Subaru Overheating issue

If it is loosing coolant and it is not a leak, it is most likely a head gasket issue. When having a bad head gasket coolant sometimes goes from the head gasket leak directly to the combustion chamber and the problem will not put oil in the coolant and vice versa. With the poor history Subaru has with head gaskets, this is the most likely cause.

I understand the poor history of the Subaru "head gasket" issue but I am not completely convinced there is not something more going on here which I haven't pinpointed yet! Especially since the overheating issue only occurs when I turn the heat control knob on (then I can watch the temp gauge needle rise). This exact same scenario happened twice so far. I will pick it up from the Subaru Dlr today and see if it happens the third time. I'll go from there?

Interesting observation: The Subaru Dlr came back the second time as I noted above, "You need to replace the head gaskets sir". However, I was not completely convinced and I didn't settle for that answer right away - or even the second time for that matter! When I picked up the car tonight, I KEPT THE HEARTER KNOB TURNED ALL THE WAY "OFF" for the entire trip home (12 miles) [I swore not to touch it - at all]! The gauge reached it's normal position (middle) within three miles (35º ambient temperature), I took back roads for safety reasons incase of an emergency pull over (no Hwys). However, it never overheated, not even close! I remained in driveway at home and left the car running for about half an hour (while watching under the hood for any strange occurrences) - but nothing out of the ordinary happened. Both fans finally started running towards the end of that thirty minutes. The temp gauge still remained in the middle and didn't budge! Wondering: How can a blown head gasket/s mysteriously repair themselves? Hum Through my internet investigation, one article pointed out, "If all the air is not burped out of the cooling system (some air could remain in the heater core), it could create a vapor/air lock when the heater knob is turned on requesting hot air in the interior, whereby, flipping a switch and create the thermostat to close (when what is inside the heater core is released. Resulting in a overheat situation, which mimics blown head gaskets! Conclusion so far: Blown HG's do not seem to be the problem, but more testing tomorrow.

