Yamaha YZF-R125 Service Manual: Overheating

Engine

1. Clogged coolant passages

• Cylinder head and piston
• Heavy carbon buildup

2. Engine oil

• Incorrect oil level
• Incorrect oil viscosity
• Inferior oil quality

Cooling system

1. Coolant

• Low coolant level

2. Radiator

• Damaged or leaking radiator
• Faulty radiator cap
• Bent or damaged radiator fin

3. Water pump

• Damaged or faulty water pump
• Thermostat
• Thermostat stays closed
• Hose(s) and pipe(s)
• Damaged hose
• Improperly connected hose
• Damaged pipe
• Improperly connected pipe

Fuel system

1. Throttle body

• Damaged or loose throttle body joint

2. Air filter

• Clogged air filter element

Chassis

1. Brake(s)

• Dragging brake

Electrical system

1. Spark plug

• Incorrect spark plug gap
• Incorrect spark plug heat range

2. Ignition system

• Faulty ECU

Overcooling

Cooling system

1. Thermostat

• Thermostat stays open