Drain the old coolant from the radiator

Start with the engine cold — never open the radiator cap on a hot engine, as pressurized coolant can spray out and cause burns. Let the car sit for at least three hours after the last drive, or work on it first thing in the morning.

Locate the radiator drain plug at the bottom of the radiator. It looks like a small valve or bolt, usually plastic or brass. Place a drain pan underneath it — you will need at least a 5-gallon capacity because most cars hold 3 to 4 gallons of coolant. Open the drain plug by turning it counterclockwise by hand or with a wrench, depending on the type. Let the coolant drain completely into the pan. This takes 5 to 10 minutes.

Once the radiator is empty, close the drain plug by turning it clockwise until snug. Do not over-tighten, or you may crack the plastic radiator. Some cars also have a drain plug on the engine block itself — check your owner's manual. If yours does, drain that one the same way into the same pan.

Key Takeaways

  • Always drain coolant from a cold engine to avoid burns and pressure release.
  • You will need to locate and open the radiator drain plug and possibly an engine block drain plug, letting old coolant flow into a drain pan.
  • Flush the cooling system with distilled water to remove sediment and old coolant residue before refilling.
  • Fill the radiator and overflow tank with the correct coolant type for your vehicle, then run the engine to bleed air from the system.
  • Check the coolant level after the engine cools and top off if needed, as the level may drop once air pockets escape.

Flush the cooling system with water

Flushing removes old coolant residue and sediment that can clog passages and reduce cooling efficiency. Close the drain plug, then pour distilled water into the radiator fill opening at the top until it reaches the filler neck. Do not use tap water — minerals in it can leave deposits inside the engine.

Replace the radiator cap and start the engine. Let it run at idle for 5 to 10 minutes while the water circulates through the system. Turn off the engine and let it cool for 15 minutes, then drain the water the same way you drained the coolant. Repeat this flush two more times — three total flushes is standard. On the final flush, the water should drain nearly clear instead of brown or cloudy.

After the third flush, close the drain plug again and leave the radiator empty and ready for new coolant.

Locate the overflow tank and inspect the hoses

The overflow tank (also called the coolant reservoir or expansion tank) is a translucent plastic container near the radiator, usually white or translucent. It has a cap and a hose running to the radiator. Before refilling, check all visible coolant hoses for cracks, splits, or soft spots. Squeeze them gently — they should feel firm, not mushy or brittle. If you find damage, the hose needs replacement before you refill.

Look at the hose clamps where hoses connect to the radiator and engine. If they are loose, tighten them with a screwdriver or wrench. A loose clamp can cause a slow leak that drains coolant over weeks. This inspection takes only a few minutes and can prevent problems later.

Fill the radiator with new coolant

Check your owner's manual or the coolant cap itself to find the correct coolant type. Common types include green (older vehicles), orange (Dex-Cool, used by General Motors), pink (Nissan and some Hyundais), and blue (some Fords and Chryslers). Using the wrong type can cause corrosion or poor heat transfer. Many auto parts stores will tell you the correct type if you provide your vehicle's year, make, and model.

Most coolant comes concentrated and must be mixed with distilled water at a 50/50 ratio. Some comes pre-mixed and ready to pour. Read the label. If mixing, pour the coolant and water into a clean bucket and stir well before pouring into the radiator.

Remove the radiator cap and pour the coolant mixture slowly into the radiator opening until it reaches the filler neck — the point where the opening narrows. Do not overfill. Replace the radiator cap. Then open the overflow tank cap and fill it to the "full cold" line marked on the side. Replace that cap as well.

Run the engine and bleed air from the system

Air pockets trapped in the cooling system prevent coolant from circulating properly and can cause overheating. Starting the engine forces coolant through the system and pushes air out through the overflow tank. Start the engine and let it idle for 10 to 15 minutes. You may see coolant bubbling out of the overflow tank — this is normal and means air is escaping.

Watch the temperature gauge. It should climb slowly toward the middle of the gauge. If it climbs too fast or the engine begins to overheat, turn off the engine when ready and let it cool. This may mean air is still trapped. Once the engine reaches normal operating temperature, turn it off and let it cool for at least 30 minutes.

Check the level and top off if needed

After the engine has cooled completely, open the radiator cap and look at the coolant level. It may have dropped slightly as air pockets escaped and coolant settled. If it is below the filler neck, add more coolant mixture until it reaches the neck. Check the overflow tank as well — it should be between the "full cold" and "full hot" lines. Top it off if needed.

Replace both caps. Start the engine one more time and let it run for 5 minutes, then turn it off and cool again. Check the level a final time. Once it holds steady, the job is complete.

Dispose of old coolant safely

Never pour old coolant down the drain or into the ground. It is toxic and harms the environment. Most auto parts stores accept used coolant for free, or will take it when you buy new coolant. Some recycling centers also accept it. Call ahead to confirm they take coolant before you bring it in.

Keep the old coolant in a sealed container labeled "used coolant" until you can drop it off. Store it away from children and pets, as it is poisonous if swallowed. Proper disposal takes only a few minutes and protects your local water supply.

Common mistakes to avoid

Opening the radiator cap on a hot engine is the most dangerous mistake — the pressurized coolant can spray out and cause severe burns. Always wait until the engine is completely cold. Another common error is mixing coolant types. Mixing green and orange coolant, for example, can create a sludge that clogs the system. Stick to one type unless your manual says otherwise.

Skipping the flush step leaves old sediment in the system, which reduces cooling performance over time. Do not skip it even if you are in a hurry. Finally, failing to bleed air means the system will not circulate coolant properly, and you may overheat on a long drive. Take the time to run the engine and let air escape.

Frequently Asked Questions

How often should I replace my coolant?

Most vehicles need a coolant change every 30,000 to 50,000 miles or every three to five years, whichever comes first. Some newer cars with long-life coolant can go 100,000 miles or 10 years. Check your owner's manual for your vehicle's specific interval.

What if coolant leaks out after I refill it?

A small amount of coolant in the overflow tank after the engine cools is normal. If coolant is pooling under the car or dripping from the radiator, you have a leak. Common sources are loose hose clamps, cracked hoses, a faulty radiator cap, or a leak in the radiator itself. Tighten any loose clamps first.

Can I use water instead of coolant in an emergency?

Distilled water works temporarily in an emergency to get you to a repair shop, but it does not protect against freezing in winter or boiling in summer the way coolant does. Water also lacks corrosion inhibitors that protect the engine. Use water only as a short-term fix.

Why is my coolant level dropping between changes?

A slow leak is the most common cause. Check under the car for drips and inspect all hoses and clamps. A faulty radiator cap can also allow coolant to escape. If the level drops without visible leaks, the head gasket may be failing — this requires a mechanic's diagnosis.

Do I need to bleed the system if my car has a bleeder valve?

Some vehicles have a small bleeder screw on the radiator or engine block designed to release air. If yours has one, open it slightly while filling the radiator until coolant flows out without bubbles, then close it. Check your owner's manual to see if your vehicle has one.