How To Get Air Out Of Coolant System

How To Get Air Out Of Coolant System

Few problems cause as many headaches for drivers as air trapped in the cooling system. While most know coolant circulating through the radiator keeps an engine from overheating, few understand how crucial removing every bubble of air is for proper function.

One tiny pocket can disrupt heat transfer enough to trigger warning lights or damage components if left unattended. Let’s explore the complex issue of air intrusion and detailed solutions to expel it fully.

Why Air is Detrimental

Glycol-based coolant works by absorbing heat from pistons and transferring it to sleeves of liquid rushing through tubes inside radiator cores. Fins cause air flow to evaporate heat away and water hoses replenish with fresh temperature-regulated fluid.

But air is an imperfect conductor – its compressibility prevents 100% contact needed to wick heat consistently.

Pockets form insulating voids in cylinder heads, water pumps or small radiator passages. Temperature sensors erroneously read cooler than actual hotspots. This stresses pistons and valves more than intended thermostat settings, accelerating wear over thousands of miles.

signs that air present

  • Overheating – Air blocks heat transfer, causing higher operating temperatures.
  • Rough idle – Bubbles can disrupt coolant flow dynamics.
  • Loss of heat – Trapped air reduces coolant’s ability to absorb and transfer heat.
  • Bubbling noise – Gurgling or fizzing from the radiator as air moves.
  • Low fluid level – Bubbles displace coolant from the system.
  • Higher temps – Thermostat may open earlier to compensate for reduced heat transfer.
  • Temperature fluctuations – Heat transfer fluctuates as air pockets move, affecting temperature readings.

Pay attention for overheating, rough idle, loss of heat, bubbling noise, low fluid levels or unstable temperatures which could indicate air is present and needs bleeding out to restore optimal cooling system performance.

Signs That Air Present
signs that air present

Entry Points for Harmful Gases

Any breach introducing regular contact with surrounding atmosphere permits air and moistures access to disrupt the sealed environment. Common culprits include loose or deteriorated components like:

  • Radiator caps failing to hold 15-psi vacuum
  • Cracked radiator tanks or corroded seams
  • Aged rubber hoses shrinking on tight connections
  • Blown head gaskets mingling coolant and combustion gases
  • Warped engine manifolds with inefficient gasket seals
  • Frayed electric cooling fan blade seals and water jackets
Entry Points For Harmful Gases
Entry Points for Harmful Gases

Proper Removal Methods

Rushing the degassing process only disperses bubbles throughout instead of concentrating efforts on specific problem zones. Patience is key to success.

  1. Drain and refill one small section at a time
  2. Leave radiator cap and reservoir off to vent during operation
  3. Tilt vehicle and rock to slosh air upward
  4. Burp highest bleed screws until clear with engine running
  5. Operate heater on max to cycle coolant and purge lines
  6. Top off reservoir every 30 minutes minimum
  7. Allow slow cool-downs between heat cycles
  8. Check for rising bubbles escaping through filler neck
  9. Replace suspect parts like caps if issues persist

With dedication following these steps, the determined do-it-yourselfer can expel air deeply lodged for long periods.

Common Methods for Bleeding Air

Now that we understand why air gets in and how to recognize its presence, let’s examine different bleeding techniques:

Radiator Cap Method

Remove the radiator cap with engine cool. This releases any pressure holding air bubbles down. Start the engine and let it run until the thermostat opens, circulating coolant. Top off coolant in the radiator as it reduces in level from air escaping.

Once bubbling stops for a minute, replace the cap and check refill needed in overflow tank. Continue idling and topping off coolant until no more air escapes and levels stabilize.

This is the simplest method that relies on coolant circulating and venting through the radiator cap to release air. It works well for basic small air pocket removal.

Radiator Cap Method
Radiator Cap Method

Reverse Flush Method

Attach a garden hose to the lower radiator hose barb fitting (never the upper hose). Submerge the opposite radiator hose end in a bucket of coolant to form a sealed loop.

