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Disappearing coolant

I am dumbfounded. The reservoir falls below the min line so I fill it to full. A day or 2 later and it’s back to the low line. It’s a 2014 Impreza hatchback sport with 75k on it.

no visual leaks or puddles

No visual puddle in the engine chamber

Pressure tested radiator… passed

Pressure tested radiator cap… passed

No Smokey tail pipe exhaust

No milky oil

No overheating alert

No sweet smell

When the car is cold, check the level in the radiator, not the coolant recovery system. Mare sure the radiator is full before you refill the coolant system. The coolant recovery system will not draw coolant back into the radiator if any leak is letting air into the system. this includes any hose to the coolant recovery tank and ant internal passages from the hose to the bottom of the tank. Sometimes thing only leak when at full temp and on the highway, like water pumps and head gaskets.

Because it is a Subaru with 75000 miles, the head gasket is always a suspect.

Yes, what @oldtimer_11 said… He is correct.

Now if the situation is like this… You fill the rad, and the overflow…and the overflow drops a bit…thats fine…as long as it doesn’t keep dropping.

What you may be witnessing is the air being purged from the rad and being replaced with the coolant that was in the reservoir. But this should stop happening after a while…let it drink the coolant it wants to…refill the res and do NOT open the rad cap…drive and test…hopefully you will see it drop again just a little and then it should stop dropping. That will be the air being burped out of the system and is normal…up to a point. It should not continually be emptying the rez…

If the rez does not stop dropping…then we will be discussing head gasket failure.

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Just the facts.
You saw res was low. You added coolant. Once.
Coolant level Dropped again
You did not say if you added coolant a 2nd time

Yes sorry. I’ve topped off the reservoir 3 times.

How much coolant would you say it drank from the rez so far? Some is ok… a lot is not…

My coolant leaked because the water pump had blown a seal. Because it only leaked when I was driving, I never had a puddle. Because it leaked slowly enough I never saw a trail.

I had the same problem on my 1997 Camry. Turned out to be the heater core. Thank goodness I didn’t have to remove the dash to change the heater core. However, I did have to replace the carpet.

There’s lots of good advice above. I’d add: you can borrow a cooling system pressure tester and pump it up and check again in the AM. You might find an external leak. Possibly even into a cylinder - with sensitive nose you might detect coolant in one or more if you remove the spark plugs one by one.

So I have topped it off maybe 3 or 4 times over the past 2 weeks. Top it off as in from the low line to the full line in the Rez. Interestingly since I’ve posted this and we have drove the car the past several days the coolant hasn’t dropped at all (Weather is still 95-105 outside). Usually it would have dropped by now.

Now we haven’t been running the ac on full blast (have no idea if that would play into it at all…). That’s the only thing I can think that we have changed in our behavior.