I bought a used subaru outback with 200,000 miles. Replaced the catalytic convertor (expensive) 1 month later, after being parked for 1 hr the car got stuck and would not shift into drive. It would only go in reverse. After 3 days of research and adding transmission fluid, I finally got the car to move forward. 15 minutes later, in stop and go traffic, the temperature gauge suddenly shoots up almost to the red zone. I immediately pull over and turn the car off. I hear swooshing, liquid sounds coming from the dashboard. There is no steam or fluid leaks coming from the car. After 15 minutes I restart and drive home (5minutes). I check the coolant reservoir and it is empty. I refilled it, but it keeps going dry after a day or so. I have not driven the car since and only start it up to move it for street cleaning. I checked the radiator and it is full. What could be the problem? No warning lights come on, no external leaks, and the car does not overheat after running for 15 minutes+.
I would check all the hoses under the hood, especially the heater hoses if you heard noise coming from the dashboard. The vehicle could have a bad heater core and that is where the noise is coming from.
Check the oil on the dipstick to see it is a milky color. This would mean that the coolant is leaking into the block, which would indicate a possible head gasket.
Water pump could also leak and not leave a trace on the ground. Look at it and see if it looks wet somewhere.
Very difficult to diagnose by remote control, but you could have a thermostat that behaves irratically, staying shut from time to time.
The usual problem with a Subaru with that mileage is a blown head gasket, which releases hot gasses into the cooling system. The swooshing sound could be hot gasses getting into you heater hoses. If the leak in the head gasket is small, it could show no “steam” or fluids coming out of the cooling sysyem or exhaust.
I would check the exhaust for visible white “smoke”.
A third pssibility is that your new catalytic converter does not allow the exhaust to flow freely. Did the dealer install it or was it an aftermarket? On a well-maintained car, the converter lasts the life of the vehicle.
In any case, I would not drive the car very far; you need to have the cooling sytem checked out and a compression test done to check for a head gasket leak. And the folks who installed the convertor should recheck what they did.
Why did the transmission need more fluid? It seems the transmission problem may not be directly related to the other problems. Do you have the maintenance records of this vehicle?
From what you say this isn’t sounding real good. There could be a headgasket leak causing the coolant problem. I suggest you have a block check done to see if exhaust gases are getting into the coolant.