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Subaru -- overheating

2003 Outback, 113,000 miles

New water pump and thermostat (replaced twice)

Below 40 degrees, starts to overheat. Is controlled by

1. making sure the heat is turned on high – if temperature gauge doesn’t come down, then

2. put blower on full and then the temperature comes down within 60 seconds.

Occasionally, even with that, the gauge may fluctuate and then I bring the speed down to 60 and it gets regulated.

It does not happen if it’s above 40 degrees and the Subaru dealer cannot replicate the problem.


Define “overheating.” Are you losing any coolant? Is the radiator boiling over, or are you just watching the dashboard gauge? Has anyone measured the ACTUAL temperature of the coolant? The gauge or sender could be faulty.

Are you sure there’s no air in the cooling system? Are the cooling fans working correctly?

Let me get this straight; above 40 fahrenheit there are NO problems whatsoever, regardless of vehicle speed, length of travel, or HVAC setting. Is that correct?

Overheating … in this case just means that the temperature gauge goes up. No coolant is lost. No radiator boiling over. Just watching the dashboard gauge. Subaru checked for air in the cooling system (it’s been in four times (Subaru - 2 times; other mechanic - 2 times) over the last 6 weeks for this problem). Subaru dealer did find air the first time; I will not take it back to the original mechanic. Subaru dealer has checked the temperature and it’s within “normal conditions”. Head gaskets have been checked and are fine. Other possibly relevant info: The second time I took it to Subaru dealer, they said the engine fan didn’t come on at first … but then did. I think it has something to do with that – otherwise, why would the heater and blower take care of the temperature gauge?

[i] Subaru dealer cannot replicate the problem. [/i] 

Is that because they don’t get to rive it under the conditions you have experienced problems, or because it only happens on occasion, or because there is something different in how the dealer is driving the car and you?

Does it overheat (please do answer mcparadise's questions) every time or just sometimes?

Subaru cannot get the temperature gauge to move during test drives and their tests indicate it is running within normal range – so they don’t know what to change/fix.
It happens to me when

  1. temperature is below 40 degrees.
  2. it happens within 10-20 minutes of driving … regardless of speed.
  3. it happens every day when it’s below 40 degrees – but just since the water pump and thermostat were changed and then the new faulty thermostat was then replaced …
  4. the gauge comes immediately down when I turn the heat and fan on
  5. I try to get it to Subaru when it’s happening, but I’m usually about 25-50 miles away and can’t get it to them when it’s happening
  6. it NEVER happens if the weather is above 50 degrees.

There is a sensor in the radiator which controls the cooling fans. If it’s bad the fans won’t come on when they should. The sensor can be tested to see if it’s operating correctly. I’m surprised the dealer didn’t do this. There’s also a relay for the fans, and it, too, could be bad.

It’s also possible that the temperature sender that works the gauge is bad, or the connection to it is dirty or corroded, and the engine is not really overheating.

It’s good that you’re monitoring this, because Subaru engines do not handle true overheating very well.

If you start the engine and let it idle (you don’t have to drive the car) it should warm up to normal temperature, at which time the thermostat will open and allow coolant to circulate through the radiator. As the coolant in the radiator heats up the fans should come on.

If everything is working correctly you could let the car sit and idle all day. The fans should cycle on and off to maintain the correct coolant temperature, and the gauge should move very little, if at all.

You can open the hood and watch all of this happening. You’ll be able to feel the radiator getting hot after the thermostat opens, and you’ll see and hear the fans when they come on. Keep your hands away from the fans; they can come on at ANY time.

Perhaps trying this, while watching the gauge, will give you some additional information.

I’m still trying to figure out what’s magical about 40 degrees.

After the gauge goes up and you turn on the blower to bring the temperature down, what happens then? Does everything become normal after 20 minutes of driving, or does the temperature go back up again if you turn off the blower?

How high does the gauge get, and what happens if you do nothing as it starts climbing?

Thank you for those explanations … Perhaps the dealer tested the sensors – I just know they said “WNR”. The “critical” temperature is changing. At first, it only happened when it was below freezing and not in the 30s, but now it happens in the 30s as well. Here’s how it went the last two days:

  1. Thursday (morning temperature 30s) temperature gauge rising – driving 100+ miles and controlled it with varying degrees of heater and fan – it gets VERY hot inside the car so I drive around with the windows open. Finished work early – excited to get to Subaru dealer to show them – temperature in afternoon rising to 46 or so. Temperature gauge stops rising about 15 minutes from the Subaru dealer – so didn’t go.
  2. Friday – Drive south about 130 miles – temperature range 46-66. Absolutly no problem.

