Drain the old coolant from the radiator

Start with the engine cold — never open the radiator cap on a hot engine, as the coolant is under pressure and can spray out and burn you. Locate the drain plug at the bottom of the radiator (consult your owner's manual for the exact location, as it varies by make and model). Place a drain pan underneath it, then open the plug by turning it counterclockwise by hand or with a wrench, depending on the design.

Let the coolant drain completely into the pan. This takes 10 to 15 minutes. While it drains, locate the lower radiator hose — this is the hose that returns coolant from the engine to the radiator. You may also need to open a drain plug on the engine block itself if your vehicle has one; check your manual. Some vehicles require you to loosen the lower hose clamp and pull the hose away from the radiator to let coolant drain from the engine side.

Once the flow stops, close the radiator drain plug by turning it clockwise until snug — do not overtighten, as you can strip the threads. If you removed the lower hose, reconnect it and tighten the clamp securely.

Key Takeaways

  • Always work on a cold engine and use a drain pan to catch old coolant, which is toxic and must be disposed of at a recycling center or auto parts store, not poured down the drain.
  • Your owner's manual tells you the correct coolant type and the total capacity your radiator holds, both of which vary by vehicle.
  • Refill the radiator with the engine off, then run the engine to circulate the new coolant and bleed out air pockets before checking the level again.
  • The entire job takes 30 minutes to an hour and requires only basic tools: a wrench, a drain pan, a funnel, and a coolant flush kit if you want to rinse the system first.

Flush the cooling system if the old coolant is discolored

If the drained coolant is brown, rusty, or cloudy instead of its original color (usually bright green, orange, pink, or blue depending on the type), the system has accumulated sediment and corrosion. Flushing removes this buildup and extends the life of the new coolant.

To flush, close the radiator drain plug and refill the radiator with distilled water only — not coolant. Start the engine and let it run for 10 minutes with the heater on full blast. This circulates the water through the system and loosens debris. Turn off the engine, let it cool for 20 minutes, then drain the water the same way you drained the coolant. Repeat this process two or three times until the water draining out runs clear instead of discolored.

After the final flush, close the drain plug and proceed to refilling with fresh coolant. Flushing adds 20 to 30 minutes to the job but is worth doing if the old coolant was visibly dirty.

Refill with the correct coolant type and concentration

Open your owner's manual and find the coolant type your vehicle requires — this is critical, as mixing incompatible types can cause corrosion or clogging. Common types include IAT (Inorganic Additive Technology), usually green; OAT (Organic Acid Technology), usually orange or red; and HOAT (Hybrid Organic Acid Technology), usually yellow or pink. Your manual also lists the total capacity in quarts or liters.

Most coolant is sold as a concentrate and must be mixed with distilled water at a 50/50 ratio to achieve the correct freezing and boiling points. Some brands sell pre-mixed coolant ready to pour straight in. Read the label on the bottle to confirm which you have. If you are mixing concentrate, use a separate container and a measuring cup to combine equal parts coolant and distilled water before pouring into the radiator.

Place a funnel in the radiator filler neck and pour the coolant mixture slowly until the level reaches the "full" mark on the radiator or the overflow tank (whichever your vehicle uses). Do not overfill — excess coolant will spill out when the engine warms up and expands.

Run the engine and bleed air from the system

Close the radiator cap and start the engine. Let it idle for 5 to 10 minutes while the cooling fan cycles on and off. This circulates the new coolant through the engine, water pump, and heater core, and allows air pockets to rise to the top of the radiator where they can escape.

You may hear gurgling sounds or see the coolant level drop slightly as air is pushed out — this is normal. Some vehicles have a small bleed screw or valve on the thermostat housing or upper radiator hose; if yours does, open it slightly while the engine idles to let air escape, then close it once coolant flows out steadily.

Turn off the engine and let it cool for at least 30 minutes. Once cool, open the radiator cap and check the level again. Top it off if needed. Reinstall the cap and you are finished.

Dispose of old coolant safely

Old coolant is toxic and must never be poured down the drain, onto the ground, or into storm drains. Take it to a local recycling center, auto parts store, or hazardous waste facility. Many auto parts retailers accept used coolant for free or a small fee. Call ahead to confirm they accept it and ask whether they want it in a sealed container or if they provide one.

If you flushed the system with water, that water is not toxic but still should not go down a household drain if it contains rust or sediment. Pour it into a storm drain (the kind in a parking lot or street) or let it evaporate in a safe outdoor location.

When to change coolant and what to expect

Most vehicles need a coolant change every 30,000 to 50,000 miles or every three to five years, whichever comes first. Some newer vehicles with extended-life coolant may go longer — check your owner's manual for the interval specific to your car. If the coolant is discolored, smells burnt, or your temperature gauge runs high, change it sooner.

The job is straightforward enough for a home mechanic with basic tools, but if you are uncomfortable working under the hood or your vehicle has a complex cooling system layout, a shop can do it in under an hour for a labor charge of $75 to $150 plus the cost of coolant. Doing it yourself costs only the price of the coolant itself, usually $15 to $30 per gallon depending on the type.

Frequently Asked Questions

Can I mix different coolant colors?

No. Different colors represent different chemical formulations, and mixing them can cause the additives to react and form sludge that clogs the radiator and water passages. Always use the exact type your manual specifies. If you are unsure what is currently in your system, drain it completely and flush with distilled water before refilling with the correct type.

What if I overfill the radiator?

Excess coolant will spill out when the engine warms up and the coolant expands. This is messy but not dangerous. Let the engine cool, then open the cap and remove the excess with a turkey baster or syringe until the level is correct. Do not leave the cap off while the engine is hot.

Do I need to bleed air if my vehicle has a sealed cooling system?

Most modern cars have sealed systems with an overflow tank that automatically draws coolant back in as it cools. These still benefit from running the engine after a refill to circulate the coolant and push air out. If your vehicle has a bleed screw on the thermostat housing, use it. Otherwise, simply running the engine for 10 minutes is sufficient.

How do I know if there is still air in the system?

Air pockets cause the temperature gauge to spike, the heater to blow cold air, or the cooling fan to run constantly. If you experience any of these after a coolant change, let the engine cool, open the radiator cap, and top off the coolant level. Run the engine again for another 10 minutes. Repeat until the symptoms stop.