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How to Bleed Your Car's Cooling System

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Air pockets in the cooling system may cause serious engine damage.

Air pockets in the cooling system may cause serious engine damage.

You need to bleed the air out of the cooling system in your car. Air pockets — or hot spots — are dangerous. Besides causing little or no heat to come out from the heater, it may cause:

  • Engine overheating
  • Crack the engine
  • Wrap the engine
  • Increase emissions

Usually, replacing a system component, replacing the coolant or servicing the cooling system in your vehicle can lead to hot spots in the system. But air may also enter the system through the coolant reservoir from time to time on some vehicle models.

This is more true with modern vehicle designs where the radiator has been pushed to a lower level than the engine's.

A hot spot is nothing more than trapped pockets of air or bubbles inside cooling system passages that prevent coolant from carrying heat away from those areas, causing the engine to overheat and corrosion to start setting in.

To bleed the air out the cooling system you can follow one of three different methods, depending on whether your particular system comes with bleeding screws or not.

If you don't know whether your system comes with a bleeding screw(s), consult your car owner's manual, or check your vehicle service manual. You can buy a repair manual for your particular vehicle make and model in many auto parts stores or online. Still, you may find a copy of the manual in the reference section of your local public library.

Either way, read on. The descriptions provided here may help you determine the method to use in your particular car.

Index

I. How to Bleed a Cooling System Equipped With a Bleed Screw

II. How to Bleed a Cooling System That Doesn't Have a Bleed Screw

III. How to Bleed a Cooling System Using Two Jack Stands

IV. How to Bleed a Cooling System When the Radiator Has No Cap

V. Do You Need to Replace the Radiator Cap?

Wait for the engine to cool before bleeding the cooling system.

Wait for the engine to cool before bleeding the cooling system.

I. How to Bleed a Cooling System Equipped with a Bleed Screw

The bleeding screw is usually located around the top of the engine. The most common place is right on the thermostat housing where the upper radiator hose connects to the engine. If you see an orphaned, single screw, your cooling system has a bleed screw—some systems come with more than one bleed screw, though. Check your car owner's manual or repair manual, if necessary.

Before you begin this procedure, make sure the engine and radiator are cool. If they are hot, wait an hour or more.

  1. When ready, remove the radiator cap
  2. If necessary, bring the coolant to the proper level by adding a 1-1 ratio mixture of distilled water and antifreeze. Fill the radiator up to the bottom rim of the radiator neck with the mixture.
  3. Add coolant to the reservoir as well. Bring the coolant level to the "Cold" mark.
  4. Replace the radiator cap.
  5. Locate the bleed screw and place a pan on the floor to catch the coolant that will come out through the screw.
  6. Turn the bleed screw 2 turns counterclockwise and attach the end of a 4 feet, clear hose to the bleed screw. The hose should fit snugly over the screw. Place the other end of the hose in the catch pan under the vehicle.
  7. Start the engine and let it idle for about 20 minutes to bring the engine to operating temperature—when the engine has reached operating temperature, you'll feel the upper radiator hose getting hot.
  8. When you see a stream of coolant free of air bubbles flowing through the clear hose, tighten the screw again.
  9. Turn off the engine, wait for it to cool and remove the clear hose and the catch pan from underneath the vehicle.
  10. Add more new coolant to the radiator—up to the bottom of the radiator neck—and the reservoir, if necessary.
  11. Test-drive your car and see if the temperature stays within the normal parameters. Otherwise, head over to the third section below "How to bleed a radiator using two jack stands."
  12. Over the next few days, check the coolant reservoir from time to time to make sure it remains at the correct level.

Warning!

Don't remove a bleed screw while bleeding the cooling system or you may seriously burn yourself with hot coolant.

Some cooling systems don't have a bleeding screw.

Some cooling systems don't have a bleeding screw.