Good one. Listen carefully. First the smaller corrections: turning the heater control on Subies ONLY opens an air vent door allowing passage of air past the always-plumbed (valveless) heater core. IOW the engine coolant system does NOT know where you've set the control. So that makes diagnosis a bit simpler; the fact that turning the heat "up" affects the tipping point of coolant thermodynamics is simply secondary, and based upon internal temperatures of the coolant bath; the instantaneous pegging of the temp readout gauge is a function of superheated gases trapped near the heat sensor probe...possibly in sync with your "jiggling" the system with the minor change in heat-sinking via your knob gyrations...but that's immaterial. No voodoo there. The important stuff: Subaru 2000-2009 2.5i SOHC motor head gaskets MOSTLY fail because too much coolant loss from a minor breach of the HG (usually at the #4 cylinder) results in sufficient accumulative loss that the motor overheats. replenishing the coolant is usually sufficient to temporarily remedy the condition unless the motor overheats long enough to "blow" the head gaskets INTERNALLY. That's the key. Like the prevalent catastrophic (Mt Vesuvius-like) overheating scenarios very common with the first gen '96-99 DOHC 2.5i, we've learned that a minority of the newer SOHC 2.5i can also have their HGs fail between the combustion chamber and the coolant ports in the head, resulting in an initially intermittent "percolation" of the hot combustion gases at high pressure (obviously) into the coolant system, "percolating" bubbles of exhaust gases into the coolant. If temps are low (ambient and/or short duration travel) the system may be able to tolerate some of this behavior, but eventually the tipping point is reached where the coolant system (mass and radiator) can't sink all the hot bubbles, and Mt Vesuvius occurs at the radiator cap. Most owners learn to allow the system to cool, then replenish the radiator...even learning to purge ALL trapped air bubbles by topping off after each high way run...only to find another explosion occurs later on. Eventually the system becomes sufficiently "porous" that constant bubbling of gases can be seem through the (carefully opened!) radiator cap at idle...or even in the expansion tank. At that point it can be difficult to drive even a short distance before another exasperating boil-over. The ONLY solution to this high-pressure HG failure is complete replacement of both HGs, paying careful attention to machine- shop service for examining the heads for warpage and cracks. You'll end up using the newer 3ply HG that seals better from the ubiquitous EXTERNAL coolant and/or oil seepages that plague all of these motors, sometimes innocuously as owners learn to keep their coolant levels up and/or tolerate the stench of an atmospheric oil leak onto the hot cat on the passenger side. Given that you have NOT evidenced a coolant leak on the driver's side, AND your description of the overheating landscape, I'm nearly 100% convinced that you have the "old-style" high pressure internal HG breach. You can get another fairly definitive test response by having a hydrocarbon sensor "sniff" the bubbles emanating from the coolant (rad cap or expansion tank if visible) to show exhaust gases present in the coolant. That nails the diagnosis. In other less defined scenarios I've seen both blocked radiators or blocked heater hoses restrict coolant flow sufficiently to cause momentary overheating either on the highway (leaves/mud/debris blocking the radiator cooling fins surface area), or even in the city (blocked rear heater hoses). So before you bite the $1500-1800 bullet, at least spray-clean the front radiator surface of debris, and disconnect and with a garden hose BACK-FLUSH the two heater core hoses under the windshield. If you see granular or sludge- like junk in the wash then you know someone tried to stop the overheating with the wrong coolant additive that collected up at the hoses to block coolant through them. (REMEMBER: the heater core is permanently plumbed in inline with the cooling system...NOT a parallel arrangement with control valves...so a blocked heater core WILL take down the system!) If you're convinced that the coolant flow is normal through the rad and core and the overheating continues then it's the HGs...especially if you see those dastardly bubbles! (Note: to aid in testing for percolation, as well for purging air from a recently-filled coolant system, I attach a wide, tight-fitting kitchen funnel to the radiator orifice, ading water/coolant to create a wide, level surface for ease of visual testing.. Get the motor to warm idle, turning the AC fans ON to keep the system fairly isothermal so that the level in the funnel doesn't trampoline up and down as the fans switch on otherwise. You'll then have a nice mini"lake" to watch for what look like air bubbles that percolate up either steadily or in response to mild throttling. But be careful: once the system's quite compromised raising the throttle can result in so much combustion gas pushing into the system that superheated coolant/gas mix can shoot up several feet and burn you! In that case replacement of HGs is critical, as you probably shouldn't drive the car except ever so gingerly over to a wrench who's VERY experienced in this work. Note also that unlike the earlier DOHC 2.5i, the newer SOHC 2.5i "takes" to HG replacement in stride (if the heads are uncracked and straight, of course!), operating another 100k miles easily. Just make sure you didn't warp the heads, as they can take only about 0.004" planing before they'll overheat from being too small! So should you do it? I like '06-'09 OB 2.5i a lot, as the VVT and simpler emissions system introduced in '06 is a nice setup, and the aluminum hood used in '06-07 is a bonus in the salt belt. So unless you need brakes, tires, wheel bearings and CV axles all at once, replacing the HGs is a good investment. But if you need to add another $2k for all the other stuff above I'd think twice. A good analogue to your situation I've used for naive customers is to imagine boiling a lidded pot of water when cooking pasta: sometimes it boils over, sometimes not, depending how much of those air bubbles can escape or be absorbed by the water/pasta mass and how much the heat is on...and the lid is cocked. Each time you drive now you're playing that boil-over tipping point game, as you no longer have a sealed "pressure cooker", eh? Hope this helps. Ern

Note that your HG breach almost NEVER occurs into the oil passages, so you won't see contamination there. Indeed the HC bubbles are invisible, but sometimes longterm percolation can result in staining the inside of the expansion tank with exhaust resudue (just like a tailpipe). Note also that unless you've replaced the OE big thermostat with a cheaper, smaller-orificed aftermarket one, the OE t-stat is NEVER the culprit in this scenario, so don't bother to replace it if it's the big OE one. I've tested so many of these and NEVER found a bad one, so don't waste your time. Also note that Subaru water pumps are also ultra-reliable, so when you replace the HGs DO of course replace the t-belt (since it comes off!), and test its tensioners, but leave the water pump alone to save a few bucks. Sounds like heresy, but remember that Subies are unlike Hondas, Toyotas and VWs that eat water pumps for lunch.

