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Discussion Starter · #1 ·
Replaced thermostat 2 months ago after started overheating. Now, sometimes , the gauge will be halfway through the "H" (well above red), but coolant/radiator/motor not hot (early morning 0*c)

Has to be a sensor doesnt it?

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If the car is not overheating, but the gauge shows that it is? Then yes, probably the sensor, though it could be a general electrical issue like a corroded ground.

Boilerplate overheat/thermostat info:

Re-check thermostat. Verify that it is a Subaru part. There are many aftermarket thermostats which fit, but don't actually trigger at the right temperature. The Subaru part really is different.

Then check your heater core hoses. Hot water comes out of the engine to go into the cabin heater, and then returns. That water then hits the thermostat, and if it is hot enough the thermostat opens and allows flow through the radiator.

If the heater core is blocked or flow-restricted by mineral deposits, then it doesn't matter what thermostat you have- it won't get hot enough to open and you'll overheat. So check those skinny heater hoses- they should both be hot.
 

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Discussion Starter · #3 ·
If the car is not overheating, but the gauge shows that it is? Then yes, probably the sensor, though it could be a general electrical issue like a corroded ground.

Boilerplate overheat/thermostat info:

Re-check thermostat. Verify that it is a Subaru part. There are many aftermarket thermostats which fit, but don't actually trigger at the right temperature. The Subaru part really is different.

Then check your heater core hoses. Hot water comes out of the engine to go into the cabin heater, and then returns. That water then hits the thermostat, and if it is hot enough the thermostat opens and allows flow through the radiator.

If the heater core is blocked or flow-restricted by mineral deposits, then it doesn't matter what thermostat you have- it won't get hot enough to open and you'll overheat. So check those skinny heater hoses- they should both be hot.
Thank you.

I've had it 3 years, bought as a 2nd car. When purchased, was told the periodic engine light was a knock sensor code, I've tried 2 new knock sensors in it, and now its on permanently... So it wont surprise me if there's a wiring issue.

The replacement thermostat was only half the size of the original, but it was the only 1 in town... If it does actually start running hot I'll source true stat.

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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Stant, and maybe 1 or 2 others, make a soob-specific t'stat . dealer prices are usually not 'horrible' either.
 

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Meh.
I has wagons.
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Scrap that junk stat pronto.
 

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1. why was the thermostat replaced to begin with? this is often proceeded by some smell, suspicion, etc. if that's the case you may be chasing another issue.

2. burp the system. wash rinse repeat until yo'ure positive it's golden. these are sensitive to this.

3. you can test the tstat operation - put it in boiling water. it either opens or it doesn't or only goes half way. very easy.

4. replace with subaru OEM tstat, XACTA Stants are similarly robust as OEM if you want options.

make sure fans are coming on, check fluid level, for leaks, etc.
 

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Discussion Starter · #7 ·
1. why was the thermostat replaced to begin with? this is often proceeded by some smell, suspicion, etc. if that's the case you may be chasing another issue.

2. burp the system. wash rinse repeat until yo'ure positive it's golden. these are sensitive to this.

3. you can test the tstat operation - put it in boiling water. it either opens or it doesn't or only goes half way. very easy.

4. replace with subaru OEM tstat, XACTA Stants are similarly robust as OEM if you want options.

make sure fans are coming on, check fluid level, for leaks, etc.
1. Was actually boiling (seized shut) the car inside 15kms

2. System was flushed when tstat replaced

3. I believe the new tstat is fully operational. I honestly don't think it's actually getting hot, I'm certain there's a faulty sensor, was just chasing a second opinion

4.The main reason I put the smaller tstat was because it was the only 1 in town the day i had to replace the seized 1.

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Replaced thermostat 2 months ago after started overheating. Now, sometimes , the gauge will be halfway through the "H" (well above red), but coolant/radiator/motor not hot (early morning 0*c)

Has to be a sensor doesnt it? . . .
How are you checking the temperature of the coolant/radiator/motor?

When does it start overheating according to the instrument panel temperature gauge? In other words, if you turn the ignition switch to ON, but not start the engine, and leave it there for a few minutes, does the temperature gauge go up, or does it stay at "C"?

If it stays at "C", and you start the engine, how long does it take for the gauge to show overheating even though the engine etc., is still cold?

If the motor and accessible coolant pipes at the top of the engine are all cold (0 degrees Celcius?) while the instrument panel gauge is indicating above normal, then it's possible that the thermostat is not a factor. I would agree with not using that smaller thermostat in any event, but the absence of any apparent engine heat while the gauge is already above normal suggests the sensor, provided the wiring to the sensor isn't faulty (as rasterman mentioned in post #2 above).