II. How to Bleed a Cooling System That Doesn't Have a Bleed Screw

Systems without a bleed screw require a slightly different method to purge trapped air. First, make sure the engine and radiator are cool.

  1. Remove the radiator cap.
  2. Fill the radiator with a 1-1 ratio mixture of distilled water and antifreeze up to the bottom of the radiator neck. Mix only distilled water with the antifreeze.
  3. Add coolant to the reservoir container to bring the level up to the "Cold" mark as well.
  4. Leave the radiator cap off and start the engine. Set the heater to MAX and turn it on.
  5. Let the engine reach operating temperature. When the engine reaches operating temperature, you'll see through the radiator neck the coolant flowing. Also, the upper radiator hose will become hot.
  6. Wait for a few seconds to allow coolant to flow. You'll feel the heater blowing only hot air. At this point, air has been purged from the system.
  7. Turn off the heater. Shut off the engine and let it cool.
  8. Then, add more coolant to the radiator if necessary, to bring the level up to the bottom of the radiator neck.
  9. Squeeze the upper radiator hose to expel air after adding more coolant, and add more coolant if necessary.
  10. Replace the radiator cap.
  11. Start the engine and let it reach operating temperature.
  12. Test drive your car and monitor the engine temperature (if the engine still overheats, use the next method to remove the remaining air from the cooling system).
  13. Check for coolant leaks and then turn off the engine.
  14. Monitor the coolant level on the reservoir over the next few days and add coolant as necessary.

III. How to Bleed a Cooling System Using Two Jack Stands

Some cooling systems are harder to bleed than others, even if they provide more than one bleeding valve. Try this method to purge the remaining air.

  1. Park your vehicle on a level surface.
  2. Make sure the engine and radiator are cool.
  3. Use a floor jack to jack up the front of your vehicle so the radiator neck is at a higher level than the engine. Then, secure the vehicle with a jack stand on each side.
  4. Block the rear wheels using a couple of wooden blocks.
  5. Set the parking brake.
  6. Remove the radiator cap and start the engine. Set the heater to MAX and turn it on.
  7. Wait until the engine has reached operating temperature.
  8. When the engine reaches operating temperature, you'll see coolant starting to flow by looking through the radiator filler neck. Wait for a few seconds for the system to expel the air. Then turn off the engine.
  9. When the engine cools, replace the radiator cap and remove the jack stands.
  10. Add coolant to the radiator to bring the coolant to the correct level, if necessary. Squeeze the upper radiator hose to expel any trapped air.
  11. Add coolant to the reservoir as well, if necessary.
  12. Test-drive the vehicle.

IV. How to Bleed a Cooling System When the Radiator Has No Cap

Some modern cooling systems come equipped with a pressurized system. The radiator has no cap. This system is easy to bleed, but you still need to follow a few key points.

  1. Set your transmission to Park (automatic) or Neutral (manual).
  2. Set the emergency brakes.
  3. Pop the hood open and fill the system with the recommended antifreeze and distilled water (50/50 blend) through the surge tank. The tank that connects to the radiator.
  4. Replace the tank's cap.
  5. Start the engine and let it idle.
  6. Turn on the heater and set it to MAX to allow coolant to run through the whole system.
  7. Let the engine reach operating temperature (about 20 minutes at idle).
  8. As the engine idles, raise engine speed to about 2000 rpm for 10 to 15 seconds at different intervals to help eliminate air pockets.
  9. Turn off the engine and let it cool.
  10. With the engine cool, add more coolant to the system to bring the level back up to the Full Cold mark.

During the next two or three days of driving your car, check the cooling system level with the engine cool and add coolant as necessary. The system will self-bleed during this period.

V. Do You Need to Replace the Radiator Cap?

A radiator cap helps pressurize the system to the correct level and keep air out of the system. As the cap's seal wears out, through, air begins to seep in and pressure to leak out. Overheating may take place.

If you haven't replaced the radiator cap within the last two years, it's a good idea to replace it now to prevent corrosion and system overheating.