Thank you for your excellent reply to my overheating issue. You were very poignant with several possibilities, evaluations and tests which warrant further investigation. Guru: You can get another fairly definitive test response by having a hydrocarbon sensor "sniff" the bubbles emanating from the coolant (rad cap or expansion tank if visible) to show exhaust gases present in the coolant. That nails the diagnosis. Skydiver1500: I like definitive answers! I would like to do this test. Is the "Sniff" test something that a rookie can do? Do I need to go to a garage for this test (hydrocarbon sensor)? Guru: In other less defined scenarios I've seen both blocked radiators or blocked heater hoses restrict coolant flow sufficiently to cause momentary overheating either on the highway (leaves/mud/debris blocking the radiator cooling fins surface area), or even in the city (blocked rear heater hoses). Skydiver1500: There absolutely is NO dirt or debris blocking radiator fins. The Subaru Dlr replaced one of the radiator fans (Sub-fan they told me) because it was shorted (however, it's working now). Everything was very clean when I got it back! I also watched both fans running too since it's return. Both heater hoses at the firewall are very warm when the car is running at normal temp and I wrap my hand around each one of them, which proves that coolant is flowing though the heater core (even though the heater control knob is off and interior fan is turned off as well). No Voodoo! Is it possible that there is an air pocket remaining it the cooling system? What is the solution to burp all the air out (purging all air out of the cooling sys)? I'm almost certain that the heater core is clear because the air coming out of the vents went from very hot to very cold during an overheating situation Sunday night, and again on Monday when I drove it in to the Subaru Dlr

The heater core sits a bit high in the coolant system, so increasing volumes of exhaust gases percolating into the coolant rise to settle in the core hoses, allowing the core to significantly cool quickly as blower air cools the gas-full core, whereas a liquid-filled core would cool from airflow MUCH more slowly (about 50:1 as I remember air vs water specific heat?). Therefore, to purge all gases (exhaust and/or air) in the system is a bit difficult, as Subie eliminated the radiator's opposite-end purge hole after about '99. MOST of the time a few highway runs and re-toppings of the radiator (squeeze the top hose to "burp it" too) are sufficient to purge all air. But to be sure, it's better to raise the front of the car so that the rad cap is higher than the rear core hoses, install a tight wide-mouth funnel on the rad orifice, and overfill it a bit with 50/50 mix at warm idle. Any trapped air bubbles should work their way to bubbling out the funnel "puddle". After a few minutes you'll be convinced that there's no more trapped air...IF you don't have ongoing HG-related percolation of course! (Note that I've seen a core hose filled with granular goup that MIGHT have operated as a one-way valve...in other words possibly allowing some hot flow under pressure, but causing blockage when static. Not sure that matters, but disconnecting and back-flushing these two hoses shouldn't be ignored, as it's easy. Note also that Subie heater cores tend to NOT block...perhaps helped by the fact that they're always in the loop, rather than operating via water valves only in the winter. Get back to me AFTER you run your test procedures. Ern

It might be easier to park facing significantly uphill to tip the cooling system rather than jacking the front end up. (We have hills here in New England!)

Is there a "best answer of the year" button? Cuz Ern deserves it! Jaw. Dropped.

I'm blushing....

I've decided to take on changing of the HG's! I am now waiting for the special tools to come in (should arrive Wednesday 3/17/16) in order for me to remove the harmonic balancer (crankshaft pulley), and the tool to hold the cam sprockets to remove them for machining. Once I get the heads off I will then find out if there was any HG leakage that caused the strange problem I was having and also which noone could answer (except for HG replacement). I will report back to you on my findings.

Oh yeah, I forgot to mention. As I was disassembling, I removed the thermostat. I put it in a pot of hot boiling water on the stove, and it opened fine. Then when it cooled off in open air (out of the water), it closed very smoothly...so the thermostat WAS NOT my problem in case you were wondering (because I was).

If the t-stat is OE size (big bore) you can reuse it. But if it's AFTERMARKET (small bore), although it may open and close at correct temperature, it will NOT allow full flow at highway speeds, and thus cause boilovers. So check it against a new Subaru one, or measure the orifice diameter...whatever. You do NT need special tools for remving the balancer. Just block it and crank the starter; it'll loosen quickly. When retightening use a big screwdriver or pry-bar to block the flywheel and then just torque it on TIGHTLY.

