Home » The heat is on – replacing your Subaru Thermostat

The heat is on – replacing your Subaru Thermostat

Another heat related issue popped up recently on Bax the 05 Subaru Impreza and since we burped him last time, I figured maybe we needed to go a little deeper to sort out this issue this time. This was also a great lesson in how easy it is install a Subaru Thermostat the 2nd time after blindly following the way they installed things before I got there.. the more you learn.

Edit: Sigh and now I’m reinstalling things for the third time..

I got off the freeway and the heat was on!

Well more to the point the heat gauge was having a bad time of things, the needle hovering near the top of things after a brief 22km run at 110kph to get to the Ettamogah pub for Mothers Day lunch. We parked, had lunch, drove back, no more issues – needle stayed halfway on the gauge where it did. Strangely I had none of this overheating behaviour when we took it out for a big spin around Beachworth and Yackandandah around Easter time so it seemed to be completely random. Another air pocket that somehow escaped the last purge or something else at play?

After reading up on it, I found there’s four possible things that might be having a bad day and in order from ‘cheap fix to dig deep’ they are:

-Radiator cap. If it’s not holding pressure, the system is operating at full capacity and there’s the chance it’s inviting bubbles into the cooling mix. Mine is a nice shiny Tridon unit so someone replaced it along the line somewhere, so I decided to skip this first one (but I will come back to it if symptoms persist.)

-Subaru Thermostat. It’s a simple part but sometimes they can go wrong. It also doesn’t help anything if you don’t install it correctly (more about that in a sec..)

-Radiator. Something might be cracked, parts might be blocked. Either way it could be having issues.

-Water pump. If it’s not pushing out water/coolant with enough pressure then things are going to get hot (and possibly go boom) pretty quick.

Of course it’s only after I bought this Tridon Thermostat did I discover that quirky Subaru’s only seem to get on well with their own Subaru Thermostats aka OEM. Which I will sub in there, if this one fails to do the trick.

Tridon Subaru Thermostat

Note that you have to buy the rubber seal for it separately, because reasons. Luckily Supercheap Auto here in Albury had plenty spare.

How to replace your Subaru Thermostat

This replacement was on a 2005 Subaru Impreza EJ25 but the steps of the process should be pretty similar for similar models. There is enough room to do this (just) without jacking the car up but parking on a slope can help.

Important note that should be common knowledge but hey, read it anyway. Only work on doing this when the car is cool. Coolant can scold you, coolant can burn you, this job can get messy, you don’t need a trip to the burns unit when changing your Subaru Thermostat, no. Even if you’re in a hurry, let it cool. Wear gloves still, employ lots of towels and rags and enjoy not getting burnt.

Firstly remove the plastic under tray. That’s six bolts or in my case, four bolts and two old cable ties

Your thermostat is in there, under that black cover that leads into your lower radiator pipe. Now you can either a) drain your radiator from an opening below (I couldn’t see mine and didn’t worry about it) or drain it from the thermostat itself. You can either take the hose off first or just undo the two bolts holding the cover slowly and prepare for it to start to come out that way. Either way, get your drip trays ready because it’s going to dump a lot of coolant! This is why you don’t want to be anywhere near it when it’s super hot and ready to sizzle flesh..

Subaru Thermostat

And there’s the old one, in the middle of the drainage action shot. Wait til it stops dripping and then grab it. It should just pull straight out.

Subaru Thermostat

Now the first time I did this, I made special note of which way the little jiggle was pointing (aka I took a photo) because I figured that if the previous mechanic had it positioned this way then this way must be the right way for installation. Especially because with this particular model, you can have the jiggle (which helps burp things along the way) any way you like. Turns out that no, this wasn’t correct to begin with (and possible goes some way in explaining why it overheated on occasion).

Subaru Thermostat

The correct way for the EJ25 Subaru Thermostat depends on the model. If the unit is pointing down like this one, the jiggle needs to be pointing directly at the radiator. If the opening for your Subaru Thermostat is at the bottom of the block but horizontally mounted the jiggle needs to be at the 12 o clock position. Or north. Or up. You’ll get it. 

So for the second time around, I moved it to where it should be pointing..

Anyway, back to the process of installation. The old one pulls right out with barely any effort and it seems at one stage someone looks to have siliconed it for some reason? Nothing I’ve come across suggests doing that so I have no idea. Either way, it looks high time for an update.