Bleeding the cooling system requires a few simple steps. Still, you may need to try more than one method, besides the one recommended by your car manufacturer, to successfully bleed the cooling system. Follow either one of the previous methods, depending on your particular vehicle model, every time you need to replace the coolant or when adding coolant to the system between service intervals. This will help you get rid of hot spots, and the potential for serious engine damage.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: How do I bleed the air if my radiator doesn't have a cap?

Answer: Check for a bleeder screw, or through the coolant reservoir--or pressurized tank.

Question: I drive a 2009 Kia Rio in Nigeria. Is it right for my mechanic to completely remove the thermostat because of our hot tropical weather?

Answer: The thermostat helps to keep the engine at operating temperature. Make sure to install the appropriate thermostat for your application. If possible, use only OEM parts. Hope this helps.

Question: Your article about bleeding air in a cooling system makes sense, but why jack front end up? Can't you just park on a hill or any high spot?

Answer: Yes you can. If it brings the input point higher than the engine, it will help to purge the system.

Question: I have a 2002 Chevy Impala. I just replaced the water pump, coolant fan, and the thermostat. Why does my car keep running hot despite its new water pump and coolant fan?

Answer: Make sure to bleed the system. Other potential problems are a restricted radiator or a faulty radiator cap. This other post may help:

https://axleaddict.com/auto-repair/What-Causes-a-C...

Question: I have a 2002 Pontiac Grand Prix. I can’t get water to come through either bleeder valves. I’ve replaced everything and still can’t figure it out. What could be the problem?

Answer: First, make sure you have enough coolant in the system. Check the capacity in your owner’s manual. If there’s enough coolant, the bleeder valve might be clogged. You may need to remove the valve to make the inspection.

Question: I have a 1986 Toyota truck two-wheel drive. The oil sending unit on the block under the oil filter went bad last summer. I replaced it but was told it needs a specific tool. It was hard to get to and used channel locks to tighten it. 5 months later it's leaking pretty bad. Could it be a bad part? Not tightened enough? Does it take a special tool?

Answer: Some models use a special socket. It definitely makes your job easier. I don’t have the manual for your model, but you should remember to wrap some Teflon tape around the threads. Using other than the special tool, if necessary, makes it easier to over-tighten and damage the unit. It easy to cross thread it when not using your fingers to snug it in place and then using a wrench to secure it. These are two possibilities.

Question: Why is the coolant in my reservoir bubbling, steaming hot, and boiling?

Answer: This could be an overheating issue, especially if the reservoir is also pressurized (no radiator cap). Check the coolant level once the engine cools. Make sure the engine has the correct amount as well as the reservoir (cool level). Verify the reservoir is able to hold coolant. Closely inspect the reservoir for leaks.

Question: I have an 09 Dodge Charger 2.7 engine. I recently had my radiator and my thermostat replaced. But because I didn't buy enough coolant for when the radiator got replaced, my mechanic didn't bleed my system. So how can I do it myself? My car has no radiator cap. I know where the hose is that flows the coolant to the radiator. So how can I bleed it myself?

Answer: This video shows how to bleed the system on a Charger. Hope it helps:

https://www.youtube.com/watch?v=mgcRD1kndu8

© 2016 Dan Ferrell

Comments

Dan Ferrell (author) on June 10, 2020:

Not unless they are failing. If they work fine, just replace the thermostat.

Sarah Crescioni on June 10, 2020:

I need to know if both fans need to be changed when replacing a new thermostat on a 2007 Macima?

Dan Ferrell (author) on April 25, 2020:

There'll be hot coolant when it returns from the engine. If the engine is overheating, there's a problem in the cooling system. Monitor the temperature gauge. If there's an indication of overheating, you need to find out the cause. This other post may help:

https://axleaddict.com/auto-repair/What-Causes-a-C...

Thoks on April 24, 2020:

Is it normal for coolant reservior to get hot?

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