I was told by my mechanic that these engines run hot and the best approach is to swap the OEM radiator with a bigger one.

He's wrong. Being that it's more obvious than a volcanic eruption next door, I'd filter future advice from this place. Do you know how many of these were "running hot" when brand new on the factory floor with 5 digit price tags?

None.
Zero.
Didn't happen.

If the cooling system is currently operational, with no issues or symptoms, and you're not trying to dance around some suspicion, then treat it like you would any other 13 year old vehicle.

Which might include replacing the radiator if you're the preemptive type, but most of the 100's of thousands out there are still running with original radiators.

 

the space between the condenser and radiator can sometimes collect dead bugs and debris - clean that out and clean the radiator fins.

if you replace thermostat and radiator cap, OEM is necessary - particular for the thermostat.

it can sometimes be tricky to get all the air out of the cooling system. Search for ideas but, getting the nose of the car up on ramps or jacks and occasionally boosting the idle speed will help purge air from the system while filling.

confirm both radiator fans are working.

if the little tube from the rad. neck to the overflow has no clamp, use 2 small zip-ties to clamp it. Take the other end out and cut the bottom of that tube at 45* angle. You can also remove the overflow and put some ice cubes in it, shake them around like crazy, to help clean its interior before re-assembly.

 

@idosubaru, this is exactly what I thought - so many cars during the years have survived with their OEM radiators. How is this possible? Apparently it's a niche business here in Bulgaria - one of the biggest Subaru shops made/developed a custom, bigger radiator for the H6 which is not yet on the market though.

@1 Lucky Texan, great advice list! Unfortunately I'm not skilled enough to do the change myself so it'll be done by a shop. And yes, only OEM parts.


So in the end I'll stick to a coolant change as initially planned.


Thank you for the inputs!

 

@idosubaru, this is exactly what I thought - so many cars during the years have survived with their OEM radiators. How is this possible? Apparently it's a niche business here in Bulgaria - one of the biggest Subaru shops made/developed a custom, bigger radiator for the H6 which is not yet on the market though.

Probably just misinformation. Are you sure it's common or are they just trying to sell you something because they're making it? If I bought everything someone told me was important, needed, and everyone else liked it I'd be broke.....or....

1. Wide spread misunderstandings about mechanical systems are common and probably vary by region or country. A/C systems, brake rotors, engine oil, stripping lug nuts, tires, brake fluid...and apparently radiators in bulgaria. And some of it comes from excellent sources of data or experience, but is misapplied.

2. If racing, offroad, towing, or other performance demands are common then additional cooling would be warranted. They will overheat if pushed hard. Maybe shops are taking this specialized need and trying to apply it more universally....confirmation bias, anecdotal reasoning...etc take hold and you've got a bunch of assumptions.

3. Maybe there's somethign about Subaru's in bulgaria. Subaru puts junk radiators in vehicles destined for bulgaria....I'm joking. Maybe people make lengthy remote wilderness voyages into mountainous, frigid, desert, dangerous ares where break downs are unacceptable. In which case items that cause stranding would be problematic and get heightened air play. But I doubt it.

There's lots of possibilities, but what you're describing is common.

 

Yeah, I'd just change the coolant, new thermostat and gasket for it, new radiator cap, bleed air out. Use the proper OEM Subaru coolant, and they do say to use the Subaru Coolant Conditioner (basically OEM stop leak to prevent head gasket failures). There is a procedure for bleeding the air out properly in the factory service manual, which you can find online. It's a whole procedure involving revving the engine a lot of times.
See if the o-ring behind the oil filter is leaking, and consider changing it and changing oil at the same time.
Don't think I'd mess with the Amsoil stuff. Not familiar with it, but likely a band-aid similar to Redline WaterWetter for hiding real maintenance issues.

While we're on this topic, sorry to hijack, but I think I have a coolant leak behind a little housing on the passenger side of my H6. Anybody know the name of that housing or part for sealing it? It looks shaped like a termostat housing, but IIRC the thermostat is on the driver side of the car.

