Coolant - how do I flush the system?

Hi people,

I've seen the post about using OAT coolant in a C3 but we have two C3s in the family & neither have red coolant:-

C3 1.4HDI 2009 has a blue/green coolant
C3 1.4 16V 2004 has a blue/green coolant

The 2004 car has coolant I need to change. I assume I should go back to OAT coolant but how do I flush the system? Do I just leave a hosepipe in the header tank for a while with the bottom hose off?

I have to admit I have been topping up the 2004 car a couple of times since having it but have used standard blue antifreeze......

Hi , to flush is to run clean water through the system fully removing any remains of the other coolant.

Hook up a hose to the mains water and the other end to various coolant hoses while some others are disconnected and keep running water until it runs clear.

If the 2 antifreeze colours mix they may affect each other so its needs to all be flushed out.

There is the heater, the block and head, the radiator and the pipes connecting these that need flushing

If you think there maybe some sludge in the radiator or heater matrix due to coolants mixing or corrosion then it will be helpful to add a flushing aid during the flush. Try this Holts Speedflush additive to get the most out of the flush.

drain water flush then fill with holts speed flush and top up with water run for 10 to 15 minutes flush again then fill with anti freeze and water job done !

If you can remove the thermostat, so much the better as this can impede the flushing.


De-ionised water (here) to dilute the concentrate OAT and mix with the flush
3 Litres of G30 Concentrate (here) to make up 6L at a 50% mix for -35c.

The cooling system on the engines take between 5.7 and 7 L, so you may need a bit more.


Some refilling tips can be found in C3 coolant tips section (here).