You replacing your head bolts Sky? Also a good time to replace your coolant hoses, heater core hoses, soft fuel lines, other items? I like SGB's iridium spark plug idea (it's on my list for the future)

Walth's right, but I doubt the heater core hoses are problematic (unless plugged with Prestone "oatmeal" sealer!). Seems most modern iridiums are equal quality, whether NGK, Bosch, Autolite or Denso, so you needn't get BLR6EIX OEunless you can grab under $30/set on eBay like I do.

Guru - The T-stat is the original BIG-bore T-stat (no small-bore aftermarket T-stat). Do you still recommend testing it against a new Subaru T-stat since the original seems to be working fine? Wralth - I do NOT intend to replace the Head Bolts! Guru - What brand of 'Spark Plugs' come as OEM on the Subaru - and are they any of the ones you listed above?

Nope...just use the OE t-stat you have. But DO GET NEW HEAD BOLTS (part of the full rebuild kit with gaskets and seals). ..and torque them very carefully. Lots 'o guys botch this. If you're unsure let your machine shop mount the heads (if willing), to ensure you don't blame them for failure later from a non-existent cracked or warped head, for example. Wrenches routinely do this full job in 8 hours, but as a first-timer expect to take 12-16 hours to do it right!

The cheapest set on rockauto is $65 for all 12 bolts. Only one, Beck Arnley, states differences in washers (big/small). http://www.rockauto.com/en/catalog/subaru,2006,outback,2.5l+h4, 1432741,engine

Guru - I feel confident about torqueing the heads myself. Just got my Snap-On Torque Wrench out of mothballs and I had it recalibrated for the occasion. Also I'm not in a hurry to get the job done as I have another vehicle to drive while the Subie is down! I'd rather stick w/ pre-stretched head bolts. Can you reinforce your opinion so maybe I'll change my mind? (Have you ever known any pre-stretched [used] head bolts to snap)?

skydiver, I've been experiencing the same issue since May. Was fine in the winter months, except after a really really long drive. Once the weather warmed up, it would always overheat after a few minutes of city driving. Guru described the problem I've been experiencing down to a T. Percolation and all. So I've been burping it with a no-spill funnel, probably once every 3 weeks. Eventually new air builds back up and it's stressful while driving wondering when it will blow next. Is replacing the head gaskets truly the only permanent fix? PS - I know nothing about cars but I am 100% certain what the issue is based on what Guru is describing.

That's the idea Tony. Permanent is relative but it's the goal of replacing the head gaskets. check out this thread: https://www.cargurus.com/Cars/Discussion-t16961_ds756392 where I posted a couple links to youtube. Brian shows a couple things, one is "temporary" but can prolong having to fix the HG's for however long? by drilling 2-3 extra bubbler holes in the thermostat. I thought that was pretty clever and very simple. It still allows the car to warm up and be pretty efficient verses taking out the thermostat. Another suggestion I've seen is putting a connector tube to jump the heater core hoses together and I think that was on here somewhere.

Walth, thanks a lot for the reply even though this thread was 8 months old. I stumbled across that same video from Brian months ago on YouTube but wasn't brave enough to drill the holes since I know nothing about cars and I wasn't 100% sure it would solve my problem. But at this point, it's either try this or give up and sell it (and lose thousands), cuz there ain't no way I'm dropping nearly 2 grand to replace the head gaskets after wasting $700 on a new radiator per the dealer's recommendation. Funny thing is, I told the dealer exactly what Brian and Guru were describing. He goes, "Don't believe everything you read online." Okay yeah, unless you're on CarGurus! Anyway, thanks again guys and Happy Thanksgiving.

