.

3. Drain and clean filter.

4. Check lines from fuel supply pump and to injectors. Replace leaking, damaged lines as needed. Bleed air at injector fuel line fittings.

5. Check and replace pump if output is less than 5 1/2 psi.

6. Check and correct as needed.

7. Check air intake cap, duct, air cleaner, hoses, and air horn. Clean/repair as needed. Replace filter if plugged.

8. Replace pump if inoperative.

9. Check wiring and solenoid. Repair wiring and connectors. Replace solenoid if failed.

10. Check wiring, wastegate vacuum actuator, and PCM with Tech I scan tool. Run vacuum check on actuator and vacuum pump with vacuum tool.
Check for intake/exhaust leaks. Replace turbocharger, or related components as indicated by diagnosis.

11. Diagnose and replace pump as needed. Replace
damaged injectors.

12. Check compression and timing. Then disassemble engine and repair as needed. Remove engine completely if diagnosis indicates crankshaft, rod, or piston problem.


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