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2003 Forester
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6 Posts
Discussion Starter · #1 ·
Have an 03 Forester and have no trouble driving around town. When I take long car rides which is essentially to and from my school (375 miles one way) the car does fine on the highway but the moment I slow down it will start to overheat and boil my coolant. It will only do this after I have been on the interstate for a while. Usually my solution is when I stop to get gas or get off the interstate I stop the car but let the cooling fans run for 15 minutes or so. By that point it will have cooled to below operating temp and I am good to go for a while. Any ideas what may be causing this?
 

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2004 FXT 4EAT
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1,079 Posts
The engine will generate a fair amount of heat while you're travelling at highway speed. And it doesn't overheat because you have plenty of air flowing through your radiator (because you're moving "fast".

It shouldn't overheat when you slow down though. Is it possible that your radiator is clogged? (not able to cool your coolant enough) Are both of your cooling fans working properly?

I'm assuming you've checked your coolant level...
 

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2003 Forester
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6 Posts
Discussion Starter · #3 ·
@GuusV,
Both fans work properly and I have plenty of coolant. Only thing I can think to try before I start buying parts would be to bypass the fan relay and add a switch so I can turn the fans on full blast whenever I need to.
 

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2017 Forester XT Touring EyeSight
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964 Posts
I don't have an explanation, but one expedient band-aid, if you can stand it, is to run the heater at max temperature and max fan speed when you are nearing overheating. This was entirely sufficient for our Nissan minivan, to prevent overheating in stop and go traffic while we were suffering from a cremated radiator fan motor. This was in moderate eastern Canada summer temperatures, and of course you need to not run the air conditioning. We were fine with not even running the heater, as long as we were moving steadily faster than 10 mph or so. And of course, we had to have a full volume of coolant, and good radiator, water pump, thermostat, etc.
 

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2003 Forester
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6 Posts
Discussion Starter · #6 ·
@Kevin
No nothing has been done recently. Never had any cooling issues before. I was told water pump, radiator and hoses were done about 5k miles ago when I bought it. Since then I’ve probably put 15-20k on it. The radiator is missing some of its fins in one spot but I can’t imagine it would be enough the cause any issues at all.
 

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2004 FXT 4EAT
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1,079 Posts
There are a couple of things I can think of:
Are there any restrictions in the fluid-flow. As in: Do the bigger hoses look like there are any kinks or twists in them?

Is your engine generating more heat than it is supposed to? How long does it usually take to get up to operating temperature? And does it stay at operating temp on the gauge while you're on the highway? Or does it go up and down? Once the gauge gets to about half-way, the combination of radiator fans and thermostat are supposed to work together to keep the temp there.

Do you push it pretty hard when you're on the highway?

It's possible that the timing is out a bit (last person that replaced your timing belt got it wrong). This can cause your engine to run hotter than it is supposed to.
 

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2003 Forester
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6 Posts
Discussion Starter · #8 ·
@GuusV,
Normally takes 5-10 mins of driving slowly through neighborhood/city streets to warm up. When I’m on the interstate it normally stays pretty close to 45% of the way on the gauge but may vary up and down just a bit from time to time. I won’t lie I let that thing eat on the highway normally sitting around 3500-4000 rpm as it is a long and boring drive haha. I don’t think the timing would be an issue considering the first few times (probably 8ish) I made the drive I didn’t have this issue. The last 3 times is when it’s acted up. Do you think it could be something as simple as air in my coolant? I haven’t tried bleeding it yet but I don’t know how air could’ve gotten in to begin with as I don’t have any leaks and don’t burn any coolant to my knowledge
 

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2015 Forester 2.5i Limited CVT
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1,790 Posts
Check the following:
Thermostat
Coolant temp sensor
Is the coolant fresh? Subaru coolant.
Any parts replaced recently? Do the hoses feel hot? Overflow bottle ok? I would flush the system with a Lisle coolant funnel and replace thermostat and radiator cap.

You can also perform a coolant pressure test. Coolant flush: or burp air. May have trapped air.
Engine oil changes on time? Check level.
 

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2003 Forester
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6 Posts
Discussion Starter · #10 ·
@Theseus161,
Thermostat should be working fine because if I’m just driving around town i havnt had overheating issues. Oil has been kept up with. ECT hasn’t given me any reason to suspect it’s faulty. I can also feel thermostat open on the upper radiator hose. I have only been using Subaru coolant. I think flushing may be the next logical step as mentioned previously.
 

