Isuzu AA-4BG1T, AA-6BG1, BB-4BG1T, BB-6BG1T Industrial Diesel Engine Workshop Manual – PDF

Follow the hard starting troubleshooting procedures on page 165. Bleed the fuel system starting at the injection pump inlet, then work toward the injectors. Inspect the fuel filter for water contamination and replace if needed. Verify injection nozzle opening pressure is at 18.1 MPa using a nozzle tester; weak nozzles will not atomize properly and the engine won't fire. Clean or replace the fuel lift pump if delivery volume is low.

Inspect the oil strainer screen and oil pump pickup for blockage or debris. Verify you are running the correct viscosity oil for ambient temperature. Remove and test the relief valve on page 124 for sticking or a broken spring. Measure bearing clearances against repair standards on page 192; worn main or rod bearings will bleed off pressure at idle. Replace the oil pressure sender to rule out a faulty gauge before tearing into the engine.

Verify the thermostat valve opening temperature; it should open between 80 and 84 °C per page 34. Remove and test the thermostat in a hot water bath if you suspect it is stuck closed. Inspect the fan belt for proper tension and the radiator core for external blockage. Confirm the water pump impeller is not corroded or spinning on the shaft. Retarded injection timing generates excess heat, so check timing if other causes are ruled out.

Adjust valve clearance to 0.40 mm cold on all cylinders per page 36, torquing rocker arm lock nuts to 21 to 30 N·m. Check the governor linkage for binding or worn bushings. Inspect the fuel system for air leaks at all line connections between the tank and injection pump. Verify idle speed adjustment on the injection pump. If hunting continues, the pump may have worn internal components; refer to calibration data on page 134.

Pull the air cleaner element first; a clogged filter starves the engine and causes rich running. Examine the turbocharger compressor wheel on page 144 for oil buildup or contact marks on the housing. Measure compression pressure warm; any cylinder below 2.5 MPa (page 42) indicates rings, liner, or valve problems dragging performance down. Verify injection nozzle spray pattern and replace any nozzle that dribbles or streams instead of atomizing.

Measure connecting rod bearing clearances against repair standards on page 192. Torque connecting rod bolts in two steps: first to 39 N·m, then rotate 60 to 90 degrees per page 105. Inspect piston pin fit and piston skirt for scoring or excessive wear. Listen with a stethoscope to isolate which cylinder is knocking before disassembly. If the knock disappears under load, suspect rod bearing wear; if it worsens, suspect piston slap from worn liners.