Whenever I fill up my overflow tank past the min line when cold, it doesn’t seem to go back down to the same point I filled it up at after taking the car for a spirited drive and allowing it to cool down overnight. To be honest the overflow doesn’t seem to move that much when the car is fully heated up and been on boost, if anything it draws the coolant back into the engine, leaving the overflow level nearly halfway between the bottom of the tank and the min line. Does anyone know why this would be ? Radiator is always full and the header is more than adequately full. It’s a 2008 wrx hatch and I do have an aftermarket rad on also.

 

In short. Are you continually topping it up or not? Yes; Problem. No; No problem.

Long version:
If you don't top it up, does it continue to keep on dropping or does settle and stay there (albeit below the min on the reservoir)?

If you don't top it it off and it doesn't continue to drop it could just be the cap venting pressure a bit prematurely.

If it continues to drop you may have a small leak somewhere (or dare I say gasket). If you have pink coolant, you'll usually get crusty pinkish/white stuff building up by the leak...if you are using a cheap pattern radiator from a brand you've never heard of; look for deposits around where the plastic ends meet the aluminium core; They have a habit of intermittently weeping when heat cycled.

Aftermarket water pumps can also intermittently weap a little, sometimes they also draw in a bit of air under certain conditions causing random coolant level fluctuations...when this happens when using pink/red antifreeze (OAT) you may find the coolant becomes discoloured and deposits start forming inside the reservoir.

If its a head gasket issue, albeit very slight; the only way to properly confirm is by testing for combustion gases in the coolant.

 

it seems to wanna settle after cool down at the same point everytime when I leave it and look the next day.

the water pump (rcm one) is only a year old so shouldn’t be that surely, unless some of the original hoses have turned bad

 

Have you got the oem radiator caps on the aftermarket radiator

 

No mate. The rad has only one cap and I used the one they provided

 

I’m sure this has a thread on here. That you should use the original Subaru caps or can cause issues.

 

Ok mate! I might give that a go then

 

Rad cap.
Subaru Impreza Turbo 02-15 Radiator Cap 1.37 BAR (importcarparts.co.uk)
Header tank cap.
Subaru Impreza Turbo Genuine Subaru Header Tank Cap (importcarparts.co.uk)
Rad cap release pressure 1.37 kpa.
Header tank release pressure 1.1