Home » Duramax LLY » Duramax LLY – ENGINE DIAGNOSIS – Hard Starting

Duramax LLY – ENGINE DIAGNOSIS – Hard Starting


1. Insufficient cranking speed (below 100 rpm cold or 180 rpm hot).
Caused by:
– Low batteries
– Bad cable connections
– Serpentine belt or alternator problem

2. Starter problem
– Poor connections
– Relay
– Solenoid
– Brushes
– Armature
– Starter drive damage

3. Insufficient fuel to injectors, or start switch has fault. Check run solenoid also.

4. Water in fuel (filter indicator light NOT on).

5. Glow plugs inoperative (most noticeable at cold ambient temperatures).

6. Air in fuel delivery lines.

7. Fuel return lines restricted or tank vent inoperative.

8. Air cleaner plugged (after extended off-road operation).

9. Injection pump timing or camshaft timing may be off if pump, cam, or timing chain/sprockets/gears were removed/replaced during service.

10. Vehicle misfueled.


1. Check, repair cables or connections. Recharge batteries if low. Replace batteries if shorted, sulphated, damaged. Replace belt or tensioner if damaged. Test alternator output and repair or replace if low.

2. Check/repair cable and solenoid connections if needed. Check starter draw; overhaul if draw exceeds 450-475 amps.
Remove and overhaul starter if brushes, starter drive, or armature are faulty.

3. Check fuel volume at supply pump and injectors while cranking engine. If volume is low, check for restrictions in lines or at filter. If volume is OK, check pressure at pumps and injectors and repair as needed.

4. Drain off water at fuel filter draincock. Then crank engine with draincock open for 5-10 seconds to purge remaining water. If water continues to appear in fuel coming out draincock, purge fuel tank and lines, and refuel vehicle.

5. Check fuse and wiring harness to plugs. Correct open or short as required. Refer to diagnosis in Section 12 if required.

6. Check for loose fuel line connections at pump and injectors. Correct if necessary then bleed any remaining air at injectors.

7. Check return lines (from engine to tank) and correct restrictions, kinks, plugged lines. Make sure tank vent is OK. Repair as necessary.

8. Remove and clean air intake components. Replace filter element if plugged or restricted.

9. Check timing marks on pump and front cover. If aligned, check marks on pump gears and cam sprockets. Re-time if necessary.

10. Drain sample at filter draincock. Replace fuel load if fuel contains gasoline, kerosene, fuel oil, or low
quality fuel.



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