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99 L 5MT
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200 Posts
I just wanna say with these engines you really do not want to test your luck here. You will blow the head gaskets fast if you haven't already. I had my main radiator fan die & my car showed overheating only 3-4 times, but it definitely blew my head OEM MLS gaskets. I wouldn't drive this at all until you make some fixes, definitely start looking at the radiator. With these symptoms it seems like it's clogged. Does your heat work well? Heater core clog maybe?
 

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2003 Forester
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6 Posts
Discussion Starter · #12 ·
@Relaxation_Lover
STi head gaskets with 25k miles and oil has no signs of coolant currently. Heat works great. Just for peace of mind I’ve been driving around town for a half hour doing everything I can to try to get it to overheat and no luck. Only time the issue seems to happen is when I’ve been cruising at high rpm for hours then come to a stop. You think there would be any reason my fans may not be turning on soon enough?
 

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2012 Forester 4 speed auto
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1,200 Posts
At highway speeds the temperature stays below the boiling point of coolant and the coolant cools the engine. When the car slows down the coolant temperature goes up beyond the boiling point. If the cooling system cannot hold pressure the coolant boils, the engine overheats. So pressure check the cooling system at 15PSI.
 

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99 L 5MT
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200 Posts
I've been there man, same gaskets, did the same thing trying to get it to overheat. It almost never showed it on the very inaccurate stock temp gauge but in reality it was overheating every time I drove it. Search around in these forums & see how common this is with these engines. The temp gauge absolutely sucks. I couldn't believe how poorly this system was designed either but be careful & check for head gasket symptoms a bit closer than normal for a few months, it might not pop it's head up until later. It's easier than you think to blow these. I still very highly recommend that the radiator is checked before you drive this again, it could save you a ton of trouble. I can't think of what else could be causing this other than a radiator issue.
 

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2020 Forester Limtied
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12 Posts
How did you check the fans? Search Youtube for "P0483 cooling fan problem Subaru Outback". That should lead to the video from Eric O. of South Main Auto about a 2003 2.5L Outback that had a check engine light on. He couldn't identify the problem but showed how to test the fuses, relays and fans. The fast track testing starts at 5:00. You'll need a ODB2 scanner for monitoring engine temperatures to see when the fans kick on and off, to clear the check engine light after disconnecting the green plugs under the dash and to see if there are any codes present that didn't trigger the check engine light. Read the second comment down from Bluewrench61 about intermittently running fans. Also Mr Subaru1387 shows how he diagnosed a blown head gasket with one of those oversized spill-proof coolant funnels (Youtube again, search on "Subaru cooling fan diagnosis", jump to 2:38) and there are head gasket test solutions available to check for leaky head gaskets. Let us know what you find.
 

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2006 Forester XT and 1999 Forester L
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3 Posts
@Mcostoff, Is the coolant overflow reservoir low when cold and overflowing when hot? Sounds alot like my ‘99 forester which had internal head gasket leaks. After a long drive 10+miles open the overflow reservoir and if it smells like exhaust the head gaskets will need to be replaced.
 

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2003 Forester XS
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65 Posts
@Mcostoff
With our 2003 XS, we had an overheating problem. We eventually did a new head gasket, timing belt and water pump. With all that problems went away. We had out favorite local mechanic on Long Island do the work, at far lower cost than a dealer, though not cheap (dealers have a habit of charging for each of these things completely separately including full labor for each, even though there is a lot of overlap). I don't know what head gasket source was used.

Did you check your water pump?
Hopefully it’s not a head gasket issue.
These head gaskets don't have a super long life. We did ours and the timing belt and water pump (the last two because it was time, the first because there were signs it was going) and our mystery overheating problems went away.
 

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2015 Forester 2.5i Limited CVT
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1,790 Posts
The Subaru oem MLS gaskets are one and done. They were made for the turbo engine but are interchangeable.

The regular non MLS gaskets have a graphite coating that doesn’t last. Some dealerships won’t install the oem MLS gaskets. Crazy.

Our 2003 XS never had a HG issue with 148k miles on the clock. Frequent coolant and oil changes.
 

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2010 Subaru Forester
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2 Posts
@Mcostoff
The water pump may be failing. I had a Ford Contour where the vanes on the pump eroded away; it overheated at low speeds, cooled down at higher speeds. Did you replace the water pump with the last timing belt change?
 
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