A flush is not just draining and refilling. Its purpose is to remove corrosion product, scale and any incompatible residue from surfaces the drain cannot reach — the radiator core, the heater matrix and the block's cooling channels. Done properly it restores heat transfer and gives the new coolant a clean start.
What you need
- Correct premixed coolant for the vehicle, plus roughly 20 % extra to account for air pockets
- Cooling system flush chemical or cleaner
- Deionised or distilled water for the rinse phase
- Collection pan, funnel, gloves and eye protection
- New thermostat, radiator cap and hose clamps if they are due
The procedure
- 1Cool down and depressurise. Let the engine stand until cold, then carefully release the radiator cap to the first stop to bleed residual pressure.
- 2Drain the system. Open the drain cock or remove the lower hose. Open the heater to full hot so the matrix drains too.
- 3Clean with flushing chemical. Refill with water plus the recommended dose of flush cleaner, run to operating temperature with the heater on hot for the time stated on the product, then drain again.
- 4Reverse flush if the heater or radiator is still restricted. Use a low-pressure hose or flush gun against the normal flow direction — heater matrix first, then radiator, then block.
- 5Rinse with deionised water until the water leaving the system runs clear and neutral. This may take three or four fills. Do not skip this step; leftover cleaner attacks the new coolant's inhibitors.
- 6Replace serviceable items now. Fit the new thermostat and gasket, radiator cap and any brittle hoses while the system is empty.
- 7Refill with premixed coolant. Fill to the cold mark, then run the engine with the heater on full hot and the cap off until the thermostat opens and the level stabilises, topping up as air escapes.
- 8Purge remaining air. Squeeze the upper hose with the engine running to help move trapped air, then fit and tighten the cap and let the system pull vacuum as it cools.
- 9Verify. Take a short drive, let the engine cool completely, then recheck the level and top up. Repeat once more after the next heat cycle.
How much coolant will you need?
| Vehicle type | Typical system volume | Approximate premixed coolant |
|---|---|---|
| Small passenger car | 4 – 6 L | 5 – 7 L including top-up |
| Mid-size car / SUV | 6 – 9 L | 7 – 10 L |
| Light commercial van | 8 – 12 L | 9 – 13 L |
| Heavy truck | 25 – 45 L | Depends on engine; allow 10 % over |
| Generator set (standby) | 10 – 30 L | Depends on radiator and block volume |
Common mistakes
- Topping up with tap water and leaving the ratio too weak. Hard water adds scale and the reduced inhibitor dose shortens the corrosion protection window.
- Leaving the heater valve closed during the flush, so the matrix keeps its old contaminated fluid.
- Filling with concentrate and topping up with water afterwards, which leaves an unknown ratio. Premix in a clean container first and pour that.
- Forgetting the air purge, which causes hot spots, a cold heater and repeated coolant loss as air expands out.
See our coolant range
Borate–organic hybrid engine coolant, premixed and ready to use, 0 °C to −45 °C.
Related topics
cooling system flushhow to flush a radiatorradiator flush procedureheater matrix flushcoolant change steps