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My Forester (2004 2L XT) has always run in the middle of the temperature gauge - until a couple of weeks ago, since when it has started running warm at slower speeds.

Now, it gets right up above 3/4 once I've been sitting in traffic or driving at slower speeds (40mph or less) for a few minutes - but doesn't go into the red, and as soon as I get onto the open road and go faster, it very quickly drops back to it's normal 1/2.

I've checked both oil and coolant, no signs of contamination in either. I can't see any blockage or obstruction that might be limiting airflow across the radiator, and the fans are working fine.

I changed the alternator belt recently, but made sure everything was properly tightened afterwards.

What else could be the issue? Thermostat? Coolant flow through the rad?

  • You mention there was no signs of contamination in the coolant but is there any sign that it's losing coolant? If not then it points (as Solar Mike says) to a thermostat issue. – motosubatsu Sep 28 '17 at 12:52
  • I don't think so - the level looks the same as usual, that was the first thing I checked! – Nick C Sep 28 '17 at 12:56
  • Where did you check the level? In the rad tank (the metal one on top of the engine) or in the plastic expansion tank? – motosubatsu Sep 28 '17 at 13:04
  • In the expansion tank - it was too hot to take off the cap of the metal one – Nick C Sep 28 '17 at 13:06
  • the expansion tank doesn't always tell the whole story - with the engine cold pop the rad cap off and have a look in there, it should be very near the top of the little metal tank (you may need a torch to see it properly) – motosubatsu Sep 28 '17 at 13:20
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I've seen something similar to this happen on a few vehicles and each time it has been the fan not running. Cars these days have electric fans that run when needed to help cool the radiator. When the car is driving down the road, the wind is enough for adequate cooling, but when sitting still the fans have to make the wind.

This can happen for a few reasons. The electric motor could be bad, or the fuse or relay could have popped. The fan is controlled by a temperature sensor and it's not always the same sensor that is used for the dashboard display. That sensor could be bad. If there are two fans, but only one is moving, the fans could be connected by a belt that is broken.

Diagnosis of this would be to drive the car until its warm, then park it, pop the hood and wait to hear the fans. If the temp gauge goes past the middle and the fans don't come on, then that's the problem. Fuses, the relay and temp sensor can all be checked with a multimeter (and some instructions), so it could easily be a DIY fix.

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Check your fan connections. It sounds like the right-hand fan isn't running (the left-hand fan comes on when the AC is switched on or when the engine is starting to really overheat).

The fan connection on the Subaru can be a little odd - the pins can get pushed out on the fan side when the connection is made, so it looks like the connection is good, but there's no pin contact. To spot this, look for a brown waterproof connector on each individual pin; if you can see it, that pin isn't in fully. Take some needle-nose pliers and push that individual pin into the connector.

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Easy to check the rad for leaves - just reread your question you have done this bit !! etc then thermostat, then flow of water through the rad - at least that is the order I would start.

Does sound like classic thermostat though - when it is slow to open or sticking a bit...

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