Turn on water and it will pump backwards through the system, pushing air out the top radiator hose and into the overflow reservoir. Top off coolant as levels drop from air displacement. Shut off when bubbles stop coming out.

This uses water pressure to actively flush air backwards, making it more effective for large volumes trapped deep in the block. Always use conditioned or distilled water to avoid mineral deposits.

Reverse Flush Method
Reverse Flush Method

Heater Core Method

Disconnect lower radiator hose and place in a clean container. Start the engine and turn the heater on high with the blower on to pull maximum coolant flow.

Top off the radiator as air escapes and the level drops. Once bubbles stop for a minute, refit the lower radiator hose and check overflow bottle level.

By running the coolant through the entire interior loop including heater core, this evacuates air pockets throughout the system very thoroughly.

Heater Core Method
Heater Core Method

Pressure Pot Method

Remove the radiator cap and connect an adapter fitting to the neck. Attach a pressure pot (such as a bleed block or vacuum filler) set to 15-20psi.

Open the valve to pressurize the system and force air out all vents and overflow. Top off coolant level and release pressure once bubbling stops.

For larger vehicles, heavy equipment, and hard to bleed cooling circuits, a pressure pot is highly effective for complete air removal. This requires low pressure air or nitrogen.

Pressure Pot Method
Pressure Pot Method

Sources of Trapped Gases

Hard to reach crannies require pinpoint focus for complete victory. Pay attention to:

  • Cylinder head galley valleys and coolant passage intersections
  • Thermostat housings and crossover tubes
  • Water pump inlets and outlet cavities
  • Heater core inlet/outlet ports behind the firewall
  • Small coolant jets underneath intake manifolds
  • Between tube sheets inside radiator cores
  • Electric cooling fan shrouds and motor water jackets

Proper Coolant Fill Procedure

While bleeding removes air pockets, following this procedure prevents future air infiltration:

  1. Clean radiator/reservoir before filling to remove scale/debris.
  2. Fill cooling system with a 50/50 premix of distilled water and ethylene glycol.
  3. Fill to the “cold fill line” indicated on most overflow reservoirs or radiators.
  4. Install new radiator and reservoir caps, ensuring a proper vacuum seal.
  5. Check fluid level again after engine cools as coolant contracts with temperature.
  6. Top off small amounts as needed to maintain proper fill concentration.

Always use distilled water approved for automotive use rather than potentially corrosive tap varieties. With air removed and system sealed air-tight, reliable cooling and protection resume.

Additional Tips

Check for noticeable air bubbles escaping when first starting the engine warm. Top off radiator as needed. Idle engines no longer than necessary to prevent overheating while air escapes. Shut off immediately if temperatures rise.

Tighten all hose clamps securely to prevent future air leakage points from developing. Flush antifreeze during scheduled maintenance to dissolve and remove accumulated rust/scale deposits.

Periodically check coolant concentration with a refractometer to ensure proper dilution isn’t altered over time.

Supplemental Bleeding Methods

For severe bubble accumulation resistant to passive techniques, upgrading tools assists the cause:

  • Vacuum bleeders suck air swiftly from screw threads and hose ports
  • Flushing machines deaerate fluid vigorously with heated pressure pumping
  • Catch bottles mounted higher than engines separate air risen upward
  • Blowing compressed air counter-currently through heater core lines
  • Ultrasonic cleaning radiators to scrub mineral deposits trapping microbubbles

With repeated cycling and pinpoint targeting hard spots, the system emerges refreshingly clear and ready to regulate temperatures perfectly.

Supplemental Bleeding Methods
Supplemental Bleeding Methods

Preventing Recurrence

Though tedious work expelling trapped air, taking shortcuts invites quicker return of the same disruption. Inspect for:

  • Failed water pump seal integrity leaking air intake
  • Loose or disconnected heater hoses dried out and porous
  • Rusted-through radiator tanks needing full replacement
  • Cracked manifolds compromising gasket sealing ability
  • Worn out cooling fans destroying their air-tight casings

By thoroughly fixing entry points and maintaining an obsessively clean system, vehicles stay bubble-free for the long haul. With diligence, mechanics can avoid battling this frustrating foe ever again.

