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2003 - Overheating issue - fine on highway, moment I slow down, starts to overheat?

('03-'05) 
5K views 25 replies 14 participants last post by  Kevin 
#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?
 
#2 ·
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...
 
#4 ·
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.
 
#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.
 
#7 ·
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.
 
#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
 
#9 ·
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.
 
#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.
 
#11 ·
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?
 
#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?
 
#13 ·
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.
 
#14 ·
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.
 
#15 ·
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.
 
#18 ·
@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.


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.
 
#19 ·
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.
 
#21 ·
Interesting nobody has suggested the radiator. My first impression was the radiator is compromised to some degree. When driving around town you are not putting a significant load on the engine; the present condition of the radiator can handle the heat load. When you run out on the interstate for multiple hours at speed, the increased airflow through the radiator compensates for the inability of the radiator to shed the heat load. And I suspect the engine is running a bit hotter than normal but not hot enough to turn on the dash board idiot light. When you slow down after a good run on the interstate, the airflow decreases but the engine is still quite hot, as is the coolant, and the radiator at this point cannot accommodate the added heat load with the reduced air flow. The coolant temperature spikes and you have an overheat condition.
 
#23 ·
Interesting nobody has suggested the radiator.
Really! - did you actually read the thread? :rolleyes:

Is it possible that your radiator is clogged?
I was told water pump, radiator and hoses were done about 5k miles ago
I had my main radiator fan die
definitely start looking at the radiator. With these symptoms it seems like it's clogged
I can't think of what else could be causing this other than a radiator issue.
 
#22 ·
The biggest problem with the head gaskets failing is due to the cylinder head configuration. The coolant does not completely drain away from the gasket area when the engine is turned off. It pools and eats away at the gasket. It is imperative that the gaskets get replaced with a premium set and an anti corrosive additive is mixed in with the new coolant! Also, water pump, thermostat and timing belt with tensioners at the same time.
 
#24 ·
Under the category of simple stuff first, you might pressure check the radiator cap. If it won't hold the correct pressure, that will allow the coolant to boil at a lower temp. The same pump used to check the system can usually be used to check the cap. You can likely get a loaner at one of the chain parts houses.
 
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