You may want to check your oil, I had a six cylinder chevy pickup that had a blown head gasket, no leaks on the driveway or road, coolant was leaking into the oil pan. Hope that is not the case for your car. Good luck .

 

How much are you adding and how often vs runtime and/or miles driven? Have you removed the radiator cap/block fill cap and looked for oil residue?

Have you checked under the water pump and the transmission right side? Does the windshield fog when air begins to circulate? i.e. check the carpets at all four corners for moisture.

Those are the things that come to mind!

 

 

You look under the car or is a MT?

 

 

I would try this. Maybe it just leaks very slowly under pressure and evaporates before you notice it.

 

 

More likely you got slow lake from head gasket. Lost of IS350 (If in your case) has that problem on our shop. The more you drive your engine hard, the more fluid you will lost

 

 

MT - manual trans. Autos have water to oil heat exchanger using engine coolant to cool the trans on the right side under the car.

Still, get it up on a lift and inspect it from the bottom.

 

 

I'd pressure test cooling system. If the coolant is leaking due to head gasket, taste the exhaust condensation it'll have trace of sweet taste.
How about engine temperature is it staying normal?

 

Did you check the engine bay ? Waterpumps are known issues on these cars and seep coolant when going bad. Ask me how I know, I just changed my water pump 2 weeks ago. Look for pick residue in the engine bay.

 

This implies the block IS NOT full of coolant. Have you ever taken the cap off and looked at the coolant level when cold? It should be full to the rim but it's not because it has a leak.

For the coolant system to keep the block & radiator purged of air there can be zero leaks. It is ONLY WHEN the engine cools down that block pulls a vacuum from the reservoir and fills it. When you have the tiniest leak, it pulls air and the block can run low on coolant.

Pressure test the cooling system and cap. Find the leak.
And thanks goes a long ways too. Just sayin... ...