Common Causes of Trapped Air:

Worn or improperly tightened hose clamps allowing small amounts of air to enter over time. Cracked or damaged hoses/reservoirs that compromise the cooling system seal.

Low coolant levels allowing air bubbles to be drawn into the system. Air pockets released from old coolant during a fluid flush that weren’t fully purged. Loose or worn radiator/engine caps that no longer maintain a proper seal.

Tips for Properly Filling the Cooling System:

Use a premixed 50/50 coolant/distilled water solution for best freeze/boil protection. Fill to the cold fill line or maximum limit indicated, not the reservoir top.

Allow ample time for air pockets to rise and escape through the radiator while topping off. Inspect for leaks once started and keep an eye on fluid levels/condition regularly. Consider an additive to prevent corrosion if your water has high mineral content.

Being aware of common entry points and symptoms can help diagnose air issues early. Proper filling also prevents future air contamination.

precautions to keep in mind when purging air from the cooling system:

Only remove the radiator cap or open bleed screws when the engine is cool to avoid scalding from hot coolant. Top off coolant slowly to allow air bubbles time to rise and escape, avoiding introducing more air.

Watch temperature and coolant levels closely after starting a warm engine. Stop immediately if overheating occurs. Have extra coolant on hand in case levels drop more than expected as air is removed.

Use rags to wipe up spills and keep coolant away from painted/plastic surfaces where it could damage finishes. Take extra care if a vehicle has aluminium components that are more prone to corrosion if mixed metals come in contact with coolant.

Beware of potential burns from touching hoses, the radiator or coolant during and right after purging while components are warm. Only run the engine as long as needed to bleed air – avoid overheating from extended idling.


Will air bleed itself out of a cooling system?

Over time, some small air bubbles may work their way out on their own. However, it’s not guaranteed and large pockets of air could remain trapped if not properly bled out. Active bleeding is required to fully remove air.

How do I get rid of airlock in my coolant system?

To remove an airlock, loosen the bleed valve or screw at the highest point of the system. Run the engine to circulate coolant and push air out. Top off coolant levels and keep bleeding until no more bubbles emerge. Proper bleeding is required to allow air to rise and fully escape.

What happens if there is air in the coolant system?

Air pockets can cause cooling system inefficiency. They impede heat transfer and fluid flow, allowing engine temperatures to spike higher than normal. This extra heat stress can lead to overheating failures over time if air is not removed.

What happens if you don’t bleed air out of coolant?

If air is not bled from the cooling system, it can cause overheating problems as the trapped pockets block circulation. Temperature fluctuations put excess strain on components and could potentially cause a breakdown. Bleeding air helps prevent overheating issues and costly repairs down the road.


Bleeding air from a coolant system is an important process to ensure proper circulation and prevent overheating issues. Trapped air bubbles can interfere with heat transfer and coolant flow. The key steps to bleeding air are:

Loosen the bleed screw or valve located at the highest point of the coolant system. This allows any trapped air to escape. You may need to loosen additional bleed points as well.

  • With the bleed screw open, start the engine and allow it to run at idle speed. Warming the engine helps the coolant circulate and move any air bubbles to the bleed points.
  • Top off the coolant reservoir as needed to maintain the proper fluid level as air is bled out. Continue running the engine until no more bubbles come out when the bleed screw is opened.
  • Once all air is gone, tighten the bleed screws securely. Check for leaks and top off the coolant as a final step.

Proper bleeding of air from the cooling system is an important maintenance task to ensure efficient heat transfer and regulated engine temperature. Taking the time to perform this process thoroughly helps prevent overheating and costly repairs down the road.

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