Subaru Thermostat

Clean out any leftover silicone in the opening and pop the new one in, making sure you’ve remembered to put on the rubber ring. On this downward facing one I held it into place with one hand, then moved the cover over and put the bolts on with the other hand to keep it there. Make those bolts nice and tight, you want a nice tight seal down there.

After that, it’s filling the system back up with coolant and burping time again! (Guide on how to do that is right here) Once again you need to get all the air bubbles out of the system so you can rule that out if things get too hot again later on. When that’s finished, bolt the under tray back up again.

After two attempts, I think things have calmed down now.



Installed the thermostat like it had previously done with the jiggle pointing in the wrong direction. Burped the system for half an hour – at one stage of the cycle a burst geysered out of the open radiator which was new, usually it just gushes out not shoot out everywhere angrily. Found out pretty quickly I needed more coolant, I’d obviously lost a lot more in the messes I’d made doing this than first thought. (Also if your heater runs cold on the hottest setting, that’s a sign that your running low on coolant.)
Filled it up, went for a run and the needle stayed where it needed to round many a corner and hill climb…until I parked it and let it idle, when the thermo fans kicked in and the temperature started to rise well past the 3/4 mark and climb from there.
Not ideal and things got very messy very quickly..


Learnt where the jiggle should be pointing thanks to this very helpful vid. Got home, and got everything opened up again quick smart, installed it pointing in the right direction.

Pointing towards the radiator if you missed that bit above

Another half hour burp (this one with no fountain effects) and a test drive kept the needle exactly where it should be. I took it for a spin, the wife took it for a spin, we idled at the lights, we picked up the kids from school, we went shopping not an issue. Looks like it’s all fixed for now but time will tell.

Still, I know how easy this is now if I ever have to put a OEM Subaru Thermostat in there next time..


So not a hint of overheating at all for the short trips around town but what about on the freeway? Well before I gave it a test, I grabbed a new radiator cap from Repco for $16 just in case. It’s cheap, takes a second to replace and if I was having the same problem again from this test, well at least I could rule the cap out.

Tridon Radiator Cap Subaru

Once on I hit the freeway and gave things a good run from town all the way to Barnawartha, which is roughly 22kms away and that’s mostly freeway at 110kph. Both times the needle has risen (when I didn’t put the thermostat in pointing the wrong way) was after a decent run on the freeway so my theory was that the system was having a really hard time of cooling things down straight after higher temps at speed. I drove all the way out there, parked at the local railway station and let it idle while keeping an eagle eye on the gauge.

As the fans kicked in while idling, the needle climbed ever so slightly and stayed bang on the middle. Usually it’s just a point or two below that but it certainly didn’t go any higher. In fact the only exciting thing that happened while I waited was this train whizzing by.

So I turned around and drove back home, the needle quite happy to stay where it was. Looks like I may have just sorted my random overheating problem!


Sigh. It was going so well for the first 200 kms of a 700km round trip to Melbourne and back for a hospital visit. And sadly it wasn’t just the thermostat that caused the issue, a completely unexpected friend joined the party causing a very stressful drive there and back.

-The first heat spike arrive 200kms in. I’d been keeping one eye on the heat gauge for all of the trip and strangely after an hour and a half of driving with cruise control on at 110kph, the needle started it’s dreaded rise to the top of the gauge. So I got of out cruise and tried to get ahead of the cars beside me so I could safely pull over to see what was going on. Putting my foot down caused the needle to drop down where it should be. Well okay then, that was random. Possible culprit – New thermostat jamming temporarily or a block in the radiator.

-No further issues all drive. The next day however in the hospital car park, it kept spiking and would only drop down at speed. Parked for four hours, I put in some more coolant before we left but noticed the overflow bottle was beyond full, a couple of inches away from the cap. ‘Well that should drain back overnight surely?’ I thought and drove back to where we were staying. Possible culprit – Was this new thermostat pushing things through too late? Was there too much stuff in the system and it did a massive purge? (Unlikely) Or maybe there was a hole in the hose from the over flow sucking in air? 

