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Unless you're towing something, the coolant gauge should never go above the center mark (3 lines). If it does, this is what you check/replace, in order:

1) Fan Operation (follow FSM)
2) Coolant hoses
3) Coolant filled per FSM procedure
4) Thermostat
5) Radiator cap
6) Heads/Head Gasket

If at any time you notice bubbling in your coolant overflow bottle, the sooner you face the fact that you're going all the way to #6, the sooner you'll find resolution. If there is no sign of bubbling or combustion gasses in the overflow bottle, that doesn't necessarily mean your heads/head gaskets are sound, as false negatives have been widely reported with the coolant/combustion gas tests.


That said, I have recently ascertained that the OEM upper LH radiator hose has a potential design flaw such that the bend nearest the radiator cap can fail and develop a pin-hole leak (or smaller) that initially may not be detectable when the engine is at idle or during a pressure test (apparently, coolant temp and RPM's need to be near the upper limit of operation). I have had two of these hoses fail - in virtually the exact same place - the first one was only discovered only after performing a stress test on the engine and the second was discovered by adding a UV dye to my coolant after I notice my overflow level was falling (I witnessed the leak in the former but never saw leaking coolant in the latter... only the dye stains).

Since the coolant is so hot in this situation that it evaporates almost immediately, the coolant leakage over time is virtually undetectable. If coolant levels aren't monitored regularly, this leakage can lead to overheating, which starts a positive feedback loop (overheating = more pressure = more leakage) that eventually leads to warping in the heads, which allows combustion gasses to reach the cooling passages, thus causing more overheating and additional warping. *** It's important to make a distinction here that this is not a failure of the head gasket, this is a failure of the head at the head gasket... therefore, it's absolutely imperative that you machine the heads for flatness, not just replace the head gasket (if you go through 1-5 without resolution).

I have not confirmed whether any of the other cooling hoses are prone to this same failure as I've only confirmed it in the upper LH radiator hose.

Realistically, the best way to prevent this is to regularly check the coolant level in the overflow bottle (at rest/ambient temp) and change the radiator hoses at the first sign that level drops significantly (i.e. an inch or so). Also, use the Subaru Coolant Conditioner anytime you change the coolant... this may seal the smallest of potential leaks. I am nearly at 100,000 miles since I machined the heads and replaced my head gaskets, and there hasn't been a single sign of overheating since. I replaced the radiator hoses prior to tearing apart the engine and replaced them again in the last 1,000 miles... so conservatively, consider the useful life around 100,000 miles on these hoses.

IMO - There's no need to flush the radiator or the heater core unless you've been using an additive other than the SCC (or confirmed the presence of debris/foreign material) or a coolant not spec'd specifically for the H6 (Subaru OEM or Peak Global). It can't hurt otherwise (provided you aren't using something that's corrosive to aluminum), but candidly, I think it's a waste of time/materials... drain and fill should be sufficient.
 

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Discussion Starter · #42 ·
Just an update - cooling system flushed, heater core flushed, thermostat replaced. No change in my issues - temp rises when idle to about 224F - can bring down by revving. No heat from vents (on lowest fan level there is heat - turn the fan up a few ticks and the heat will die out). Next steps? Was considering bypassing the heater core with a U shaped hose to see if that fixes the overheating issue and confirms I need a heater core.
 

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The reason why you don’t get consistent heating is because combustion gas is being driven into the coolant passages. This creates air locks and prevents circulation of the coolant through the heater core. I’ve had various manifestations of this issue due to head gasket failure twice and a cracked cylinder head. Poor heater function was always associated. Sadly, it appears that you do have a head gasket leak.
 

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@huskylord Just to give you some HG info since you are in Toronto - when I was searching in 2016 this is what the price range looked like (from dealership) $2.5k (Mississauga) to $5k (Budds) - used engine was also suggested ($1.5k + 10 hrs labour). John from Brampton Subaru was helpful - most shops were not experienced on this type of repair on the H6 engine. Ended up taking it to Autologix in Kitchener - Adam there knows the H6 engine very well.

Getting my heater core flushed tomorrow and will report back how that went wrt my overheating problem.
Thanks Darena,

The past few months I've been ignoring the problem, and then got stuck in a left turn that didn't move, but did see my temp guage start climbing and climbing. Got better the moment I turned my heater all the way up and started moving. I'm likely to change all the radiator components, but fear its the HG.

Please keep me updated with your progress.

Thanks again to everyone on this very informative thread.
 

