To bleed a cooling system, refill it with the specified coolant, then run the engine with the heater on full while topping up until no more air bubbles come out, using the bleeder screws if your engine has them. Trapped air causes hot spots, overheating, a gauge that jumps around and no heat from the heater, so bleeding is essential after any coolant drain, water pump, thermostat or radiator job.
Why air gets trapped
Draining coolant lets air into the engine, heater core and radiator. Air collects at high points, such as the cylinder heads and heater core, and stops coolant flowing past the temperature sensor and thermostat.
How to bleed a cooling system
- Engine cold, parked level or nose-up. Raising the front helps air rise to the filler.
- Fill slowly with the coolant type and mix your manual specifies. A spill-free funnel that seals to the filler makes this much easier.
- Open bleeder screws if your engine has them, and close each one when coolant runs out without bubbles.
- Start the engine with the heater on full and let it reach operating temperature so the thermostat opens; keep topping up as the level drops.
- Squeeze the upper radiator hose gently to push air out, avoiding hot parts and the fan.
- Finish: once no bubbles appear and the heater blows hot, fit the cap, let the engine cool, and recheck the level in the reservoir.
Never open a hot radiator cap. Some vehicles have a specific bleeding procedure or need a scan tool to run the electric water pump; follow the manual where it differs.
Common mistakes
- Mixing coolant types or using plain water, which reduces corrosion and freeze protection
- Overfilling the reservoir past the hot mark
- Not rechecking the level after the engine has cooled down once or twice
- Opening the cap while hot, which can scald
Signs air is still in the system
- The temperature gauge rises and falls
- No or intermittent heat from the heater
- Gurgling from behind the dash
- The coolant level drops after a few drives
After any cooling system work, check the level again after the first few drives and look for drips at the joints you disturbed.
If the level keeps dropping with no visible leak, the problem may be a leak rather than air; see coolant leak and engine overheating.
Related
Bad water pump and thermostat symptoms · radiator fan not working · DeWitts Corvette radiators on our DeWitts page.
More guides: overheating then going back to normal · heater blowing cold air · bad thermostat symptoms.
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