I have a 95 Subaru that is overheating. I have brought it to different mechanics who have tried to fix it but can't figure out exactly what is wrong. I can usually drive the car for a while before it starts to overheat.

My reservoir is always full as it should be but I have to pour coolant directly into the radiator in order to drive it. I can sometimes go for a whole day without it overheating, then other days it will overheat a couple times a day (usually in the morning). I noticed when it heated up last night that the heat turned completely cold about the time that it started to heat up. I have had a new thermostat put in but have been told that that could mean it was a faulty thermostat.

I brought it to a mechanic who said it wouldn't heat up when he drove it but he noticed that there was gunk built about around the radiator where the coolant would enter, so he took out the radiator and pressure washed everything. He then test drove it again and drove it all over the mountain and it never heated up. The next morning when I got it back it overheated on me.

I have been told that there could still be some sediment blocking the cooling system. I was also originally told that it could be the be head gasket but other mechanics didn't seem to be convinced that that was the problem.

If anyone has any ideas as to what could be the problem please let me know!

  • Do you see green sludge under the reservoir cap when you are going to add coolant liquid? If yes clean it and check it next day and tell me how much new green sludge you found. When the car is overheated touch different spots of your radiator, left up , center down, right up,.... and tell me if you have a cold spot, be carefull to not to burn your self just a quick touch first. The third question is about your heater, does it blow hot air when the car is overheated, it should burn your hands somehow, does it? You must not be able to hold your hand in front of that hot air, and....
    – Iman Nia
    Commented Dec 9, 2016 at 23:16
  • Do you add engine oil? How often do you add coolant liquid and how much each time? Can you bleed the coolant system yourself and did you do it recently specially when you add a lot of cooland liquid, and the last question, do you see coolant leaking on your drive path? Check it when the engine is off and when it is on for about 20 minutes. Tell me all the answers and I will tell exactly what is your problem
    – Iman Nia
    Commented Dec 9, 2016 at 23:20

5 Answers 5


Subaru Thermostats are different from those you buy at OReilly and other auto stores. You NEED to buy the OEM thermostat from Subaru; the design is different.

Before bothering with the thermostat, have a test done for exhaust gas in the overflow. You may have the classic Subaru issue of failed headgaskets. I'm ignoring the mechanics report of 'he drove it all over the mountain', as we have no guarantee that this actually happened.

If in doubt, take it to a Subaru specialist or dealer. Yes, it will cost for diagnosis - but worth it, as you can then decide whether to drop the money on new - expensive - headgaskets. The engine will need to come out for a complete overhaul. The 'head gasket in a bottle' solutions are a waste of money on Subarus, because of the location of the head gaskets.

  • 2
    ..a waste of money on Subarus? ....does this imply head gasket in a bottle actually works with some other engines? I always assumed the liquid-fix stuff was just preying off of poor peoples' last hopes
    – elrobis
    Commented Dec 9, 2016 at 18:42
  • It must work on something, otherwise it wouldn't be sold! :p
    – PeteCon
    Commented Dec 9, 2016 at 20:32
  • The liquid stuff gives you a few months (or maybe even a year or more) to come up with the money to replace the head gaskets or buy a new vehicle; that's what it's good for. I don't see any basis for claiming that it works better on other vehicles than on Subarus. Commented Dec 9, 2016 at 21:27

If you need to add coolant before driving it, you have a coolant leak. It's either leaking inside the engine (head gasket or oil cooler), or leaking outside (radiator, hose or water pump). Find that leak. Outside leaks are easy to find, just look for a puddle under the car. For inside leaks, look at your oil for a change in colour and/or stuff floating around, and look at your exhaust fumes for a white colour.

Being an older vehicle, it is entirely possible that your cooling system is obstructed by deposits. Your best bet would be to drain the entire system and fill it up with a chemical cleaner for rust and lime deposits. Then run the engine until it reaches normal temperature (so your thermostat opens), then drain the entire system again and refill with brand new coolant. You may need to repeat the cleaning a few times (you can reuse the same cleaner after you drain it, just use a paint filter to keep particulates out).