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Hi All, just reviving this thread - I am the OP - overheating issue is coming back but I am catching it early and want to nip it in the bud and hope it is something simple. Recently I have noticed hat after the car is warmed up and I stop at a light my temp will slowly rise - after a minute or so it is up to 106C and goes up one tick over normal on the gauge on the dash. If I rev the engine it will go down to 92C or so. Fans are working correctly and kick in when needed - no obvious leaks. I also note I am not getting a lot of heat from the cabin. Temp stays in the 90's C while driving. I don't 100% trust my dealers and the independent that fixed it last time in 2+hrs away so would like to try some basic trouble shooting or selective maintenance first. Was thinking to try flushing the heater core again?
Thanks in advance.
Daren
Clogged radiator? Reduced capacity to dissipate heat, the falling ambient temps masked it from mid-sept until now. Flow text the radiator. It should be able to flow an average home garden hose volume of water.

If the gaskets were replaced once before and it comes down to needing gaskets again I'd get a JDM engine. The only way I'd want to install a 3rd set of gaskets is with a thorough understanding of cause and effect and contributing factors to failure, which seems uncertain right now.
 

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Thanks Darena,

The past few months I've been ignoring the problem, and then got stuck in a left turn that didn't move, but did see my temp guage start climbing and climbing. Got better the moment I turned my heater all the way up and started moving. I'm likely to change all the radiator components, but fear its the HG.
When does it run hot? Ignoring loss from boiling over the overflow tank, is it consumting coolant over time?

Coolant Leak

Radiator fans are classic contributors when issues happen at a stop like in this instance.

Radiator clogged.
Cap
Thermostat - can be boiled in a pot of water to check operation. Last tribeca tstat I suspected and replaced, I tossed it into a pot of boiling water and saw it only open about 1/3 of the way and it was caked up in deposits, so I knew I wasn't just guessing.

I'd rule those things out.
 

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Just an update - cooling system flushed, heater core flushed, thermostat replaced. No change in my issues - temp rises when idle to about 224F - can bring down by revving. No heat from vents (on lowest fan level there is heat - turn the fan up a few ticks and the heat will die out). Next steps? Was considering bypassing the heater core with a U shaped hose to see if that fixes the overheating issue and confirms I need a heater core.
Darena4, Did you end up needing a heater core? Did that finally solve the overheating problem? I'm having the same issues with my O6 Tribeca and am stumped.
 

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Darena4, Did you end up needing a heater core? Did that finally solve the overheating problem? I'm having the same issues with my O6 Tribeca and am stumped.
it’s not the heater core - absolutrlu

Answer the questions I posed in previous post.

It’s coolant loss (leak) or headgaskets. Rule those out and diagnosis them first.
 

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it’s not the heater core - absolutrlu

Answer the questions I posed in previous post.

It’s coolant loss (leak) or headgaskets. Rule those out and diagnosis them first.
No external coolant leaks, no signs of hydrocarbons, radiator cap holding pressure and throttle body gasket replaced. Overheats while idling only after long drives. Also, no heat from heater when the car is overheating.
 

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No external coolant leaks, no signs of hydrocarbons, radiator cap holding pressure and throttle body gasket replaced. Overheats while idling only after long drives. Also, no heat from heater when the car is overheating.
If true, it’s the headgasket.

“No signs of hydrocarbons” - How was that determined? Auto store hydrocarbon tests routinely give false negatives and aren’t reliable for Subaru EZ and Phase I EJ25D engines.
 

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Have the gaskets been previously replaced?

While it’s running and starts overheating check for bubbles in the overflow tank. Check it every time, not just once and done.

If it consumes coolant internally then you’re slowly loosing fluid and it will progressively get worse over time. At first it’s such a small amount - it’s just enough to cavitate or otherwise disrupt coolant flow but not seem like fluid loss. This is additionally complicated because overheating can result in coolant pushing out the overflow, making losses hard to determine.

Unfortunately if you’re overheating in a manner where it’s not drivable then it may not be driven enough to incur measurable coolant loss or other symptoms to make it obvious or easier to diagnose.

it needs the headgaskets replaced. To diagnose find a good shop with a gas analyzer or it’ll need to be driven until symptoms progress to a level more notable. But it is headgaskets so keep testing them.

If you are driving it then it’ll get progressively worse. Loads (hills, towing, AC usage) and heat usually exacerbate the issue.
 

