How to Change the Coolant Without Causing the Engine to Overheat

Updated on January 21, 2019
Dan Ferrell profile image

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

Failing to bleed air from the cooling system can lead to engine overheat.
Failing to bleed air from the cooling system can lead to engine overheat. | Source

You can change the coolant in your own garage without leaving behind air pockets—a common problem in many modern vehicles that leads to the engine overheating.

As the protective qualities of antifreeze degrade over time, acid content increases and rust spreads out. Soon after, corrosion begins to destroy your water pump, radiator, and surfaces throughout the engine.

That's why car manufacturers suggest a two- or four-year service interval or more, depending on your particular vehicle make and model and the type of antifreeze your vehicle needs.

So the sooner you change that old coolant in your car, the better. You'll only need a few common tools.

And you can work on this car maintenance project on a Saturday morning with the help of this guide and without fear that your engine will overheat afterward.

Just follow these steps.

NOTE: The following procedures don't apply to hybrid vehicles.

I. How to Drain Coolant
II. Refilling the Radiator With Coolant
III. Extending the Service Life of Your Car's Cooling System
Replace the coolant on time to avoid sediment and corrosion in the cooling system.
Replace the coolant on time to avoid sediment and corrosion in the cooling system. | Source

I. How to Drain Coolant

Before you start, find a well-ventilated area with a level surface—if you need to raise your car—and with enough room around your car to comfortably change the coolant.

  1. Wait for the engine to cool before removing the radiator cap--or the reservoir cap if there's no radiator cap. This will prevent hot coolant from gushing out and burning your skin.

  2. Find the petcock towards the bottom of one of the radiator tanks. On some vehicles, you need to raise the front of the car to gain access to the petcock from underneath. If this is so, use a floor jack to raise the front of your car and secure it with a jack stand. Also, chock the rear wheels and set the parking brake.

  3. Place a drain pan under the car toward the radiator drain valve.

  4. Loosen the petcock with your hand or a pair of pliers or a wrench.

    Some radiator drain valves have so little room around them you can hardly reach them with your hand or tool; others tend to rust over time, so you don't want to remove them for fear of causing some damage. If the drain valve on your car is hard to reach or is rusted, try removing the lower radiator hose instead to prevent an expensive repair. To do this:

    • Loosen the clamp holding the hose at either end, whichever is more accessible to you—depending on the type of clamp, you'll need a screwdriver or a pair of pliers for this.

    • Then, after removing the clamp, break the hose grip at the fitting using a pair of slip-joint pliers by carefully twisting the hose just a few degrees.

    • Once you break the grip, manually twist the hose back and forth as you pull it off its fitting. Let the old coolant drain into the drain pan.

Drain the radiator coolant completely.
Drain the radiator coolant completely. | Source

5. Now, remove the coolant reservoir, empty the coolant into the drain pan. Flush the reservoir and reinstall it.

6. Loosen the radiator cap and wait for the coolant to drain completely. Draining the radiator and overflow reservoir usually removes about 50% to 70% of the coolant in the system.

7. Find the block drain plug or plugs. They're located on the side of the engine. Consult your car owner's manual or your vehicle service manual.

On some engines, it's almost impossible to remove the drain plugs without some special tools or equipment because of their location or because they are stuck to the block. Still, other engines lack a drain plug. For these cases, you can follow the next alternative method.

  • After adding fresh coolant to the radiator and reservoir (see step 9), use your car for a week.
  • Then, drain the radiator and reservoir again and add more fresh coolant to the system. This will allow the new coolant to mix with the old coolant. Changing the radiator and reservoir coolant within a few days of difference will help replace most of the fluid in the system.
  • Follow the procedure to purge air from the system as well. See section II. Refilling the Cooling System.

8. After removing the drain plugs, wait for the coolant to drain completely. Before replacing the plugs, apply sealer to the plug's threads.

9. After draining the coolant completely, flush the radiator using a garden hose to remove sediments. Run water through the radiator until water flows clear through the radiator drain valve. If you think you need to remove rust and scale from inside your radiator, get a radiator cleaner product from the auto parts store and follow the instructions from the manufacturer.

10. After flushing the radiator, tighten the petcock or reinstall the lower radiator hose.

II. Refilling the Radiator With Coolant

With the coolant removed and the radiator and reservoir cleaned, it's time to refill the cooling system. Before you start, though, check and see if your system has a purge valve (aka bleeder screw). This is a small, single screw, usually located on the thermostat housing, which connects to the upper radiator hose on the engine side. Check your car owner's manual or vehicle repair manual.

If you don't have the vehicle repair manual, you can get an inexpensive copy for your particular vehicle make and model through Amazon. This Haynes manual, for example, comes with procedures for many maintenance, repair, and troubleshooting tasks you can do at home. So the manual pays for itself pretty soon.

