How to Get Air Out of an Engine Cooling System

The most reliable way to remove air from a cooling system is to vacuum-fill it with a cooling system vacuum tool; if that’s not available, bleed it by parking nose-up, setting the heater to hot, filling with the correct coolant using a spill-free funnel, and opening any bleed screws until a steady, bubble-free stream appears, then run the engine to operating temperature and recheck the level when cold. Air pockets can cause overheating, temperature swings, weak cabin heat, and even engine damage, so proper bleeding—following the procedure specific to your vehicle—is essential.

What Trapped Air Does and How to Spot It

Air locks prevent coolant from circulating, creating hot spots around the thermostat, cylinder head, and turbo (if equipped). Recognizing the signs early helps you avoid damage and ensures you’re actually solving an air problem instead of masking a leak.

• Temperature gauge spikes or fluctuates, especially after service or a coolant drain.
• Heater blows cold at idle but warms up with higher RPM, or never gets hot.
• Gurgling sounds behind the dash or near the heater core, visible bubbles in the reservoir.
• Coolant level drops after a drive and rises when hot, with no visible external leak.
• Warning messages or codes related to coolant temperature (for example, thermostat performance).

If these symptoms persist after a careful bleed, suspect a leak, a weak radiator cap, or combustion gases entering the cooling system (head gasket, cracked head, or liner issue).

Tools and Supplies

You can do an effective bleed with basic tools, but modern vehicles bleed fastest and safest with a vacuum fill kit. Use only the manufacturer-specified coolant chemistry.

• Vacuum fill tool (e.g., AirLift-style) and compressed air source, or a spill-free funnel kit.
• Correct coolant premixed to the proper ratio (typically 50/50) with distilled water if needed.
• Basic hand tools to access and operate bleed screws; pliers to squeeze hoses.
• Low-profile ramps or a jack and stands to raise the nose of the car if needed.
• Shop towels, catch pan, gloves, and eye protection.
• Scan tool or service function access if the vehicle uses an electric water pump or active thermostat.

Having the right equipment reduces mess, speeds up air removal, and helps confirm the system is leak-free before you add coolant.

Method 1: Vacuum-Fill (Preferred)

Overview

Vacuum filling draws a deep vacuum on the cooling system, both proving it can hold pressure (no leaks) and pulling coolant in without trapping air. This is the method most shops use on modern vehicles and those with complex cooling circuits (turbochargers, auxiliary pumps, battery heaters, etc.).

Step-by-step

Follow these steps carefully to vacuum-fill and purge air efficiently.

1. Start with a completely cool engine. Set the cabin heater to its hottest temperature and the fan to low to open the heater valve/loop.
2. Remove the radiator cap or expansion tank cap. Install the vacuum tool adapter on the filler neck or degas bottle.
3. Close the tool’s coolant valve. Connect shop air per tool instructions. Draw vacuum to approximately 20–25 inHg.
4. Close the air valve and watch the gauge for 2–3 minutes. If vacuum decays, find and fix leaks before continuing.
5. Place the tool’s pickup hose into clean, premixed coolant. Open the coolant valve and allow the vacuum to pull coolant in until the gauge returns to zero and flow stops.
6. Top off to the “COLD” mark. Reinstall the cap loosely if specified by the tool, or leave the funnel adapter on for observation.
7. Start the engine and let it idle. Keep the heater on hot. As the engine warms and the thermostat opens, watch for any small residual bubbles; top off as needed.
8. Allow the cooling fans to cycle once. Verify strong cabin heat and a stable temperature gauge.
9. Shut the engine off, fully install the cap, and road test. After complete cooldown, recheck and adjust the level to the cold mark.

This process quickly eliminates air pockets and doubles as a leak check, reducing the chance of repeat bleeding.

Method 2: Bleed Without Special Tools (“Burping”)

With Bleed Screws

Many engines include one or more bleed screws at high points in the system (near the thermostat housing, upper radiator tank, or heater outlet). Use them to vent air while filling.

1. Park with the vehicle’s nose slightly elevated. Start with a cold engine and set the heater to maximum hot.
2. Remove the cap on the radiator or expansion tank. Open the highest bleed screw(s). Place a catch pan below.
3. Slowly fill the radiator or expansion tank with premixed coolant until a steady, bubble-free stream flows from each bleed screw; then snug the screw(s).
4. Install a spill-free funnel on the filler neck or tank. Fill the funnel about one-third full to act as a high-point reservoir.
5. Start the engine and let it idle at 1,200–1,500 rpm. Watch the funnel for bubbles. Briefly crack the bleed screw(s) again to release trapped air as the thermostat begins to open.
6. Gently squeeze the upper radiator hose to encourage bubbles to rise; avoid burns by keeping clear of moving parts.
7. When you have solid heat from the vents, a stable temperature gauge, and no more bubbles, tighten all bleed screws to spec.
8. Remove the funnel, top off to the cold mark, and install the cap. Blip the throttle to 2,000 rpm a few times to move remaining micro-bubbles.
9. Allow a full cooldown and recheck the level. Adjust to the cold mark and verify no coolant odor or damp areas around hoses and bleed points.

