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Discussion Starter · #1 ·
I’ve got a frustrating overheating problem that I haven’t been able to resolve yet…. Here are the details, any insight would be GREATLY appreciated!

2001 Outback 2.5. Recently replaced head gaskets, timing belt, water pump, thermostat, radiator cap, upper and lower hoses. Thermostat and radiator cap are both Subaru. Head gaskets were replaced because they were leaking oil. It runs fine except will run very hot at high RPMs, usually when driving on the highway. Can drive all day on back roads, no issues. When it’s heating up you can turn the heat on full and it will keep them temp at a reasonable level. Sometimes if you turn the heat off then back on a moment later, air is cool. Not freezing, not hot. When this happens, temp goes way up. Pull over, turn the engine off for 30 seconds – 1 minute, start back up, turn the heat on, it is ok and the temp stabilizes. No leaks, no smell that I’ve noticed. I did notice when its running at a high temp the overflow fills right up but doesn’t overflow, just close. One you are off the highway and the temp is back to normal you can drive all day, no issues.

Exhaust leaking into coolant? Blockage in the radiator? Too much water in system vs coolant in the system? Air? I have burped the system.

I was going to test for the exhaust leaking and clean off the radiator (just in case) this weekend.

Any other ideas?

Thanks!
 

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Meh.
I has wagons.
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Are the fans coming on properly?
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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who did the work listed?

air or 'gas' seems to be pushing coolant into the o'flo. If you can see bubbles in the radiator while idling, you are a good candidate for the chemical HG test as sold at autoparts stores. It is a tube with a liquid in it that will change color when gas from the radiator is pulled through. Results are iffy when no bubbling is present though, and often Subaru gaskets only fail at speed/under load.

one could argue though, if you see bubbles whle idling, where else could that gas come from? (localized boiling maybe?)

when the system cools, does it pull the coolant back in from the o'flo tank?
 

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Discussion Starter · #4 ·
Yes, fans come on and stay on while its running hot.

I did all the work.

When the system cools the coolant is drawn back. I'm not loosing any coolant.... very strange.

When I bled the system, I was seeing some bubbles but, if I remember correctly, they stopped at some point. I let the car run about 20 minutes with the cap off to bleed it.
 

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Sometimes if you turn the heat off then back on a moment later, air is cool. Not freezing, not hot. When this happens, temp goes way up. Pull over, turn the engine off for 30 seconds – 1 minute, start back up, turn the heat on, it is ok and the temp stabilizes.


This reads like there is either a huge air bubble in the system and gets stuck in the heater core occasionally or the core is clogged.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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you could check the coolant system vaolume listed in the manual, then see if that precise amount can be drained out. If less, you had a 'bubble' of air/gas.

also, just doublechecking but, the t'stat was from the dealer, installed with the return spring up and jiggle pin forward? Anyway, you could pull it , put in a pan of hotwater on the stove, check with therm to see if it opens at 170* F.
 

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Discussion Starter · #7 ·
Yes, I picked up the tstat at the dealer and it is installed in the correct direction... I originally picked up the non-oem tstat but changed it out just to be sure.

Something I left out in my original post was before I changed the HG I had used a sealer for a temp fix until I could get the HG done... hmmm, maybe thats creating a clog somewhere?
 

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Something I left out in my original post was before I changed the HG I had used a sealer for a temp fix until I could get the HG done... hmmm, maybe thats creating a clog somewhere?
if you did not flush the cooling system when you drained it that could easily be the issue. that stuff, some of them any way, do not dry and clog until it reaches the air. so a system with sealer in ti left open to the air will dry and harden. and maybe in places you don't want it to.

back flush the heater hoses and core and flush the rad. it should flow out the bottom of the rad as fast as you can put it in the top with a garden hose.
 

