Hi I keep topping up the coolant in my overflow reservoir but every week or so when I check the coolant reservoir is down very low (but not completely empty).

No matter if I check the coolant overflow reservoir when the car is hot or cold it will be almost empty.

I've checked for small leaks in the cooling system and I can't find any. Car runs 100% fine, with no overheating. No water in oil to suggest it's a blown headgasket. No leaks in the coolant overflow resevoir either.

Any ideas?

 

You don't have to have a mix of oil/water or water/oil to have a bad head gasket. It could be lifting just enough to pressurize the system and cause the coolant loss.