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Thanks for your reply. Headgaskets is my guess too. Was hoping to hear an update from darena4 since he had replaced the head gaskets, yet continued to have the exact problem I'm having. Again, you're probably right, but would hate to go thru a head gasket repair, or JDM engine only to find out it was the heater core causing the problems all along. Will start checking for bubbles for definitive diagnosis. Thanks again.
 

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Thanks for your reply. Headgaskets is my guess too. Was hoping to hear an update from darena4 since he had replaced the head gaskets, yet continued to have the exact problem I'm having. Again, you're probably right, but would hate to go thru a head gasket repair, or JDM engine only to find out it was the heater core causing the problems all along. Will start checking for bubbles for definitive diagnosis. Thanks again.
Totally the right move to be thorough! always diagnose and don’t guess.

It’s the head gasket - keep checking for bubbles after those long drives once it starts overheating.

They progressively get worse over time - there are bubbles, it’s just a matter of time before you see them. Also keep testing it if you have the store bought block test kits. preferably as soon after an overheating event as you can. Or use a gas analyzer - that would probably confirm it but they’re thousands of dollars. I haven’t been involved with them enough to know if they can also give false negatives like the Liquid block tests but they’re definitely far better.

Keep in mind, people often don’t reply or resolve old threads. A response would be unlikely to be helpful. Likely to be anecdotal, misconstrued, and applicability to yours would be an order of magnitude less uncertain than my diagnosis.

If someone said “yes flushing or changing my heater core solved it”. The next step is to question that interpretation and still diagnose yours - same place you’re at now. if by some odd circumstance that’s correct, how would you confirm application to yours? flushing and changing the heater core won’t fix yours. You can attempt to diagnose it now if you think there’s merit to that posters experience and it did fix it. Pretend he said “yes, the only Subaru in existence to have fixed those symptoms via a flush or heater core!” and carry on to diagnose.

Subaru heater cores never get confounded with head gaskets by experienced Subaru people. They don’t cause overheating. except indirectly such as leaking causing fluid to get low. You’ve mentioned no smell, no wetness in cabin, no external leak, fluid isn’t low. And Subaru heater core to overheating correlations don’t happen in the real world.

Totally get it, it’s an enormous expense and inconvenience. But don’t let it cause to many rabbit holes either. One could question the water pump, fans, radiator...is your radiator clogged and so heat soaked after long drives it can’t dump heat?
 

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I'm new here.

I have an 06 Tribeca H6 3.0. with 270,000 kms (167k miles). I bought it a month ago at an auction for $3,000.

After 7 minutes of driving, It's temperature is spiking with 1-3 bars over the mid-range 3 lines. All this happens after a few minutes. If I'm accelerating, then it cools down a bit. There is heat in the vehicle but very moderate, sometimes it blows great heat at normal temperature but not when the temperature is high.

I was driving the other day and noticed the upper rad hose (long 36 inch hose) sprung a leak and the engine started smoking (from the coolant burning) It came loose from its upper clip and began rubbing on the pulleys. I replaced the rad hose and I changed the thermostat (not the OEM Subaru one). I noticed the old one looked brand new.

I burped it and bled the coolant from both air release valves. The relief valve near the engine firewall heater core had air. I filled it until coolant started spilling out then closed the valve (when the coolant spilled, it turned on the engine light with a flashing cruise control indicator and traction control light was on. I let it sit and it later went away. I then filled it up and bled the air from the radiator valve on the passenger side of the vehicle. Then ran it until warm revving the engine lightly until both fans engaged. I took it for a drive and the temp gauge spiking started happening again. The rad hose was not solid or under a lot of pressure but it had a couple bubbles that i squeezed out.

I have yet to change the rad cap or flush the radiator but I suspect its a head gasket issue. Maybe thats why it was in the auction. Also, come to think of it, it looked like it had some sort of metallic additive in the coolant overflow tank. Is that how the additive looks?

My question is: "Is it difficult to change the head gasket on an H6 Boxer engine and will I need to take the engine out of the car?"

I will first do the following steps before I attempt to change the head gasket.

1. Burp the car coolant system again
2. Replace the thermostat with a Subaru OEM
3. Replace the radiator cap with a Subaru OEM
4. Check the oil cap for coolant
5. Check the overflow tank for oil.
6. Flush the heater core.
7. Remove and flush the radiator.

If all that doesn't work, I've pulled a few engines before, so I'll wait for it to warm up then I'll change the head gaskets over a weekend.

Apologize for the long post - Just providing some insight for other users.

Stay tuned.

2006 Subaru Tribeca Boxer H6
DB
 

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I'm new here.