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Discussion Starter · #9 ·
Another interesting development today from the ride home... most of my ride is highway so I put the heat on as soon as I got on. About 10 minutes in, started to run hot even with the heat on so I decided to take the back-roads so as not to risk over heating. I left the heat on and a few minutes later the temp was back to normal. When I stopped at an intersection I noticed almost immediately the temp started to go back up and the heat cooled off. As soon as I started back up everything was back to normal. Just another piece to the puzzle.
 

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Did the temperature swing like this prior to the work? It does seem like a head gasket failure.
 

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2003 Outback, 5 MT, 134K, HG changed.
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Did the car overheat severely before you did the HG job? If it did, did you have the heads checked and machined for flatness? I wonder if the heads may have warped before, and there is still a leak somewhere, even though you reported you're not loosing any coolant.

The other part I don't understand is why it would over heat at a steady cruising rpm on the highway, but be fine on back roads. I figured all the starting / stopping / rpm changes on back roads would cause over heating, whereas a steady load and lots of air passing over everything on the highway would make things cooler. Am I thinking about that all wrong?
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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Another interesting development today from the ride home... most of my ride is highway so I put the heat on as soon as I got on. About 10 minutes in, started to run hot even with the heat on so I decided to take the back-roads so as not to risk over heating. I left the heat on and a few minutes later the temp was back to normal. When I stopped at an intersection I noticed almost immediately the temp started to go back up and the heat cooled off. As soon as I started back up everything was back to normal. Just another piece to the puzzle.
definitely triple-check the electric rad fans. They better be working or you get no cooling unless you're moving.
 

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Discussion Starter · #14 ·
Did the car overheat severely before you did the HG job? If it did, did you have the heads checked and machined for flatness? I wonder if the heads may have warped before, and there is still a leak somewhere, even though you reported you're not loosing any coolant.

The other part I don't understand is why it would over heat at a steady cruising rpm on the highway, but be fine on back roads. I figured all the starting / stopping / rpm changes on back roads would cause over heating, whereas a steady load and lots of air passing over everything on the highway would make things cooler. Am I thinking about that all wrong?
It never really "over heated" but did run pretty hot a few times. The first issue with the HG was a oil leak that turned into coolant leaking into the oil.

I'm confused as well, I'm trying to put all the clues together and come up with what I hope is the right answer... we'll see.
 

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Discussion Starter · #15 ·
Sounds like coolant flow issues. Higher speeds the coolant isn't circulating fast enough.bad radiator etc
I'm hoping this is the case VS a cracked block etc... I'm going to try to chase it down this weekend and hopefully figure it out.

Could also be a small leak high in the system. Someplace where it can suck in air but isn't going to piss fluid?
 

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Make sure that the thermostat used was OEM subaru part the cheap aftermarket thermostats are known for killing subarus due to them not working well in the subaru system. Next would be a radiator which is plugged up and restricting flow of the coolant causing you issues at higher speeds when the cooling should be far better than the slower speeds.
 

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If your not seeing a drop in coolant then chances are your system is sealed and no leaks. A block crack is rare but not unheard of but I would expect you would see a drop in coolant levels if that were the case. The easy things and assumption would be some sort of flow issue at the higher speeds causing the temp rise vs it seeming to be OK at the slower speeds.
 

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One more possibility........You say you had lower radiator hoses replaced......If they are not stiff enough, the hose can collapse under suction from the water pump. This would only happen at high RPM, and would account for the heater not putting out hot air, until the engine is shut down and restarted......(Vacuum would be relieved when the engine shuts down).
If you can easily close the hose with your hand, this could be the issue. I don't know the design of these hoses, but if they are supposed to have a spring inside, and it was left out, that would be your problem. Good luck.
 

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Discussion Starter · #20 ·
Update: Did the block test it was negative (good news). Changed out the lower radiator hose, flushed the system and replaced the fluid. Drove 18 minutes to the highway (with the AC on) no over heating, on the highway for 5 minutes and the temp started to creep up. Put the heat on and the temp never got into the red, and did come back down to about 3/4. Got off the highway about 10 minutes later. About 2 minutes after that the temp came down. Shut off the heat. No problems for the rest of the drive, about 45 minutes. ???
 
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