My overflow tank will be a tiny bit low every week or so. It's only a few ounces, but it's still going down. The level in the radiator itself stays 100% full, it's just the tank. I can sometimes smell coolant under the hood, but never inside the car. I performed a block test and all cylinders are 2-5%, so it's not a head gasket. Where could it be going, and what should I check? 08 si.

 

If you run the car hot a lot, then you get evaporation of the water content in the coolant. The water vapor will be lost through the evap( emissions ) system. You shouldn't be losing that much though. Check for leaking around your intake gasket, there is a water jacket in the intake mani for winter pre-heating. Might be a seeping leak.

 

You need to be more specific as to what model you have to get more accurate information.