How to Change the Coolant in a 2019 Yamaha YZ125: Step-by-Step Guide

Shop parts for a 2019 Yamaha YZ125 Dirt Bike.

Proper maintenance of your 2019 Yamaha YZ125 is essential for optimal performance–especially keeping the cooling system in top shape. This exclusive MotoSport guide will walk you through the process of changing the coolant on your YZ125, as well as share some key model-specific notes all owners should know.

Tools & Materials Needed

  • Fresh coolant (compatible with Yamaha specifications)
  • Funnel
  • Drain pan
  • Ratchet and sockets (for drain bolt and radiator cap)
  • Clean rags
  • Gloves and eye protection

Important Safety Steps

  • Only change coolant when the engine is completely cool. Opening a hot radiator can cause burns from steam or escaping hot coolant.
  • Work in a well-ventilated area and avoid contact with coolant on skin or eyes.

1. Remove the Radiator Cap

  • Ensure the YZ125 is on a stable stand.
  • Locate the left-side radiator cap at the top of the radiator.
  • Turn the cap slowly to release any residual pressure.

2. Drain Old Coolant

  • Place a drain pan beneath the right side of the engine, under the coolant drain bolt (typically located at the bottom water pump cover).
  • Use the correct socket to remove the drain bolt and carefully let the old coolant completely drain out.
  • You may notice a small copper washer on the bolt–inspect it for damage and replace if needed to prevent leaks.
  • Some YZ125 owners remove the lower radiator hose to ensure the system drains fully–this is optional but helps when fully flushing the cooling system.

3. Reinstall Drain Bolt

  • Once draining is complete, reinstall the coolant drain bolt. Tighten securely but do not overtighten–refer to your manual for correct torque specs if needed.

4. Pour in New Coolant

  • Insert a funnel into the radiator fill neck and pour in fresh coolant slowly, filling to the top of the radiator core.
  • Gently shake or tap the bike to help any trapped air escape.
  • If the coolant level drops after settling, carefully top off to the correct level.

5. Bleed the Cooling System

  • Start the engine with the radiator cap off and let it idle for a few minutes. Watch for air bubbles escaping–this means air is purging from the system.
  • As the coolant warms, keep an eye on the level and add more as needed. Once bubbles stop, the system is nearly bled.
  • Shut off the engine and allow it to cool before replacing the radiator cap securely.

6. Inspect for Leaks & Test Ride

  • Check around the drain bolt, hoses, and radiator cap for leaks.
  • After a short test ride and full cool down, re-check the coolant level and top off if necessary.

YZ125-Specific Tips & Notes

  • The 2019 YZ125 features a lightweight, highly efficient radiator system. Keeping the cooling system clean of debris and using fresh, manufacturer-approved coolant helps prevent overheating and ensures engine longevity.
  • Monitor the radiator shrouds and guards–damage or blockage limits airflow and reduces cooling effectiveness.
  • If you regularly race or ride in muddy or sandy conditions, inspect the radiator fins and flush out mud or dirt during coolant service for maximum performance.
  • The YZ125 uses a two-stroke engine, which is lighter and less complex than larger four-strokes but still relies on efficient cooling to prevent piston and cylinder damage from heat.

Disposal

  • Always dispose of used coolant responsibly at an authorized recycling or hazardous waste station. Never pour it down household drains or onto the ground.

Regular coolant changes are a simple, effective way to protect your YZ125’s engine and ensure you get the most enjoyment from every ride.

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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.