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How to bleed a radiator

A performance V8 engine

Your car’s cooling system is designed to contain one thing: coolant — and that’s it. Air in your radiator is a bad thing, as the bubbles interrupt and obstruct the flow of coolant that your engine needs to stay at proper operating temps. If air gets in, you have to get it out. So let’s talk about how to bleed a radiator.

How Does Air Get Into the Radiator?

Air can get into your radiator in a few different ways. A botched coolant system flush job, a broken or blown head gasket, a cracked radiator hose or a radiator cap that doesn’t seal tightly can all let air in.

How Can I Tell If There’s Air in My Radiator?How to bleed a radiator

If your car starts overheating and cooling off over and over, and everything else looks fine — good coolant level in the overflow tank, no leaks visible from the hoses or radiator itself — that is a strong sign that air has been trapped in the system. The swings in temperature happen when the air bubbles move and coolant either gets where it’s going or is obstructed. Over time, this can cause heat-related engine damage.

How to Bleed a Radiator:

This can be done by one person, but it’s better with two — one handling things from inside the vehicle and one keeping an eye on the coolant level moment to moment.

The Basic steps:

  1. Jack up the front of your car, and place jack stands on both sides.
  2. Put a pan under the car to catch any dripping coolant. Note that coolant is toxic and can be deadly for pets.
  3. Start pouring coolant into the radiator through a funnel. Stop when the coolant is at the top of the radiator neck.
  4. Start the car.
  5. Turn the heater all the way to the max temperature and put the fan on low.
  6. Watch the coolant level. As the engine gets to a normal temperature, the level will begin to drop. That’s when you want to slowly add coolant back in and keep it topped off.
  7. Keep an eye on the heat coming from the HVAC in the car, which should be constant. Also, check the engine temperature gauge, which should be in the normal operating range. If you do have a partner for the job, make sure they keep you informed about any changes.
  8. When the coolant stops dropping, turn off the engine, remove the funnel, and put the radiator cap back on tightly.
  9. Start the vehicle, and let it idle for a few minutes, watching for any changes in heat output from the vents or in engine temperature on the gauge.

If all is good, then take the vehicle for a short drive, and continue monitoring the engine temperature. When you come back, turn off the engine and let the vehicle cool. Then, and only then, you can remove the radiator cap and check the coolant level one last time. If it’s where it was last time, congratulations! You’ve bled your radiator, possibly saving yourself from some long-term headaches.

If issues persist, consider seeing your local NAPA AutoCare to diagnose any bigger problems before they damage your car.

Check out all the engine heating and cooling products available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to bleed a radiator, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photos courtesy of pxhere.


Mike Hagerty View All

Mike Hagerty is an automotive journalist whose work has been featured on radio, TV, in print and online since 1997. He's the Publisher and Editor of, and contributes car reviews to the Los Altos Town Crier and Previous outlets have included KFBK and in Sacramento, California, the ABC television affiliates and Hearst-Argyle and Emmis radio stations in Phoenix, Arizona; AAA magazines for Arizona, Oklahoma, Northwest Ohio, South Dakota and the Mountain West and

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