Using the bleed screws strategically purges high-point air; patience matters—wait for the thermostat to open and the fan to cycle.

Without Bleed Screws

If your system has no dedicated bleeders, use gravity and a raised fill point to move air out through the filler neck or degas bottle.

1. Park nose-up on ramps or a gentle incline. With a cold engine and heater set to hot, install a spill-free funnel on the filler neck or tank.
2. Fill slowly until the funnel remains half full. Start the engine and idle; maintain the coolant level in the funnel.
3. As the engine warms, lightly squeeze the upper radiator hose to release trapped air upward.
4. Once the thermostat opens and the radiator fans cycle, watch for a diminishing stream of bubbles in the funnel.
5. When bubbles stop and heater output is consistently hot, remove the funnel, top off to the cold mark, and install the cap.
6. Road test, then allow the engine to cool completely and recheck the level. Top up the overflow bottle to the indicated mark.

This approach uses the funnel as the highest point so air migrates out naturally; it may take longer than vacuum-fill but works reliably if no leaks exist.

Make/Model-Specific Notes

Some vehicles require extra steps. Always check the service manual for your exact engine code.

• BMW/MINI with electric water pumps: Use the electric bleed routine (ignition on, heater max, accelerator to the floor for ~10 seconds, pump runs ~12 minutes). Top off afterward.
• Ford EcoBoost and other turbo engines: Multiple circuits and high-mounted degas bottles often self-bleed best with vacuum fill; locate any hidden bleeders near the turbo coolant lines.
• Subaru flat-fours: The radiator cap is often below engine high points; use a funnel and the upper-tank bleeder when present, or vacuum fill for best results.
• Honda/Toyota four-cylinders: Commonly include a bleeder on or near the thermostat housing—open during initial fill until a solid stream appears.
• VW/Audi and other VAG models: Complex auxiliary loops benefit from vacuum fill; some require running an auxiliary pump via a scan tool service function.
• Hybrids and vehicles with electric heater pumps: Some have separate cabin or battery cooling loops. Follow service-mode bleeding via a scan tool to run pumps and open valves.
• Thermostat “jiggle pin” types: If you replaced the thermostat, orient the jiggle pin/vent hole at 12 o’clock to help self-bleed.

Skipping manufacturer procedures can trap air in auxiliary circuits, causing intermittent overheating or no-heat complaints.

Aftercare and Verification

Bleeding doesn’t end when you shut the hood; the system may release residual micro-bubbles over the next heat cycles. Confirm everything is stable.

1. Check the coolant level cold after each of the next 2–3 drives; top up to the cold mark as needed.
2. Verify consistent heater performance at idle and cruise, and a steady temperature gauge.
3. Inspect for seepage at hose joints, thermostat housing, water pump weep hole, and bleed screws.
4. If the level keeps dropping or bubbles persist, pressure-test the system and cap. Consider a combustion leak test if air returns quickly.
5. Monitor for related fault codes and address sticking thermostats or weak caps promptly.

Stable levels, solid cabin heat, and a steady temperature gauge indicate the air is gone and the system is healthy.

Common Mistakes to Avoid

These pitfalls can reintroduce air or mask underlying problems, leading to repeat overheating.

• Opening the cap on a hot engine—risk of scalding and sudden coolant eruption.
• Using the wrong coolant type or mixing incompatible chemistries, which can form sludge and restrict flow.
• Ignoring small leaks; even minor seepage prevents a stable fill and keeps pulling in air.
• Forgetting to turn the heater to hot, leaving the heater core air-locked.
• Overfilling above the cold mark, which can cause overflow and reintroduce air as the system purges.
• Incorrect thermostat orientation or omitting bleed holes where required.
• Using tap water that introduces minerals; use distilled water to mix concentrate.
• Neglecting auxiliary circuits (turbo coolant lines, throttle body warmers, battery heaters) that can trap air high in the system.

A careful setup, correct coolant, and attention to leaks make bleeding faster and more reliable.

Safety and Environmental Notes

Coolant is hazardous when hot and toxic if ingested. Protect yourself and handle fluids responsibly.