There have been cases of an "over sensitive" temperature sensor unit that would indicate overheating when that was not in fact the case. Also, a high reading on the gauge corresponds to a low resistance in the sensor, which could be a faulty sensor, but also a short to ground in the wiring.

idosubaru mentioned the radiator cooling fans, and that could also be a lead. If the instrument panel temperature gauge is showing above normal, and the engine itself is still cool, are the fans running?
 

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Does cabin heat follow the heat gauge trend?

When it's in the red - is there any cabin heat?

Got it you're paying good attention, taking good notes which is good. It's hard when we only read words - there was an overheating event, an attempt to repair it and now there is another cooling system related failure. That's possible but coincidental.

I'd look to repair quick. Overheating or not being able to tell if you're overheating is 4 digit dangerous.
 

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Discussion Starter · #10 ·
How are you checking the temperature of the coolant/radiator/motor?

When does it start overheating according to the instrument panel temperature gauge? In other words, if you turn the ignition switch to ON, but not start the engine, and leave it there for a few minutes, does the temperature gauge go up, or does it stay at "C"?

If it stays at "C", and you start the engine, how long does it take for the gauge to show overheating even though the engine etc., is still cold?

If the motor and accessible coolant pipes at the top of the engine are all cold (0 degrees Celcius?) while the instrument panel gauge is indicating above normal, then it's possible that the thermostat is not a factor. I would agree with not using that smaller thermostat in any event, but the absence of any apparent engine heat while the gauge is already above normal suggests the sensor, provided the wiring to the sensor isn't faulty (as rasterman mentioned in post #2 above).

There have been cases of an "over sensitive" temperature sensor unit that would indicate overheating when that was not in fact the case. Also, a high reading on the gauge corresponds to a low resistance in the sensor, which could be a faulty sensor, but also a short to ground in the wiring.

idosubaru mentioned the radiator cooling fans, and that could also be a lead. If the instrument panel temperature gauge is showing above normal, and the engine itself is still cool, are the fans running?
Checking coolant temperature with my hand on radiator cap. The gauge is above the red, into the "H", but isnt even hot enough to scald my hand.

It hasn't actually overheated with new tstat. And there is no regularity to the new fault, seems be more likely in freezing temperatures...

Fans are working.

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Discussion Starter · #11 ·
Does cabin heat follow the heat gauge trend?

When it's in the red - is there any cabin heat?

Got it you're paying good attention, taking good notes which is good. It's hard when we only read words - there was an overheating event, an attempt to repair it and now there is another cooling system related failure. That's possible but coincidental.

I'd look to repair quick. Overheating or not being able to tell if you're overheating is 4 digit dangerous.
Cabin heat isnt what it used to be. Warm at best, even with the climate set at 25*c.

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Checking coolant temperature with my hand on radiator cap. The gauge is above the red, into the "H", but isnt even hot enough to scald my hand.
That might not be a conclusive test of a bad sensor. If the car has been, and is, out in near or below freezing temperatures, it might take a long time for the radiator cap to get scalding hot. Also, there might not be a lot of coolant flow to and through the radiator when the ambient temperature is low, so the cap might not reach the same temperatures as in the engine itself.

The question is whether or not the sensor is faulty. To check it, we would have to know if the temperature of the coolant at the sensor is what the sensor is indicating.

When the cooling system and sensor are working properly, and the gauge is up at the red zone, the coolant temperature is above 120 C. To check if the sensor is working properly, the idea would be to check the temperature of the coolant where the sensor is located. It's mounted into the water (crossover) pipe that bridges across the top of the engine. It's not readily accessible, but an infra-red thermometer can be used. (https://en.wikipedia.org/wiki/Infrared_thermometer ) On your 2000-2004, the sensor is on the back of the water pipe, opposite the pointer in this diagram:



Check the temperature in that vicinity after the pipe itself has had time to heat up and the gauge is reading normal, or above. If the gauge is at normal, I would expect the water pipe at the sensor would be somewhere in the range of 80 - 90 C, depending on how long the gauge has been stable at the normal position, and how cold the water pipe was to begin with.

And there is no regularity to the new fault, seems be more likely in freezing temperatures...
When the thermostat was replaced, what was used to refill the cooling system?

Fans are working.
Fans should be working as soon as the gauge is even a pointer width above normal, if not before then. When do your fans turn on?

Has to be a sensor doesnt it?
It's possible, but based on the information we have so far, I don't think that can be said with some confidence.
 

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'03 outback limited, '01 Outback Limited, '01 Legacy L wagon, '96 Legacy Brighton wagon
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Have you actually verified that the thermostat is opening? If not, stick it in a pan of hot water ( with cooking temp probe) to see what temp it opens at. I ALWAYS do this with new 'stats before installing - and have had more than my fair share be faulty from the very start.
 
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