It has been a year and a half and it is now happening again! (same exact presentation.) It started happening again when it started getting cold out and is still going on now. Took it to the mechanic that swapped out the radiator and he's stumped.

No leaks, but couldn't the head gasket be leaking on the inside? How would I know if the head gasket needs replacing?

Been noticing lately that my heater stopped working and temp climbs to red. Came to find out that I had no coolant in engine.

Filled it up. Problem is that it still happens.

Now, radiator full, Heater works fine, but if I drive it for a while, temp climbs and heater stops working. Took it in. Mechanic said coolant pump was leaking. replaced it, and temp sensor.

still happens!

Here is how I can reproduce problem: Drive with high RPMs for about 15-20 minutes. I put it in Low 3, and got on the highway for about 10 minutes. This kept the RPM gauge up to about 4 or 5. temp gauge climbs rapidly once it decides to start.

I drove home after that point never letting the rpm gauge go higher than 1 and a half. (if I did, temp would rise rapidly). coasting in neutral makes it go down. The heater being on has no effect, however, once the temp gauge climbs rapidly, all heat from heater vanishes.

The heater starts working again if the temp goes down (meaning I lowered rpms long enough to let things cool down) when I got home I arranged it that I got back in the driveway with the temp gauge almost in the red. I then put the car in neutral and raced the engine, keeping the RPMs up to 5 for 2 minutes. It had no effect (temp did not climb) I let the engine cool down a little and tried again in Neutral. no go. unable to make temp climb while parked.

  • Which engine does your Subaru have? The 2.5L? What mileage is on the engine? Commented Mar 28, 2012 at 17:29
  • Did the mechanic replace the thermostat as well? Was it bled properly after all of those items were replaced?
    – Dude318is
    Commented Mar 28, 2012 at 23:06
  • So the radiator swap mentioned below didn't solve the problem long term?
    – Bob Cross
    Commented Dec 8, 2013 at 21:33
  • The thermostat and radiator cap was replaced. No changes
    – KevinDeus
    Commented Dec 12, 2013 at 7:57
  • 1
    Did you get it solved? Do you still need more information? Commented Dec 15, 2013 at 18:09

7 Answers 7


They make a head gasket leak detector kit to determine if it's leaking. I would be surprised if you mechanic didn't have one. If he or she doesn't they are not that expensive.

Here is an example below

enter image description here

See my answer here for more info.

  • My mechanic did leak detection tests and said there were no leaks. As I only see this issue when the system is under load, I'd have to say that this method would only detect issues while the car is moving.
    – KevinDeus
    Commented Dec 15, 2013 at 23:03
  • 1
    @KevinDeus This is different from a leak test, did he use one of these or a pressure tester? The pressure tester won't always catch a head gasket leak. Combustion leak detectors will detect even a seeping head gasket if the directions are followed. This tool measures the amount of CO2 that's in the cooling system. Commented Dec 16, 2013 at 0:50

It sounds like a bad head gasket if everything else is in working order. Hot gases are introduced into the cooling system under load(hence why it goes down when you let off).

  • would it be a good idea for me to buy head gasket sealant as a precaution? (as in.. will it ruin an engine that is not leaking?) barsproducts.com/catalog/view/…
    – KevinDeus
    Commented Dec 7, 2013 at 23:45

Sounds like either air in the cooling system or a bad thermostat (although it'd be a weird failure mode for the thermostat). Strange that you can't reproduce it when the car's parked, though. How are your motor mounts? I'm just wondering if there could be a problem with one of them that allows the motor to move more than it should under load. This could stretch or kink some of the coolant lines (possibly), which might explain why your interior heat goes away too. Maybe take a look at any coolant lines you can identify and see if they show signs of creasing or twisting.

  • It would make sense why it would not happen while the car is in park. There is a difference when the engine is under load, for one, it heats up more. (my reason for driving in L3).
    – KevinDeus
    Commented Mar 30, 2012 at 17:36

We wound up swapping out the radiator, and that solved it.

  • Based on your edit above, this didn't solve the problem - any other information?
    – Bob Cross
    Commented Dec 9, 2013 at 12:44
  • no new behaviors have manifested. It presents the same now as it did before. (in my description above)
    – KevinDeus
    Commented Dec 12, 2013 at 7:53
  • I guess its worth it to note that this started happening after a cold snap started here. I have been reading that cold weather can cause problems with the head gasket
    – KevinDeus
    Commented Dec 12, 2013 at 7:56

there is no independant thermostat bypass in a subaru. Get or make a u-bend of 16mm metal pipe and disconnect the two heater hoses at the firewall behind the engine and join them together with the u-bend and two hose clamps. The overheating problem will go away for good. But of course, now you have no heater. The best solution if you want your heater to work is to cut both the heater hoses at a suitable spot next to each other and to fit a T-piece in each hose and join them together. Then water can bypass the heater element keeping the thermostat functioning properly, and enough hot water will still flow through the heater element to warm up the car's interior.


i put in 2 tee's between the heater core lines and connected them . this way you always get hot antifreeze to the thermostate . after a while the air worked it's way out off the heater core .


I did have the same problem, my car "subaru legacy outback 98 2.5l" did overheat vile driving n when I parked the engine will cool down. I fixed the problem by take oute the radiator n flushed it n I did find a crack in the radiator by the in pass on 4-5 inch. I welded it vit the same type of plastic that the radiator I made of n putt it back inn.. YouTube is nice 😁😁😁 then I did ask my father for son help n hi did say that boxars engine is hard to get the air out of the cooling system so I recommend me to open the hoses that goes in to the heater, first the one that comes out to giv it free pass to left the air get oute n this you must do wile the engine is running.. When the water is flowing you must remember to fill up with coolant on the radiator n close it n next is the thank by it to the middle. The engine will drain it out... Totaly.. Then after given I free air pass you just putt the hose back n then open the inpass hose, you may have the def-heat button? Push it in or activate it. The water will not return by the hoses an flood backwards... N remember, huge pressure here 😜😜 putt the hose back again after few sec n tighten them n refill the radiator, or stoop the engine first 😜 now you just fill all up n left the engine get the try. I did also think my water pump was broken but I did well to the end 😊


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