Fixing Air in a Car Radiator: Troubleshooting and Bleeding the Cooling System

Introduction

If you’ve ever noticed gurgling noises, bubbling coolant, or inconsistent engine temperatures, you might have air trapped in your cooling system. We recently helped a client identify this issue with his Mercedes-Benz E-Class W212 (2013) when he came to drop off wheels for refurbishment. We noticed a noise near the coolant reservoir; however, the principles outlined here apply to many vehicles with similar cooling system layouts. By referencing your vehicle’s manual or general cooling system diagrams, you can adapt these steps to most models.

In this guide, we’ll cover:

• How to identify air in your radiator
• A step-by-step bleeding process
• Common mistakes and what to avoid
• The role of bleed screws
• How automatic self-bleeding systems work
• Why choosing the correct coolant type matters

1. Does Air in the Radiator Make Noise?

Symptoms of Air in the Cooling System:

• Gurgling or bubbling sounds from the dashboard (especially when revving or using the heater)
• Sloshing noises when accelerating or braking
• Heater blowing cold or inconsistent air
• Fluctuations in the temperature gauge

Where is the Gurgling Noise Coming From?

In our case, the gurgling was emanating from the area near the coolant reservoir (typically located in the engine bay) as coolant flowed through it. This suggests that air was trapped in the cooling system, causing turbulence in the expansion tank. You may also hear a more pronounced gurgling when lightly revving the engine to around 2,000 RPM, as increased coolant flow can force any trapped air through the system.

Conclusion:

Air trapped in the radiator, heater core, or hoses can indeed cause noise and improper coolant circulation.

2. Does Air in the System Affect the Heater?

Yes. If there is air in the heater core, the coolant cannot circulate properly, leading to cold air being blown from the vents even when the heater is on.

Solution:

Bleed the air out of the system to ensure proper coolant flow. This applies to most vehicles, not just Mercedes.

Note: This home-based solution takes approximately three hours to complete.

3. The Role of Automatic Self-Bleeding Systems

Some modern vehicles are equipped with a self-bleeding or self-venting system designed to purge air automatically. These systems often include:

• Vents or channels in the coolant reservoir and hoses
• A special thermostat that facilitates air removal

While these systems can help, you may still need to manually bleed the system if large amounts of air enter it. Consult your vehicle’s manual to determine if your car has an automatic bleed feature, and remember that even self-bleeding systems may require topping up the coolant and running the heater to assist in removing air. This procedure can also be performed at home, although you will need to lift your car.

4. The Role of Coolant Bleed Screws

Some vehicles, including certain models of Mercedes-Benz, BMW, and other manufacturers, are equipped with bleed screws located on the radiator, thermostat housing, or on the hoses. These screws are the preferred method for removing trapped air.

How to Use Bleed Screws:

• Locate the Bleed Screw(s): Check your owner’s manual or inspect the radiator or thermostat housing for small plastic or metal screws.
• Open the Screw Slowly: With the engine cold, open the screw slightly to allow air to escape. If coolant begins to dribble out steadily, close the screw.
• Run the Engine: If your manual instructs you to do so, run the engine with the screw open until no more bubbles appear.
• Close the Screw: Once air stops escaping, gently tighten the bleed screw, taking care not to overtighten.

If your vehicle does not have bleed screws, follow the expansion tank method described in the following sections.

5. Can I Run the Engine with the Coolant Cap Off?

Yes, but only until all bubbles have disappeared, and with caution. Do not drive with the cap off, as the system is designed to be pressurised, and leaving the cap off for too long can cause coolant loss or overheating.

Steps:

• Start with a cold engine and open the coolant reservoir.
• Idle the car and allow bubbles to escape.
• Turn the heater on to maximum temperature to help push air through the system. Ensure that the air conditioning is off and that the fan speed is kept low.
• Lightly rev the engine to around 2,000 RPM to speed up the process.
• Observe the coolant level and top up as needed.
• Check for any remaining bubbles in the expansion tank.
• Once bubbling stops and the coolant level stabilises, securely close the cap before the coolant reaches operating temperature (90°C).

6. Coolant Type: Why It Matters

Always use the type of coolant specified by your vehicle’s manufacturer. For Mercedes‑Benz models, look for the MB‑spec coolant (e.g. MB 325.0 or 325.5), or other brands that meet the specification. Mercedes vehicles are engineered with specific cooling system requirements, making it essential to use the correct coolant type.

Using the wrong type of coolant in your car can lead to:

• Corrosion in the cooling system
• Reduced cooling efficiency
• Potential engine damage

Check your owner’s manual or the manufacturer’s website to confirm the correct coolant.