 

While you've got the coolant drained - replace the thermostat with Subaru OEM and the oil cooler gasket, and the oil cooler gasket hoses if needed. Gotta drain the coolant to do those and the oil cooler gaskets always leak on these over time.

While we're on this topic, sorry to hijack, but I think I have a coolant leak behind a little housing on the passenger side of my H6. Anybody know the name of that housing or part for sealing it? It looks shaped like a termostat housing, but IIRC the thermostat is on the driver side of the car.

Oil cooler hoses. Replace the coolant hoses to it and the oring for the cooler itself. The oring only leaks oil, but given how rarely they make 200k or age without leaking I would replace that while you're attentive to the cooler and hoses. Requires a 26mm socket, it may be a deep socket, i'm forgetting which one I use for that.

or it could be an external headgasket leak. see below.

and they do say to use the Subaru Coolant Conditioner (basically OEM stop leak to prevent head gasket failures).

Subaru markets that for 4 cylinder engines and it helps with external coolant leaks only, not external oil leaks or internal coolant leaks. It will work in the H6's but is rarely needed and doesn't have to be put in ahead of time, so there are options. I would personally wait for symptoms on an H6.

Initial external coolant leaks of factory installed headgaskets (the only thing that stuff works for) respond with almost 100% success rates to adding the conditioner after they are spotted. So putting it in preemptively is personal opinion as there's little mechanical advantage, probably not enough to outweigh the downsides to additives in the event of future coolant issues.

H6's usually blow internally in which case the conditioner doesn't matter, and if they do start leaking externally they start as just a vague wet spot, very slow progression, and takes a long time to even progress to a drop or more than a drop, takes months and years. You have gobs of time to detect it and add the conditioner.

 

I'm the owner of one of these h6 powered outbacks that "runs hot." I have not been able to determine why it runs hot though. It could be mechanical, it could be normal, or it could be I'm an idiot and I'm pushing it harder than I should. According to Subaru, anything under and up to the line before it gets to "H" is acceptable, if conditions warrant it. I don't feel good about it though...

The engine really only runs hot when it's over 100° ambient air temp and I'm running the car moderately uphill, or at higher RPM's.

I've been looking for a source for an oversized radiator for my car. Where I live, it's hilly and it's hot for 6 months out of the year. My circumstances might be "extreme" but I would feel better about having the increased cooling capacity a larger radiator could afford me.

Could you please let me know where I can find more information about these radiators?

 

Ido - on the oil cooler seal, have you had regular success changing that? I'm torn between living with a small seep there vs changing it because I've read of people not having success changing it. Why would it be particularly tricky? Is there sealant involved or ???

 

Don't know. 100% success rate here.

What might be problems...aftermarket gasket, improper torque, trying to tighten that 26mm fastener without a proper tool...?

The only thing I can see being tricky is trying to replace it without disconnecting the coolant hoses. I think I've done it once and it's really not ideal nor provides great access for cleaning the surfaces and getting the gasket in there and cooler bolted up without debris on them. Draining and disposing of coolant is a hassle (there's no where to take coolant in my county) so maybe people try to twist the oil cooler out of the way with the hoses connected to replace the gasket, to avoid replacing the hoses.

Otherwise just disconnect and remove it all from the car so you can clean the surfaces well, make sure all the debris around the area isn't flaky/falling off while you're working/installing, and bolt the clean cooler on top of the gasket clean. have hoses on hand if you think they're old and brittle so you're not stuck monkeying with old hoses.

 

some diagrams at www.opposedforces.com are helpful

 

@aesthetic.rake it's not on the market yet AFAIK. I'm not allowed to post links but you could do a search for sti.bg in FB and then you can send a PM for additional information I guess since the page is in Bulgarian.

 

you guys heard of this stuff ?? https://www.redlineoil.com/waterwetter-coolant-additives

 

check www.opposedforces.com for some diagrams and part numbers.