Cooling System Bleed Methods
Bleed screw
Depends on car model
Jack stand
Works on most models
Funnel method
Works on most models
No bleeding
Many pre-1995 models don't need bleeding

These are three common air purging methods you can use in your vehicle:

Purge Valve Method

Before you start, check and see if your system has a purge valve (aka bleeder screw). This is a small, single screw, usually located on the thermostat housing, which connects to the upper radiator hose on the engine side. Check your car owner's manual or vehicle repair manual.

The purge valve will expel any air that gets into the system after adding the new coolant. This will prevent the formation of air bubbles or hot pockets inside the engine that cause overheating.

Jack Stand Method

Another way to purge air from the system is to use what I call the jack stand method. All you have to do is raise the front of your car to position the radiator neck (fill point) at a higher level than the heater core — this is the small, radiator-like component (usually located underneath the dashboard on the passenger side) that provides warm air to the vehicle interior. This will prevent the coolant from trapping air bubbles. If you do this, secure the front of the vehicle with a couple of jack stands, block the rear wheels with wooden blocks, and engage the parking brake.

Special Funnel Method

Also, a highly recommended method is the use of a special funnel that makes it easier to fill up the radiator or cooling system without introducing air into the system. Check the Amazon ad below and the video so that you have an idea of how it works. Follow the instructions that come with the funnel, if you decide to get one.

Use the most convenient method for you, but make sure to purge the air from the system; otherwise, air bubbles will prevent proper coolant flow, cause engine overheat, and, possibly, severe engine damage.

Spill-free Funnel

Lisle 24680 Spill-Free Funnel
Lisle 24680 Spill-Free Funnel
* 45-degree elbow allows use on radiator caps that are placed at an angle. * The E-adapter is used on the surge tanks on many Ford applications. * The spill-free funnel eliminates trapped air pockets which usually cause erratic cooling system and heater performance. * Extension allows access to caps located under shrouds or in fender wells.

Once you are ready to fill the system with coolant, follow these steps:

  1. Set the heater temperature control to the maximum.

  2. Slowly add a 50/50 mixture of distilled water and antifreeze to the radiator until it reaches the bottom of the radiator neck.

    NOTE: If you live in an area with extreme climate conditions, you'll need to increase the amount of antifreeze to the mix. Also, some car manufacturers recommend using a premixed coolant to add to the system, while others recommend a specific type of antifreeze that you need to mix with distilled water before adding it to the cooling system. Check your car owner's manual or vehicle repair manual for the right premixed-coolant or antifreeze you need for your specific car model.

  3. Fill the reservoir with fresh coolant up to the Low mark.

  4. Install the radiator cap to the first lock position. Start the engine.

  5. Let the engine idle--for about 15 or 20 minutes--until it reaches operating temperature. At this point, the thermostat will open; air will escape through the radiator cap.

    If you're purging the air through a bleed screw, follow these steps:

    • Make sure the engine is warm, and the heater is set to Max.

    • Without starting the engine, connect a clear hose to the screw and place the other end of the hose into the catch pan.

    • Crack the screw open using a wrench and start refilling the system with coolant.

    • Let coolant flow through the valve until you see coolant flowing free of air bubbles. Then tighten the screw.

    NOTE: On some models, the valve bleeding procedure will differ from the one described here. Check your car owner's manual or vehicle repair manual for specific instructions for your car make and model.

  6. Turn off the engine and let it cool.

  7. Top off the coolant in the radiator to the correct level--usually about an inch below the bottom of the radiator neck. Then, squeeze the upper radiator hose to expel the air and add more coolant if necessary.

  8. Replace the radiator cap. Make sure it's fully locked.

  9. Add more coolant to the reservoir up to the Low mark, if necessary.

  10. Replace the radiator cap.

  11. Start the engine again and let it reach operating temperature and check for leaks.

Refill the radiator and reservoir with fresh coolant.
Refill the radiator and reservoir with fresh coolant. | Source

III. Extending the Service Life of Your Car's Cooling System

When you need to change the coolant in your car, don't forget the most critical part, bleeding the cooling system, especially on car models with low-profile radiators. Trapped air will seriously disrupt the system and may cause costly damage.

By replacing the coolant at the manufacturer's recommended service interval, you are removing dirt and rust particles that eventually corrode, block radiator and heater small passages, and cause serious system damage. So follow this guide every time you need to service the cooling system.

What Method Do You Use to Bleed Your Car Cooling System?

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2016 Dan Ferrell


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      15 months ago

      There is another method I use to bleed my system. And that is by disconnecting a small hose that connects to the engine block in a relatively high spot. Sometimes, disconnecting the upper radiator hose from the engine and holding it partially on with an small gap works as well. When coolant starts to come from this gap as you pour it in, the system is full. Regardless, some experimentation may be needed to find the best place to allow air to escape from.

      Whichever way you choose to bleed your cooling system, ALWAYS know both EXACTLY how much coolant it holds and how much you have filled it. This way, you will know how well you have bled the system. It is impossible to PERFECTLY bleed your cooling system. But you won’t overheat if you get close to filling it completely. For instance, 1.25 gallons of coolant in a 2 gallon system is a recipe for disaster. But 1.85-1.9 gallons will likely be fine, and will probably bleed itself without incident.


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