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How to Eliminate Air Lock in a Car Cooling System

Updated April 17, 2017

Coolant from the radiator runs in a continuous loop through the hoses, into the engine passages and then returns to the radiator for cooling. In a sealed cooling system the coolant flow runs at a constant pressure and volume. Sometimes air can be trapped in the radiator, hoses, or passages and interrupt the flow of coolant. Such a condition can cause the thermostat to not open and close properly, or the water pump impeller to spin uselessly in a dry chamber. Sudden and catastrophic overheating can result in minutes, damaging the engine permanently. Any vehicle owner can purge their cooling system of unwanted air with a few simple tools and tips.

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  1. Park the vehicle in neutral or park and set the emergency brake. Raise the hood. Remove the radiator and overflow cap, if so equipped (some radiators come sealed). Fill the radiator with coolant to a point just below inlet neck. Fill the overflow reservoir to prescribed limit indicated by a line on the side of the plastic container. Replace the caps. Start the engine and let it heat up to normal operating temperature.

  2. Refer to your owner's manual to determine if you have an air bleeder valve (bolt or petcock) on the water box next to the thermostat housing. This will be a small air bleeder bolt that might have a hex-head fitting, requiring an Allen wrench. Remove the bolt. Fill the overflow reservoir to its limit, and then begin to fill the radiator, while watching for any coolant that seeps out of the bleeder hole. If it does, replace the bleeder bolt and tighten. Replace the radiator cap.

  3. Watch the bleeder hole while you add coolant to the radiator. If no coolant seeps from the bleeder hole, start the engine and turn the interior heater on to its maximum setting. Wait for the engine to reach normal operating temperature so the thermostat opens (the upper radiator hose will be hot). Watch the bleeder hole while you add coolant to the radiator. Stop when you see coolant leaking, and replace and tighten the bleeder bolt. Top-off the overflow reservoir and replace both caps.

  4. Lift the front part of the vehicle with a floor jack and place two jack stands under the frame near each wheel. This process will work if no bleeder hole exists on your vehicle. Place the shift selector in park or neutral with the emergency brake set.

  5. Remove the radiator cap and let the engine heat up until the electric cooling fan cycles on. Turn the interior heater on its maximum setting. Fill the radiator with coolant and wait until the coolant starts to bubble or churn. The upper radiator hose should feel hot to the touch. If not, wait until the thermostat opens and heats the hose. The coolant level should drop at this point. Top off the radiator with coolant and replace the cap.

  6. Warning

    Never remove the radiator cap on a hot engine. Always wait for it to cool down. Raising the front of the vehicle will elevate the radiator higher than the thermostat position, making it possbile to remove air from the system.

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Things You'll Need

  • Socket set and wrench
  • Floor jack
  • Jack stands
  • Coolant
  • Owner's manual
  • Allen wrenches

About the Author

Chris Stevenson

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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