Changing the engine coolant in the Yanmar 3GM30F series Engine This article provides a set of directions for flushing the engine antifreeze in the Yanmar engine. It specifically addresses the Yanmar Japanese Yanmar 3GM, although this procedure should be the same for the European Yanmar 3GM as well. The newer 3YM series might be somewhat different.

Submitted by Karl Mielenhausen
“Silver Lining” – Hull 690


Equipment List:

  1. 0.75 gal Dexcool-type Extended Life Antifreeze (ex. Prestone 5yr/150000mi) Or 1.5 gal of pre-mix version of above
  2. Note: Engine requires 0.9 gal but another 0.5 gal is in the water heater loop. Coolant system flush (must be safe for aluminum) (ex. Prestone Super Flush)
  3. Pliers
  4. Funnel
  5. Bucket or catch-basin
  6. Water (Makeup water should be soft water or distilled water)



  1. On a cool engine, loosen the filler cap and drain old coolant into container. On my engine, there are 3 drain points. Most volume came out of the block drain fitting on the port side just aft of the oil filter. Approx. 16 oz came out of the 2 drains on the heat exchanger, starboard side.
  2. After draining the engine, remove one of the hot water tank hoses from the top front of the engine. Keep the 2 coolant drains on the engine open. Put on the radiator cap and put your finger over the opening on the engine where the hot water heater hose was connected. Clean off the hot water heater hose and then blow on the hose and the hot water heater loop is emptied (about 1/2 gallon).
  3. Remove and clean the overflow tank. It pulls straight up off of its mounting bracket.
  4. Close all drains and fill with water. Replace filler cap. Run engine until thermostat opens and temperature stabilizes. Then shutdown and allow to cool.
  5. Drain (ie. Repeat steps 1 & 2)
  6. Refill with coolant system flush chemical plus water as required to fill. Run engine until thermostat opens. Shutdown and allow to cool.
  7. Drain (ie. Repeat steps 1 & 2)
  8. Refill with water to flush out the cleaning chemical (ie. Repeat step 3). Do this 2 or 3 times until drained water is clean and free from ethylene glycol (green) antifreeze. Note: if you are staying with the EG antifreeze, 1 or 2 flushes should be sufficient.
  9. Drain for the last time.
  10. Refill with the 50/50 mix of Dexcool-type extended life antifreeze. Fill overflow tank slightly above the “min” mark.
  11. Start engine, check for leaks and proper temperature. After shutdown, recheck level in overflow tank.