LS400 1990-1997

Cooling System Bleeding


After working on the cooling system, even to replace the thermostat, it must be bled. Air trapped in the system will prevent proper filling and leave the radiator coolant level low, causing a risk of overheating.

To bleed the system, start with the system cool, the radiator cap off and the radiator filled to about an inch below the filler neck.

  1. Start the engine and run it at slightly above normal idle speed. This will insure adequate circulation. If air bubbles appear and the coolant level drops, fill the system with an antifreeze/water mixture to bring the level back to the proper level.
  3. Run the engine this way until the thermostat opens. When this happens, coolant will move abruptly across the top of the radiator and the temperature of the radiator will suddenly rise.
  5. At this point, air is often expelled and the level may drop quite a bit. Keep refilling the system until the level is near the top of the radiator and remains constant.
  7. If the vehicle has an overflow tank, fill the radiator right up to the filler neck. Replace the radiator filler cap.