-Next more, bottle still hadn’t drained. I knew there’d be enough coolant to get us home (and with mostly freeway driving things would keep cool enough) and hoped whatever was causing the blockage there would come unstuck at high speeds. But of course it didn’t. The bottle filled, the small release hole got a workout and when we finally got home a few hours later, the mess I was expecting was waiting for me.

Worse, the overflow tank was still overflowed by a wide margin.

So I carefully drove it to Subaru to order a new OEM Thermostat and radiator cap. Unfortunately they’re a few days away so I limped it to the supermarket and left things there while I shopped for dinner. While I was trying not to think of it, I remembered reading that part of the purpose of the radiator cap was to create a sealed system that would suck in and deposit in and out of the overflow as needed.
And even though the cap was brand new, could this be what was causing this headaches?

In the supermarket car park I put my gloves on and carefully swapped out the new cap to the old that was still floating around in the back. Then I tried to drive home, only for the needle to rise and not push down no matter what I did, so I parked it and took a walk just to be safe. I walked back to Subaru, bought some more coolant (I know I’m going to need more of this soon enough). I let things cool down over the next 20 minutes, opened up the overflow and swore very loudly. Because it looked like this now:

Now while it doesn’t look like much in there, the level is roughly halfway between empty and full where I want it to be after cooling down. The old cap I replaced just to be on the safe side was still working and created the correct environment to suck in the overflow again and had down this while I’d gone on my angry walk to try and calm down.

It drove home just fine and the needle didn’t move. I topped things up this morning before work and all is good, for now.

Somehow my new radiator cap ended up causing more problems than my old one did.

I’m still swapping both the Subaru Thermostat AND the radiator cap when they come in though (and keeping an eye on the overflow tank across the week). Third times a charm yeah?


So an OEM Subaru Thermostat – how different could it be to something generic off the shelf from a parts shop? Turns out quite a bit when you put them side by side. The one on the left is the one from the nearest Subaru dealership, the other the one that was originally in the car when I was first discovering problems.

Yeah that’s quite the difference there, including both height and weight. And not only am I unsure where that generic one came from, I’m also not sure when they/whoever put it in and pointed it the wrong way. Going through the service receipts I’ve found that in the last two services they’ve pressure tested the radiator and found no problems but they did that because there was evidence of dried coolant everywhere like there is now. But while there’s a cost for a new radiator cap on one sheet, there’s no listing of this thermastat in there so that remains a mystery.

This weekend it goes in along with the new radiator cap and hopefully this marks the end of the messy engine bay and rising heat needle!

So in it goes:

And unlike the other two, this one goes in and fits snugly so no need to hold it in place when you bolt the cover back up. I also discovered this third go around that yes you can do this job without jacking the car up or working on a slope but for the love of god, give yourself some room to work. It can and will get messy, give yourself some work room.

And now with a Subaru OEM cap on the radiator we’re pretty much back to how it came out of the factory back in 2005.


Strangely the new radiator cap did not work. The overflow filled but didn’t drain. So I swapped back to the older ones and still had the issue, even though I know one of them was working okay. However the last time I changed it over, I took a closer look at the actual hose itself from the radiator to the overflow and while it’s hole free, I did notice it came off the outlet under the cap pretty easily.
Could this original hose cause air to be sucked in instead of coolant? Just in case, I zip tied the hose there tightly. There’s just enough room under the cap to do this.

And in the week that it’s been on there, the overflow has gone back to behaving itself. The level in it is between the markers which I’m very happy to see.
Maybe it was the hose adding to the issues on and off the whole time?


After a 600km+ round trip from Albury to Bendigo and back again, I can happily report that the problem now seems to be completely fixed. Expansion tank fills up as things expand, radiator sucks it back in as it needs to. So if you’re having similar problems where your coolant is coming out of the expansion tank but not going back into your radiator, try these steps:

  1. Check/replace your thermostat and if you do replace it, make sure you use a genuine OEM Subaru part. Anything else is just going to lead to future headaches.
  2. Make sure it’s pointing the right way too! (See above)
  3. Burp your system properly
  4. Check/replace your radiator cap
  5. Check/replace the hose from the coolant expansion tank to the radiator. If need be, tighten at the radiator end with a cable tie to stop air getting sucked in instead of coolant.
  6. Keep an eye on coolant levels as you go (it’s not a bad idea to have some and a funnel in the back of the car while you work on this problem..)

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