You're correct in that "bleeding" a t-stat is a dying wish...often used by wrenches to get an HG case ready to trade in and dump at auction. I've NEVER seen it work longterm. Interestingly I serviced a non-overheating but coolant-dripping '06 OB like yours last week, prepping my friend that he should budget for HGs. After replacing oil-soaked valve solenoid pressure switches (look at yours?) I found a small puddle of coolant under the front. Lo and behold: a tiny crack near the upper hose/cap area. This is a very rare case where replacing the radiator may solve the problem...except his 11 yr olf t-belt is cracking...so we're getting in there anyway; but only after being assured the HGs aren't leaking as well, now that the cooling system is back to full pressure. Getting sick of this...ah yes...it's Thanksgiving! Let's all stay safe, grateful, and kind.

Yeah no doubt it's temporary. It's just how long before it goes. If you aren't looking to have it fixed then I would just go ahead and trade it in before it happens. Personally, I'd rather spend the 2k and fix it verses buying different and losing some large sum of money on trade in. Read up on negotiating dealers or even buy one from Guru here ;) Happy thanksgiving everyone!

I HAVE A 96 OUT BACK AT REVVING UP AND DOWN I HAVE CHANGED IT 3 TIMES AND AT STILL DOING THAT SO WHAT ELSE COULD IT????? ANYBOBY KNOWS ANY THING WHAT WRONG WITH PLEASE GMAIL ME AT OZZY79949@ GMAIL.COM

Did you read the thread? At 22 years old it's probably time to toss this relic.

I have a 2006 Forester that over heats. I have tried everything!! New head gaskets about 4 yrs ago and within the past year i have replaced the thermostat, water pump, spark plugs and wires, there's no trapped air, and the fan seems to be running fine. Finally, about 3 weeks ago I had it in the shop again for new tires, wheel bearings, oil change, and inspection (which it passed). I am at a loss...

Three things to check: 1. determine that a LARGE hole (OPER spec) t-stat was installed, as cheap aftermarket ones restrict coolant flow on the highway. 2. radiator isn't blocked with front external debris, nor clogged from previous use of stop- leak sludge to address prior HG concern. 3. BACK-flush REAR heater hoses as the core is IN LINE with the entire cooling system; so blockage there also starves heat-sinking. 4. Perform a percolation test to see if you've an INTERNAL high pressure HG leak. This is unlikely if the new HGs were installed properly 4 years ago. The first 3 possibilities are remedied at minimal expense. The 4th is too high ($1.5k) to invest in a 12 winter old Forester. Report back. Ern

I have a 2015 Forester with 37k miles and everything is original. Just adding WS washer fluid- I smelled coolant. It's not visible- just an odor back near the heater core. The coolant level is fine. I don't think there is anything pressing. But, I'd love to know what I should be looking for in the future. Thanks in advance.

Not using coolant? Then are you smelling musty water puddling under the right side near the tire from normal A/C drain hose effluent?

hey all I have a 99 subaru forester with 187xxx km in the last 3 months i have replaced headgaksets with felpro with proper procedure machined heads and exhuast valves due to a cracked valve. oe thermo replaced water pump timing belt radiator just swapped last week. coil pack plugs cables injectors radiator cap 3 times battery intake manifold gasket exhuast gasket and everything possible but still air is getting in the system heater draws cold then boom over heat but the weirdest part is that no coolant is really leaking from what i can tell if so very minimal because usually it over flows the over flow reservoir and boils over from there. PLEASE HELP also bled the crap out of this before you try to say air lock. and headgaskets installed by an amazing shop.

I would take it back to the Amazing Shop! and tell them they have a problem that they need to fix under warranty.

Was the thermostat installed backwards?

I have taken the thermostat out completely and still it is getting air in the system I supplied the felpro headgasket kit brand new and now they will not cover labor costs or parts to take the engine out under warranty even though they never let me know of that. So I am in a bit of a rut.

Check for blocked rear heater core hose blockage. Backflush them to clear completely. This solved one that stumped me many years ago. Did Amazing Shop send the heads out to a reputable machine shop for testing? Hope they didn't shave them more than 0.004".

TheSubaruGuruBoston HELP! I’ll try to be specific and to the point. 2006 Impreza 2.5i (non turbo) overheating. Sometimes has heat sometimes not. Test drove it today after replacing radiator (after thermostat didn’t resolve the issue). It was fine with the heat on and actually had heat for about 4 miles to our destination. On our way home temp spiked. Any suggestions? It’s my daughter’s car and she is so frustrated as she saved up and was told this was a good buy. Actions taken: New heads and valves New thermostat New radiator Burped many times. Now seeing a lot of bubbles. No puddles though. Percolating right? Especially when the fan and/or heater is on. Water pump and belts had already been done recently. Please help!