  • The problem is almost surely the head gaskets, and adding chemical cleaner is only going to make it worse by eliminating the crud around the gaps and letting more coolant flow into the oil or exhaust. Commented Dec 9, 2016 at 21:29
  • You can't say it's the head gaskets for sure. Besides, if cleaning it makes it worse, the leak will be that much easier to find :)
    – tlhIngan
    Commented Dec 9, 2016 at 22:59

Modern vehicle cooling systems operate with the coolant pressurized when it is hot, in order to increase the boiling point of the coolant. This is why the cap should never be removed hot, as you will get a fountain of scalding hot liquid shooting up out of the radiator as the pressure drops and the coolant suddenly begins to boil.

If you have a leak, this internal pressure never builds up. Instead the liquid is forced out of the cooling system as the engine warms up. Also the pump generates pressure to push coolant through the system, and the pump can also push liquid out of the system.


Also because the pressure doesn't build up, the boiling point is lower than in a normally operating cooling system, so the water is boiling into steam which pushes more coolant out, until the coolant level matches the height of the leak and then only steam squirts out the leak hole.

Ethylene glycol meanwhile has a higher boiling point than water, so it is concentrating in the system as the water boils off. If you are adding 50/50 mix to top it off, the glycol ratio is going to climb as the water keeps boiling off. The glycol has about half the heat capacity of water, so the engine runs hotter and is even more stressed as the glycol concentration rises.


Because the system is pressurized, the reservoir is not directly connected to the cooling system but rather has a spring valve regulator on it. If the temp gets way too high, coolant is pushed into it. If the cooling system develops a strong enough vacuum, it will suck from the reservoir into the system.

Since you have a leak, the cooling system never develops a strong enough vacuum as it cools to suck from the reservoir. Instead air sucks back inside through the leak hole.

If the cabin heater stops working that means the coolant level has dropped to the point that the coolant pump is no longer able to pressurize the fluid and push it through the heater. The pump is sucking air and has stopped doing anything useful.


Engines are meant to have a cooling jacket at all times around the cylinder walls, and only the walls normally get really hot. Since the cooling has failed so badly, that liquid jacket may not be present at all or no longer circulates, so the metal is getting extremely hot, with the heat spreading out from the cylinder into other areas. It is very possible that the metal is warping from the high temperatures and making the head gasket surface uneven, making any leaks worse.

Probably the engine block and head are being warped and ruined from this extreme thermal stress, pushing it beyond the point where only a head gasket replacement would fix it.

The engine may now need to be pulled completely out of the vehicle, and the block and head resurfaced in a machine shop to flatten out the warping, so that a new head gasket will seal properly.

  • Subaru engines have two head gaskets...
    – PeteCon
    Commented Dec 10, 2016 at 4:01
  • This is a generalized answer about how cooling systems work and also how failure works, which can probably be reused to answer a whole bunch of other questions that essentially ask the same thing. Commented Dec 11, 2016 at 3:02

Run through the gamet here...

  • check air flow restrictions

  • pressure test the cooling system. Note dont exceed cap psi.

  • check fans properly operate

  • check thermostat hot upper / cold lower hose

  • make sure its bleed properly

  • Do a block test / exhaust gas

  • A UV leak dye test is another option of the pressure tests fail. Should ppint it out if it isnt obvious by sound of hiss or or by sight of coolant or vapo rescaping.

That process should reveal whats going on. Also. Make sure the ratio is right. 50/50.

Report back with results. Hope this helps!


I had overheating prob on 2003 2litre forester...Expert after expert couldnt solve it..Until a freind and i thought airlock..When we tried to drain ..rad...it was full of gum..It was two year old rad replacement for original that blew hole..We tried cleaning it with limited results..Bought a new rad and been running as should .even in 35 degrees summer...I had been told by several engineers .that no way was rad issue...But hey presto after over a year of hassle it was as simple as new rad

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