2010 Subaru Legacy 2.5 liter, non-turbo.

When the overheating issue first started, my first thought was thermostat because if I popped the hood during the overheating and grabbed the bottom radiator hose, it was cold as if there was no coolant flowing through it.

Replacing the thermostat did not fix the issue, so then I did the water pump (and the other components of a full timing kit), again, same issue.

So today, replaced the radiator. And at first all seemed very good. Revving the car in idle, and letting it idle for about an hour did not produce any overheating. The bottom hose got just as hot as the top hose, and running the heat produced very hot air.

But during its first real test drive tonight it overheated after about 20 miles total, which was spread out over 3 hours, and involved a decent amount of time with the car not running in parking lots.

During this overheating, the air did not blow hot at all and the bottom hose was not hot at all, so all of the issues that were fixed and working perfectly about 5-6 hours earlier, were now issues again.

I'm at a complete loss at this point. I have tried to find a diagram of the full path that the coolant takes in this car but to no avail. I have found a diagram, but I have no idea of its source or if it relates to my specific Subaru model.

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Other information:

  1. Each time I have had to drain the coolant during one of these replacements (so 3 times total now) I have used completely new coolant and discarded the old coolant.

  2. The truly proper way to fill the coolant for this car is to use a vacuum pump, but I have followed the second best method utilizing a radiator funnel to allow the system to suck in extra coolant as opposed to air, and running the heat until all bubbles stopped coming from the radiator.

  • Why did you discard the old coolant, did it look bad (dirty)?
    – HandyHowie
    Mar 8, 2023 at 7:50
  • Agree with HandyHowie here ... antifreeze actually never goes bad. It just becomes contaminated. Also, are you sure you put the thermostat in the correct way? Mar 8, 2023 at 11:26
  • @Pᴀᴜʟsᴛᴇʀ2 Yes I made sure the new thermostat went in exactly as the old one, with the spring towards the engine and the jiggle valve up.
    – Tyler N
    Mar 8, 2023 at 19:51

1 Answer 1


Well you have done the usual stuff so it's time to dig deeper...

  1. Check all of your coolant hoses. They can collapse internally especially on older vehicles like your 2010 model.

  2. While hard to check, another possibility is a leaking/blown head gasket that is allowing hot combustion gasses to infiltrate into the water jacket. Generally you'll see oily or black residue in the coolant when this is the cause.

  3. It's possible there is a blockage in one of the engine's cooling passages. Unfortunately this can be hard to detect and fix without a teardown but this can be related to the hoses where part of the inner lining of the hose break loose and migrate to someplace inside the engine.

  • I have ordered a chemical test kit (Lisle Combustion Leak Detector) to check for a head gasket leak. I will go from there once I get the results.
    – Tyler N
    Mar 8, 2023 at 19:54

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