I have an 06 Tribeca H6 3.0. with 270,000 kms (167k miles). I bought it a month ago at an auction for $3,000.

After 7 minutes of driving, It's temperature is spiking with 1-3 bars over the mid-range 3 lines. All this happens after a few minutes. If I'm accelerating, then it cools down a bit. There is heat in the vehicle but very moderate, sometimes it blows great heat at normal temperature but not when the temperature is high.

I was driving the other day and noticed the upper rad hose (long 36 inch hose) sprung a leak and the engine started smoking (from the coolant burning) It came loose from its upper clip and began rubbing on the pulleys. I replaced the rad hose and I changed the thermostat (not the OEM Subaru one). I noticed the old one looked brand new.

I burped it and bled the coolant from both air release valves. The relief valve near the engine firewall heater core had air. I filled it until coolant started spilling out then closed the valve (when the coolant spilled, it turned on the engine light with a flashing cruise control indicator and traction control light was on. I let it sit and it later went away. I then filled it up and bled the air from the radiator valve on the passenger side of the vehicle. Then ran it until warm revving the engine lightly until both fans engaged. I took it for a drive and the temp gauge spiking started happening again. The rad hose was not solid or under a lot of pressure but it had a couple bubbles that i squeezed out.

I have yet to change the rad cap or flush the radiator but I suspect its a head gasket issue. Maybe thats why it was in the auction. Also, come to think of it, it looked like it had some sort of metallic additive in the coolant overflow tank. Is that how the additive looks?

My question is: "Is it difficult to change the head gasket on an H6 Boxer engine and will I need to take the engine out of the car?"

I will first do the following steps before I attempt to change the head gasket.

1. Burp the car coolant system again
2. Replace the thermostat with a Subaru OEM
3. Replace the radiator cap with a Subaru OEM
4. Check the oil cap for coolant
5. Check the overflow tank for oil.
6. Flush the heater core.
7. Remove and flush the radiator.

If all that doesn't work, I've pulled a few engines before, so I'll wait for it to warm up then I'll change the head gaskets over a weekend.

Apologize for the long post - Just providing some insight for other users.

Stay tuned.

2006 Subaru Tribeca Boxer H6
DB
That’s a great plan. Also check for leaks and fluid level over a period of time. I like to know how much it’s loosing.

It sounds like HG. Someone tried fixing it, knew/suspected HG or something ominous and traded it in. I’ve seen, and bought, and repaired numerous H6s from lots and auctions.

they’re significantly more work than 4 cylinder engines.

1. Check the timing cover bolts for rust. If they are rusty they can be a multi hour time sucker trying to remove 50 out of 60 stripped bolts. 2 minutes each means 100 minutes of stripped bolt party. I’ve got a method to remove them quickly and replace them with high grade metric bolts.

2. Resurface Subaru heads as a rule. don’t “check the heads for flatness”, epic waste of time on Subarus. They all have high and low spots you can see during the machining process even if they “pass” a flatness test. In my experience H6s are considerably worse on average than H4s.

2. Get the FSM online, I can probably send it to you as a thank you for your due diligence diagnosing and designing a plan of action! But it’s easily found online.

3. It’s fairly standard for H6s to be good candidates for a JDM engine replacement. Quicker and presumably you might get a warranty. A few years ago they were cheap - $600-$900 or something. They have become second fiddle to newer engines but are still out there. It’s a beastly job
Compared to H4s. $400 parts and machining....$600 engine and lots of hours saved....

4.It was clearly (or may be when you’re done diagnosing) sent to auction due to the head gaskets. Those are a prime candidate to throw a rod through the block in the coming months. I would:

a. Have an oil analysis done to check for bearing material.

b. check for additional signs of significant overheating - melted knock sensors/plastic close to engine block, damaged fender/light/battery from excessive coolant spraying from overheating events boiling out and spraying from the overflow tank. Lots of residue all around the overflow suggesting many overheating episodes. All of which I’ve seen before. If any of this is seen, JDM looks more attractive.
 

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Small leaks can be hard to spot and introduce air. They can burn off, blow off, puddle up, only leak under certain conditions. Not often and you’d probably smell it if it’s bad. But best to rule out even a low chance intermittent hard to find leak.

yours has alll the telltale markers of headgasket and you’ve described and looked over it very well, but diagnosing online without seeing the vehicle....isn’t diagnosis!
 

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Thank you for the quick response.

