Serial number breaks by model year and the ECM terminal layout for gasoline, LPG, and dual fuel K-engines are settled in one source: the 223-page Komatsu K21, K25 service manual PDF (OEM ID SM301), covering 2005 through 2008 forklift truck engines. Engine control circuits are drawn as wiring diagrams for each fuel type, including the 2007 EPA Tier 2 versions, so you can trace a sensor lead to its ECM pin. Disassembly, inspection, and reassembly take up a solid share of the book, backed by a DTC table and symptom charts for the self-diagnosis system. Set the top ring closed gap at 0.28 to 0.43 mm, the second at 0.45 to 0.60 mm, and bring lubricated K21 rod nuts to 31.4 to 37.3 N·m. Guess a ring gap and you pay for two teardowns. Bookmarked, text-searchable pages keep every clearance and torque value within reach at the bench, so your engine goes back together once and runs right.
What's Inside This Komatsu K21, K25 Manual
| System | Pages | Key Topics |
|---|---|---|
| Introduction | 3-7 | Information Resource, Diagnostic Procedures, Diagnostic Equipment, Computerized Sensors, Testing and Adjustment Procedures |
| Controls | 8-12 | Precautions (on Board Diagnostic System of Engine, Precaution), Special Service Tools |
| An Overview of Major Improvements From Current H-Type Engines to the New K-Engines | 13-14 | Cylinder Head, Cylinder Head Gasket, Rocker Cover, Rocker Cover Gasket |
| Applicable “EPA” Lift Truck Models | 15-18 | AX20 K21 Engine Models, BX20 K21 K25 Engine Models, AX50 K21 Engine Models, BX50 K21 K25 Engine Models, Serial Number Breaks by Model Year, K21 Engine LPG, K21 Engine Dual, K21 Engine Gasoline, K25 Engine LPG, K25 Engine Dual, K25 Engine Gasoline |
| General Information and Specifications | 19-43 | How to Use This Manual (Description, Definition of Terms, Definition of Units), Tightening Torque, Main Specifications, Trouble Shooting |
| Mechanical | 44-90 | Precautions (Draining Engine Coolant, Disconnecting Fuel Piping, Removing and Disassembling), Removal and Installation of Engine, Engine Service |
| Fuel | 91-123 | Engine Types, Engine Outside Drawings, System Diagram, Unit Layout, Service Data and Specifications (LPG Fuel System Trouble Diagnosis) |
| Lubrication and Cooling Systems | 124-132 | Lubrication System (Specifications, Adjustment Value, Oil Pressure, Tightening Torque, Trouble Shooting, Lubrication Schematic Diagram, Oil Pump) |
| Engine and Main Body | 133-140 | Cylinder Block, Cylinder Head, Piston and Rings, Connecting Rod, Crankshaft, PTO System |
| Valves | 141-143 | Camshaft, Timing Chain, Valve Springs, Valve Oil Seal, Valve Lifters |
| Lubrication System | 144-145 | Oil Pump, Oil Strainer, Oil Filter, Engine Oil, Dipstick |
| Intake / Exhaust | 146-149 | Intake Manifold, Exhaust Manifold, Electric Throttle Control Actuator, Air Flow Sensor |
| Engine Control System | 150-177 | ECM Functions, Ignition Timing Control, Fuel Injection Control, Fail-Safe Function, Heated Oxygen Sensor Heater |
| Diagnostic System | 178-182 | Self-Diagnosis, DTC Error Codes, Service Support Tool (SST), Diagnostic Monitor |
| Engine Components and Sensors | 183-193 | Crankshaft Position Sensor, Camshaft Position Sensor, Throttle Position Sensor, Mass Air Flow Sensor, PCV System |
| Coolant System and Fuel System Description | 194-223 | Coolant System Diagram, Thermostat, Returnless Fuel Supply System, Fuel Injection Pulse Width Theory, SST Exercise Program |
Quick Reference Specifications
| Specification | Value | Page |
|---|---|---|
| All Models | ||
| Water pump retaining bolt M8 torque | 15.7 - 17.7 N·m | p. 129 |
| Piston ring closed gap (Top) | 0.28 - 0.43 mm | p. 52 |
| Piston ring closed gap (Second) | 0.45 - 0.60 mm | p. 52 |
| Oil pan drain plug tightening torque | 29.4 - 39.2 N·m | p. 125 |
| Spark plug gap | 0.035 in. (0.9mm) | p. 32 |
| Spark plug torque | 19.6 - 29.4 N·m | p. 54 |
| Water pump retaining bolt M6 torque | 6.37 - 7.45 N·m | p. 129 |
| Fuel pump discharge rate | 19.8 - 30.4 gallons/hour (75 - 115 L/h @ 12 Volts) | p. 187 |
| Fuel filter filtration area | 170 - 201 in.2 (1,100 - 1,300 cm2) | p. 187 |
| K21 | ||
| Connecting rod nut (lubricated) | 31.4 - 37.3 N·m | p. 54 |
| Engine Oil Capacity (including oil filter) | 3.7 (3-7/8, 3-1/4) US qt, Imp qt | p. 29 |
| K25 | ||
| Engine Oil Capacity (including oil filter) | 3.7 (3-7/8, 3-1/4) US qt, Imp qt | p. 30 |
Komatsu K21, K25 Common Problems This Manual Covers
Komatsu K21 K25 engine with an ignition or fuel system fault that cranks but will not start
Check the ignition system first, then the fuel system and compression, using the cranked but not started flow in the engine troubleshooting starting on page 37. Pull the plugs, set the gap to 0.035 in. (0.9 mm) per page 32 and torque them to 19.6 - 29.4 N·m. Verify the fuel pump discharges 19.8 - 30.4 gallons/hour at 12 volts (page 187).