If the gauge goes up, I turn on heater to full. If that takes care of it, I leave the heater on full. If the gauge continues to go up with the heater on, I turn the fan on full. That will ALWAYS then bring the gauge down to its normal level within 30-60 seconds. When it’s very cold, I don’t then mess with the heater or fan – just keep them both on. If it’s 40ish, I will keep checking by turning of the heater or fan … the gauge fluctuates about 1/4".
I’m too scared continue to let the gauge climb. When the faulty thermostat was installed, it would almost get to the orange area – but that was before I figured out the heat/blower stuff. It’s normal range is about 1/8" below the marker next to the icon – when it’s rising, it will go about 1/4" above that – I don’t let it go above that.

Has anyone tried replacing the radiator cap? This inexpensive part is often overlooked as a source of cooling problems. I’d buy one from the local parts store and see if it makes a difference.

You may be cooking yourself for nothing. Perhaps the heater is not what’s bringing the engine temperature down. Maybe the thermostat and fans are doing it, and it’s coincidental that it happens as you play with the heater and blower.

Maybe the new thermostat opens at a higher temperature than the old one (not the faulty one) and that’s why the gauge is moving higher now. Once it opens, however, the gauge should stop moving around and stay in one place.

Maybe you got a second faulty thermostat.

We still have to verify correct operation of the cooling fans.

The outside temperature may have nothing to do with this problem. It seems to be less and less relevant as time goes on.

If this were my car I’d start it, open the hood, let it idle, and watch to see what happens. No heat, no blower, no nothing. Watch the gauge, feel the radiator so you know the thermostat is opening, and watch the fans. The fans should come on before the gauge reaches the red. Long before, actually.

If the radiator does not boil over, the engine is not overheating. It may be getting slightly hotter than before, but that’s not the same as overheating.

Re: radiator cap. I haven’t been told that the radiator cap has been replaced and I haven’t done it. I will try that.

Re: my heater/fan solutions. You could be right that the thermostat and fans are doing it, but I’ve been playing around with this for about 600 miles and turning off and on the heater/fan and it always takes the gauge down.

Re: the thermostat … could be right.

Re: the fans – you’re right – still not verified.

Re: the outside temperature. Yes – it may be less and less relevant, but one thing is true to date. If it’s a cold day, it happens; if it’s a warm day, it doesn’t.

Re: Your test. I will do that.


Post back here with any new information. Maybe someone will be able to help. I’m a Subaru owner but not an expert. There are a few Subaru experts lurking around, however, and I’m sure they’ve been reading everything you’ve posted. Maybe they’re just waiting for that one little tidbit of information.

Here’s an update – you were right. I was cooking myself for nothing.
On Sunday, drove 130 miles with temps between 31-36, so expected the gauge to rise. It did, but I resisted the urge to turn on the blower and within 30 seconds or so, it would come back down of its own accord.
So, I know a bit more now. The fan doesn’t come on at the right time in colder weather, but it indeed does come on; in warmer weather, the fan appears to come on at the right time because no gauge fluctuation is noticed.
That means that when I next go to the Subaru garage, I should asked them to:

  1. Check the cooling fans radiator sensor.
  2. Check the fan relay.
  3. Check the temperature sensor in the gauge.

Sounds like you’re narrowing it down. I’d test the radiator sensor that controls the fans. If they work the relay is probably OK, but it never hurts to check.

Well … you’re narrowing it down; I’m just driving! I’ll let you know what happens. Thanks for listening and continuing to ask questions.

I have the same problem, when it’s below 70 degrees. I can control it with the heater, pretty much, but in Arizona that’s not much fun. I’m on my third thermostat and the car has burned thru 3 fans. 2001 Forester, 250,000 miles. I think maybe someone might have replaced the thermostat backwards during a radiator flush?

I bought one of those radiator fluid test kits. There was enough exhaust in the water to turn the foam on the top of the fluid green, but not the fluid itself. Also, I’ve had the fan sensor replaced.

Cylinder compressions are normal, I think one cylinder was 5 PSI lower than the other 3. Also, I see the same effect, if I’m doing 75 the heater alone may not bring the temp down, and I’ll have to slow down a bit. I still think the thermostat might be backwards, will have to do some work this weekend to check.

Hi D Jackson.
My Car (though an Accord) is giving exact same problems as described by you. Can you please reveal if you were able to get the overheating issue resovled by changing the sensor.

hi.i saw this thread and thought i woulld join in.i am having the same problem with my legacy gl 2.0 wagon 2003
i also use the heater trick to help with the cooling process.but when the guage is on the red mark and i stop the car and i put my hand on the radiator cap its barely warm and when i put my hand on the lower radiator hose towards the thermostat its almost cold and the upper hose fromthe radiator is warm but not hot.
irecently put a new thermostat in and tested it before installing it.
i dont have a book for the car so i dont know where the sensor is located?
no powerloss and no smoke and no boiling over but the guage is reading fully hot. im not a car mechanic but i think its the sensor.any help on this would be great.
cheers from winter wonderland Sweden