Coolant flush: step-by-step guide

Confirm the specification in your owner’s manual before buying. Product examples are not a fitment guarantee.

When you need a coolant flush

• It's due per your maintenance schedule
• You're switching coolant technology
• The coolant is discolored, rusty or murky
• Incompatible coolants have been mixed
• After certain repairs (radiator, head gasket); follow the shop's advice

Drain-and-fill vs flush vs machine exchange

What you need

• Correct coolant for your spec, plus distilled water
• A drain pan and sealable containers
• Gloves and eye protection
• A funnel (spill-free funnels help with bleeding)
• Basic tools for the drain plug or hose clamp
• Optional: a cooling-system cleaner labelled compatible with your system

How to flush coolant step by step

1. Engine fully cold. Park level and support the vehicle safely if you need to go underneath.
2. Remove the reservoir or radiator cap slowly.
3. Open the radiator drain (petcock) or remove the lower hose, and drain into a pan.
4. Close the drain. Fill with distilled water (and a cleaner if you're using one, per its label).
5. Run the engine to operating temperature with the heater on hot, then let it cool fully.
6. Drain again. Repeat the water rinse until it drains clear.
7. Refill with the correct coolant at the correct ratio (premix, or concentrate plus distilled water).
8. Bleed the air. See how to bleed coolant.
9. Run the engine, check for leaks, let it cool and top up. Recheck over the next few drives.

Account for leftover water

After rinsing, some water stays in the engine block. If you refill with concentrate, that water dilutes it. Check the final mix with a refractometer, and adjust or use the calculator below.

Disposing of old coolant

Never pour coolant down drains or onto the ground. Take it to a recycling or household hazardous waste site, or a shop that accepts it. See coolant disposal.

Flushing when switching coolant types

If you're moving from one coolant technology to another, for example from an older conventional coolant to the long-life type your manual now allows, a single drain-and-fill isn't enough. A significant share of old coolant stays in the engine block and heater core. Do several water rinse cycles until the drain runs clear, or use a machine exchange, so the new coolant isn't diluted with the old inhibitor package. Check the final concentration with a refractometer.

Using a cooling system cleaner

Chemical cleaners can help loosen light scale or residue, but they're not a cure for heavy contamination. If you use one, pick a product labelled compatible with your engine materials (especially aluminium), follow its run time exactly, and rinse thoroughly. Cleaner left in the system can attack seals or neutralize the new coolant's inhibitors. If the coolant contains oil, find the source before cleaning. See the oil in coolant guide.

After-flush checklist

• The level is correct when cold, after two or three drive cycles.
• The heater blows hot (no air pockets).
• No leaks at the drain plug, hoses or clamps.
• The temperature gauge reads normally.
• The freeze point is tested and on target.
• Old coolant is stored for recycling.

How long a DIY flush takes

Most of the time is cool-down. Each rinse cycle requires warming the engine so the thermostat opens, then letting it cool before draining. Plan for several hours, or spread it over a day.

Flushing an older car vs a modern one

Older iron-block engines with brass radiators can shed rust and scale during a flush, so expect discolored rinse water and repeat until it's clear. Modern aluminium engines usually rinse clean faster, but heater cores and electric pumps can trap air, so allow time for bleeding.

Short checklist

• Cold engine
• Correct coolant
• Drain pan
• Disposal plan