Manual Section: Engine Troubleshooting p. 37Engine overheat with insufficient coolant circulation, water leakage, or noise from the water pump
Check for water leakage and poor coolant circulation using the coolant troubleshooting on page 129. Test the thermostat: it should open at 169.7° F (76.5° C) per page 32. Adjust fan belt deflection to 11 - 13 mm. When the pump goes back on, torque the M8 retaining bolts to 15.7 - 17.7 N·m and the M6 bolts to 6.37 - 7.45 N·m.
Manual Section: Engine Coolant System Troubleshooting p. 129Low oil pressure with oil leakage and the Engine Warning Light (MIL) turned on
Check the oil level and look for oil leakage before you chase the Engine Warning Light. Work through the lubrication troubleshooting in the section starting on page 124. Inspect the oil pump, oil strainer and oil filter. When you drain the oil, torque the oil pan drain plug to 29.4 - 39.2 N·m (21.7 - 28.9 ft/lbs) per page 125.
Manual Section: Lubrication and Cooling Systems p. 124Cylinder, piston and piston ring wear showing up as low compression pressure on test
Measure compression to confirm it, then pull the pistons. Check ring end gap in the bore against page 52: 0.28 - 0.43 mm for the top ring and 0.45 - 0.60 mm for the second. Measure ring-to-groove clearance on page 73: top 0.045 - 0.080 mm, second 0.030 - 0.070 mm, oil 0.065 - 0.135 mm. Replace any rings or pistons that are out of spec.
Manual Section: Mechanical p. 52Bottom end knocking from crankshaft bearing or connecting rod bearing slapping
Inspect the crankshaft and connecting rod bearings for poor function using the knocking diagnosis in the engine troubleshooting starting on page 37. When you reassemble, torque the main bearing caps to 83.4 - 93.2 N·m (61.5 - 68.7 ft/lbs) and the connecting rod bearing caps to 31.4 - 37.3 N·m (23.2 - 27.5 ft/lbs) per page 75. Lubricate the rod nuts before you torque them.
Manual Section: Engine Troubleshooting p. 75LPG engine with rough idle, hesitation or hunting, and a vaporizer that is not working properly
Check the LPG system for fuel leakage first. Then inspect the vaporizer for an operating malfunction using the LPG Fuel System Trouble Diagnosis chart. That chart lists inconsistent idle, hunting, low output and poor starting next to vaporizer faults. Rule out fuel delivery before you condemn compression or start taking the engine apart.
Manual Section: LPG Fuel System Trouble DiagnosisFrequently Asked Questions
How do I clear fault codes for the K21 and K25 engines?
To clear diagnostic memory, use the Service Support Tool (SST). In the 'Self-Diagnostic Results' menu, select 'Eliminate the Memory of Self-Diagnostic' and press 'YES'. The monitor will then revert to the 'Self-Diagnostic Results' menu. p. 214
What is the torque specification for the cylinder head bolts on the K21 and K25 engines?
The cylinder head bolts should be tightened to a torque of 50.6 ft/lbs (68.6 N·m) (7.0 kgf-m) during the final retightening step (step 5) of the tightening procedure. p. 26
What are the replacement specifications for the fuel pump on the K21/K25 engines?
The fuel pump is a small, lightweight turbine type with a discharge rate of 19.8 - 30.4 gallons/hour (75 - 115 L/h @ 12 Volts). It is located inside the gasoline fuel tank. p. 187
What is the spark plug gap specification for the K21 and K25 engines?
The standard spark plug gap for both the K21 and K25 engines is 0.035 in. (0.9 mm). p. 32
What is the valve clearance specification for the K21 and K25 engines when the engine is hot?
When the engine is hot, the valve clearance for both intake and exhaust valves should be 0.38 mm (0.015 in). p. 32
What exactly do I receive with this Komatsu K21 Service Manual?
A 223-page Service Manual in searchable PDF format, available the moment you complete checkout. View on computer, tablet, or phone, with no shipping wait.
Are there any print restrictions on this Komatsu K21 manual?
Printing is allowed without restriction. Pull the pages you need for the job and bin them when they get greasy.
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