I've taken your good advice in looking for small leaks. I found a couple leaks. After the upper rad hose blew out - I started smelling coolant. I figured it was just burn off. That was 1 week ago, Im still smelling coolant. I decided to spray off the engine and clean the rear part of the engine. I gave it a good pressure wash yesterday. Today, after smelling coolant, I looked and noticed that I had 2 small leaky hoses, The lower rad hose was leaking from the clamp connecting the thermostat housing. Also, the small return hose (metal and rubber) going from the thermostat housing and running to the back of the engine was also leaking. I've also noticed that a lot of corrosion on the metal portion (the part that travels over the exhaust). I will also change these hoses (if available) and install an OEM Thermostat and rad cap today.

I will bring it into my garage and take a few pictures.

I also noticed some oil leaking from the valve covers (if thats what they are... are they actually on the sides of the engine?). They are just above the exhaust and thermostat cover.

I also think they dumped the car off at the auction due to the HG. I don't mind fixing it. It's a real beast on the road and tackles everything with ease.

Will update in 48 hours.

2006 Subaru Tribeca Boxer H6
DB
 

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Thank you for the quick response.

I've taken your good advice in looking for small leaks. I found a couple leaks. After the upper rad hose blew out - I started smelling coolant. I figured it was just burn off. That was 1 week ago, Im still smelling coolant. I decided to spray off the engine and clean the rear part of the engine. I gave it a good pressure wash yesterday. Today, after smelling coolant, I looked and noticed that I had 2 small leaky hoses, The lower rad hose was leaking from the clamp connecting the thermostat housing. Also, the small return hose (metal and rubber) going from the thermostat housing and running to the back of the engine was also leaking. I've also noticed that a lot of corrosion on the metal portion (the part that travels over the exhaust). I will also change these hoses (if available) and install an OEM Thermostat and rad cap today.

I will bring it into my garage and take a few pictures.

I also noticed some oil leaking from the valve covers (if thats what they are... are they actually on the sides of the engine?). They are just above the exhaust and thermostat cover.

I also think they dumped the car off at the auction due to the HG. I don't mind fixing it. It's a real beast on the road and tackles everything with ease.

Will update in 48 hours.

2006 Subaru Tribeca Boxer H6
DB
1. Address those leaks first. Coolant leaks are far more likely to get worse quickly, not be minor, and prone to serious complications (stranding, overheating and collateral damage) than signs of oil on the block.

2. Oil - that area is below the oil fill.Could be overspill and grime coagulating down below.

If it is a leak, first- you haven’t mentioned low oil or dripping so it could be (and almost always is in the beginning) a very slow leak. Monitor oil level and drips/loss over time. You can wait years or even ignore some very minor seeping

that area could be timing chain or valve cover. Need a pic or clearer description. Timing cover goes across front of engine left to right and meets the drivers side valve cover which goes front to back - in that area. The two sort of come together there.

valve cover is easy -I replace spark plugs at same time as there is a significant time savings. But shops are less likely to pass that saving on if you’re having it done somewhere else.

chain cover is a larger job. Needs pulled, cleaned, resealed.

both valve covers and chain cover has to come off for a HG replacement so I wouldn’t worry about the oil leak until much later unless you confirm it is more than just some residual long term seeing.
 

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Update: I replaced the thermostat and tightened the hoses. No leaks. After burping the car, the overheating started after 15 minutes of driving.

I ordered a set of head gaskets from PartsAvatar (Head Gasket Set - HS26534PT by FEL-PRO on PartsAvatar.ca). I will also order OEM head gaskets from Subaru. They arrive Monday. I will also replace the timing chain and all assembly parts.

I will document the entire procedure on YouTube and post pictures.

Below is the leaky pipe hose that i mentioned along with the leaky valve cover.
2430
2431


Wish me luck!


2006 Subaru Tribeca Boxer H6
DB
 

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Yep, it was almost inevitable.

That metal pipe is a coolant line but in the photo looks like oil on it. Is it leaking coolant or wet with oil?

Get an FSM, Resurface the heads and reuse the OEM head bolts unless they’re corroded or pitted. All Subaru heads have some high and low spots and the H6s I’ve done always need more material removed than H4s. Ignore FelPros one size fits all “use new head bolts” comment. Confusing, asinine to print that.

That being said, dealerships have very good success rates not resurfacing with OEM gaskets. Just seems easy to do when the ball is in my court and customers and book time don’t influence turn around time.

I assume you will, but replace the spark plugs with OEM NGKs while it’s apart if they’re not new.
 
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