So my car is always low on coolant so my last two oil changes I measured it. For 5000 miles it's about a cup. I can't see any leaks, and passenger side foot well is dry. Just did my timing belts and didn't see anything on the water pump. 

Maybe a pinhole in the rad?

Perspiration in its socks?

Sort of why we need to check and top up, I know, it must be going somewhere ! We have a 16 year old that has never been changed or topped up since factory...noticed other day the level has dropped and one of the hardest to access hose has green whiskers ....so , not gonna make 17 years.

Your case is good contender for a chunk of Sealwel

Edited October 17, 20205 yr by Step-a-toe

Intake manifold gaskets.
One of the 7 cooling system hoses.

Radiator in poor condition.

Check it at least once per day.  Even then, some of these can fail worse suddenly, leaving you low on coolant and over normal temperature.  If that happens, sooner or later, head gaskets.

Check things until you find it and fix it.

 

Not that previous owners of any old cars I have bought would know, a radiator cap is a service item. Easily and cheaply replaced. Can do wonders

you could try the Subaru Coolant Conditioner (rebranded Holt's Radweld) - 1 bottle only, installed per directions ONLY.

On 10/18/2020 at 1:03 AM, DaveT said:

Intake manifold gaskets.
One of the 7 cooling system hoses.

Radiator in poor condition.

Check it at least once per day.  Even then, some of these can fail worse suddenly, leaving you low on coolant and over normal temperature.  If that happens, sooner or later, head gaskets.

Check things until you find it and fix it.

 

I've replaced all the hoses and I rebuilt the intake manifold last year after an injector o-ring died. Possible I messed up the gasket, would explain why I can't find the leak. It's such a slow leak I don't see any smoke.

The radiator looks good, I can't see any coolant residue on it. 

I'm just worried it's the heater core, so I keep looking for something in the engine bay to blame it on lol.

Yes, it takes a pretty good leak to get visible smoke from the intake gasket leak.   The gaskets I use are the Subaru made ones, they are not cardboard.   I've had those cardboard aftwrmarket ones fail at least twice in me.  And they were  Fel Pro brand.