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2001 outback AWP 2.5 auto 175,000 2nd owner AND 2007 Outback 2.5i 113,000 2nd owner
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Discussion Starter #1
I've read several threads about Subaru thermostats; overheating, sensors, fans, etc, but can't find one that duplicates my issue.

This is a 2.5 with 136,000. HGs were replaced before I bought it but #3 has a minor external coolant leak (after adding Subaru conditioner leak slowed considerably).

The temp dashboard temperature gauge read right in the middle when I first got the car about two months ago with 132K on it.

Last weekend on a 800 or so mile road trip I noticed the temp gauge reading only a bit above the "C" marks, and curiously would read lower at 65 - 75 mph and nudge up towards the middle when driving slower or idling. Never a sign of overheating with the needle moving in a narrow range in the black right above the white "C" marks at its lowest and a little more into the black at its highest. This is with ambient temperatures in the 10F to 35F range.

So today, assuming the thermostat was stuck partially open and not doing its job in regulating the coolant temperature. I replaced it with a new Subaru thermostat and drained and replaced the coolant.

The result was exactly the same temperature gauge behavior.

Could the engine actually be running in the normal range with a faulty sensor, or do I have other problems? Like water pump or radiator? The old fluid seemed normal with just a few flecks of the conditioner in it.
 

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2001 VDC/SC One of a Kind
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14,473 Posts
There are two temperature sensors on the engine. One is for the computer to regulate fuel/ignition, the other is for the gauge. They are located in the crossover pipe under the intake manifold, passenger side. The two wire is for the computer, the single for the gauge.
 

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To add to cardoc's reply, I believe the 2000+ has only one temperature sensor unit on the water (crossover) pipe, at least that's what I see in the 2000 and 2002 FSMs. The sensor has a three pin connector -- one connection is ground, another goes to the ECM for engine control, and the third goes to the temperature gauge. I'm not sure if internally there's only one sensor element feeding both outputs, or if there's two separate temperature-sensitive elements.

A good tool for this is an infra-red thermometer. When the engine should be fully warmed up, the thermometer can be used to measure the temperature on the crossover pipe near the sensor. It will be cooler than the coolant that's inside, but still well above what "C" on the gauge would suggest. (I would imagine a fully warmed up engine would show at least 150F on the pipe.) If the measured temperature is reasonably high when the gauge still shows close to "C", it's very likely the sensor for the gauge, or the wiring to the gauge, is problematic. There have been a few cases reported here of the sensor itself failing.

Also, the sensor is a negative coefficient resistor which means that as it warms up, the resistance goes down. If the gauge remains at or close to "C", the resistance might not be dropping as it should. Another test could be to measure the resistance between the sensor terminal and ground when the engine is cold, and then again after it has been fully warmed up. There should be a significant drop.

And if a scanner could be connected to the OBD terminal, the engine coolant temperature as seen by the ECM could be read out. If it's showing a good reading, then it again suggests that the temperature gauge sensor, or circuit is faulty.
 
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