• Work only on a cool engine unless a procedure specifically calls for a warm bleed, and use gloves and eye protection.
• Support the vehicle securely if raising the front; use jack stands on solid ground.
• Keep coolant away from pets and wildlife; ethylene glycol is sweet and deadly. Consider pet-safe propylene glycol where compatible.
• Capture and recycle used coolant according to local regulations; never pour down drains or on the ground.
• Beware moving belts and fans when the engine is running; keep clothing and tools clear.

Following safety practices prevents injury and environmental harm while you restore proper cooling performance.

Summary

To remove air from a cooling system, vacuum-fill with a dedicated tool for the quickest, most thorough result; otherwise, bleed by parking nose-up, using a spill-free funnel, opening any bleed screws, running the engine with the heater on, and topping off once bubbles stop. Recheck the level after cooldowns, fix any leaks, and use the manufacturer’s procedure—especially on vehicles with electric pumps or complex circuits—to ensure lasting, trouble-free operation.

How to burp a coolant system with a funnel?

Out. You just pour the coolant into the funnel. And it bubbles down into the coolant. System without any spillage once you’ve filled the coolant system You can also run the engine.

How do I know if there’s air trapped in my cooling system?

Symptoms of an air pocket in a car’s cooling system include engine overheating, fluctuating temperature gauge readings, the car’s heater blowing cold air, and gurgling or bubbling noises from the engine bay. Reduced engine performance, visible coolant leaks, and an inability to accelerate properly can also occur because the air blocks proper coolant circulation. 
Common Symptoms

• Engine Overheating: An air pocket can prevent coolant from circulating, leading to localized hot spots and overall overheating. 
• Fluctuating Temperature Gauge: The needle on your temperature gauge might swing high and low, indicating air is interfering with coolant flow and confusing the sensor. 
• Poor Heater Performance: Trapped air in the heater core will prevent hot coolant from reaching it, resulting in cold air blowing from the vents. 
• Gurgling or Bubbling Noises: As air and coolant move through the system, you may hear bubbling or gurgling sounds from the dashboard or radiator area. 
• Reduced Engine Performance: Sluggish acceleration or overall decreased power can happen when the engine overheats due to insufficient coolant circulation. 
• Coolant Leaks: In some cases, pressure fluctuations caused by an air pocket can push coolant out of weak points in the system, like worn hoses. 

Causes

• Improper Coolant Replacement: Air can enter the system during a coolant flush if it isn’t properly bled afterward. 
• Component Replacement: Replacing components like the thermostat or water pump can introduce air if the system isn’t bled. 
• Leaks: Minor leaks in hoses, the radiator, or water pump can allow air to seep into the sealed cooling system. 

What to Do
If you suspect an air pocket, it’s important to have it addressed promptly by a professional, as continued air in the system can lead to more severe problems like water pump damage or a blown head gasket.

Will air bleed itself out of a cooling system?

Will a cooling system bleed itself? Your coolant system can bleed some between the overflow tank and thermostat.

How do I get air out of my coolant system?

To bleed air from a coolant system, raise the front of the car, top off the coolant, and start the engine with the radiator cap off. Turn the heater to its highest setting to open the heater core, then allow the engine to idle until it reaches operating temperature, which opens the thermostat and releases trapped air. Continue adding coolant as air bubbles stop, then replace the radiator cap and test drive. 
1. Prepare the Vehicle and Coolant 

• Safety First: Park the car on an incline, with the front end raised as high as possible. This helps air rise to the radiator cap. 
• Fill the System: With the engine off, remove the radiator cap and fill the cooling system with the proper type and amount of coolant. A spill-proof funnel can be very helpful to prevent spills and monitor the level. 

2. Start the Engine and Run the Heater 

• Start the Car: Start the engine with the radiator cap still off. 
• Engage the Heater: Turn the car’s heater on to the highest temperature setting and the blower fan on a low speed. This ensures the heater core opens and allows air to escape from that part of the system. 

3. Let the Engine Reach Operating Temperature 

• Idle and Monitor: Let the engine idle until it reaches its normal operating temperature. The thermostat will open, allowing more air to escape from the radiator and engine block. 
• Top Off Coolant: As the engine warms up, the coolant level will rise. Continue to add coolant to the funnel or radiator as needed until no more air bubbles are visible. 

4. Finalize the Bleeding Process 

• Check for Bubbles: Continue to watch the funnel or radiator for at least 15 minutes after the thermostat opens to ensure no more air bubbles are escaping. 
• Replace the Cap: Once no more air bubbles are seen, remove the funnel and secure the radiator cap. 
• Test Drive: Go for a test drive, keeping a close eye on the engine’s temperature gauge. 

5. Monitor After the Test Drive 

• Recheck Coolant Levels: After the test drive and when the engine has cooled down, check both the radiator and the coolant reservoir. Top off the coolant if it has dropped below the proper level.