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07 outback overheating

My Subaru just had the thermostat replaced, and it seemed to work until the wife took it she said it over heats when not driving the fans are working, it’s over flowing out the reserve tank the fluid looked good when changed the thermostat so I don’t think it’s the head gasket my next guess would be either the water pump… After I last changed the thermostat and test drove it I felt the upper radiator hose it was hot but the bottom one was cool/warm any ideas? I would really appreciate it thanks for your time

"My Subaru just had the thermostat replaced…"
Who Replaced It?

Is it in backwards?
Was the cooling system filled & purged properly?

I’ve run into faulty new thermostats right out of the box

In fact, some of these bad thermostats actually CAUSED overheating

With the engine cold, remove the radiator cap.

Start the engine, and while the engine idles watch the coolant in the radiator.

If bubbles begin to appear in the coolant as the engine idles, that’s an indication of a blown head gasket.


“Is it in backwards?”

Those were my thoughts also.
I have seen more than a few thermostats that were installed backwards.

It’s possible the cooling system was not bled out properly after the instillation of the thermostat. Generally, you’d open the rad cap, put an adapator on that connected to a funnel, and you run the car… Bubbles with come up because of the air in the system, once the bubbles stop and rad fans come on, and the car is at operating temp, generally that’s a good indication. That would be my first step, it also happens that it could have an air lock in it as well. Either way I’d try bleeding out the cooling system again, and keep your eye the temp gauge.

This may not work for you, but if you have questions about your thermostat,try placing the working end in a pan of boiling water to see if it opens (see last paragraph).

On our old BMW 2002, thermostats were: 1.) expensive (and replacing them required draining the coolant, too), 2.) difficult to replace (our first thermostat repair showed the factory had installed the clamp screw with the screw head facing toward the engine block and I still can’t figure out why of how they did that), 3.)rarely capable of lasting over 20,000 miles (although we sure enjoyed the car and it lasted 200,000+ miles, it vas ein Deutsche auto – basically solid, but with one or two characteristics that were most annoying).

A fellow mechanic suggested placing the working end of the thermostat in a pan of freshly-boiled water to see if it opened properly. Some of ours did not, and although pressurized water boils at a higher temp, the parts people never refused me when I returned the parts. I also kept a spare thermostat in the trunk toolbox.

Depending upon how cool that lower radiator hose is I might guess at the thermostat or a clogged radiator.

I certainly would not figure the water pump or head gaskets at this point. Water pump also brings up another issue; whether or not the timing belt job has been performed.

First eliminate the new thermostat and air pockets in the cooling system as a cause. If you prove those are both ok, then you could have a head gasket that’s leaking post-combustion CO2 from the cylinders into the coolant. Bubbles are an indication, but the standard method is a chemical test of the coolant.