I will do that but my heater is working fine and the weird thing is air is getting in the system

Lots of bubbles probably means a leaking head gasket.

Yeah...sounds like percolation.

Any suggestions? Mechanic is stopping over tomorrow to look at it. I’m praying for a simple solution. I just need to catch a break! And not a literal one!

Be prepared to spend $1.5-1.7k for a good HG job, including machine shop work and new belts and pulleys. Hard to digest for a 13-winter Subaru unless it's in particularly good physical condition. But if you need tires, brakes, and maybe some suspension or exhaust work as well (or soon), you may want to part ways.

Hey Ernie. Long time no post? We need you here to stave off Mark's misinformation campaign.

does mark post much anymore? I don’t follow on here too much, just responses to these thread like questions

I've been visiting 4 of the 'Stans in Central Asia this past month, including the other-worldly really weird Ashgabet in Turkmenistan. Phew.

Hey all so i continued to do everything by passed the heater core and still air in the system subaru canada is even stumped how am i supposed to fix this if no one knows wtf is going on. I can put a new engine in but im worried it may do the same thing!>?

Hmmm.... Ask your wrench if he torqued the head bolts very carefully.

Strange vacation destinations but I am sure you have a lot of interesting stories to tell.

Yeah. You should check out Ashgabat for post-apocalyptic weirdness with a night-time Disneyland surreality.

Just found this thread and read through it as GF has called with overheating 2005 Outback 4 hours away. This car has been doing OK, no noticeable coolant loss or overheating, fans run....heater outputs quite hot air. I checked fluid levels yesterday. It *has* gotten Minnesota cold in the past few days. One question: In re OEM stat vs others, what dimensions am I looking for? Thanks!

The OE stat is most probably ok, as they never fail. Check radiator front surface for debris blockage, as well rear heater hoses for blockage (back-flush if necessary). Purge all air from coolant system. I over-heating repeats check for percolation.

I got a response by email but can't find it on this board: A CarGurus user (TheSubaruGuruBoston) answered a question you are following: "Subaru Overheating issue". They wrote: "The OE stat is most probably ok, as they never fail. Check radiator front surface for debris blockage, as well rear heater hoses for blockage (back-flush if necessary). Purge all air from coolant system. I over-heating repeats check for percolation." GF is apparently getting home OK. Some helpful person helped her refill and purge..... On our Nissan van we had overheating due to the radiator being blinded with debris which apparently passed through the condenser but was caught on the rad. It wasn't easy to see with things assembled. So I have looked for this but will look again. What's this about "rear heater hoses?" This vehicle, I'm pretty sure, has only one heater core. Or so I'd assumed. I've not been into a Subaru in a long time. Lots of opinions around about head bolts, preferred gasket sources, and all that. SubGuru, have you don't a post that addresses this stuff and/or steps through the job? Daresay I have the skills to do it but at 68 I don't really have the joints and eyes for it......

Hmm that’s odd .. wonder if it’s hit a maximum and isn’t loading anymore responses. Usually it just takes a while to finally load the new responses.

XR826: Once radiator is clean disconnect rear heater hoses and back-flush with garden hose. If crap is exuded that might be the culprit. Repurge. This is all easy stuff just requiring a bit of diligence. Raise front or park uphill to help purge heater core. If you're convinced that the radiator is ok and the heater core isn't plugged and it still overheats you might check the t-stat to ensure it's the original wide bore ONLY if you suspect someone may have put in a narrower bore aftermarket one in the past. But again, the OE t-stats never fail...but that's where naive wrenches go first. While parked a bit uphill (or jacked up), you can attach a tight-fitting kitchen funnel to the radiator hole, fill it 2.3 way with water, turn on the a/c (to prevent fan cycling on and off and its consequent contraction/expansion dynamics), and watch the "lake" for percolating bubbles AFTER you're sure all trapped air is purged. Patience required here.... Eventually you'll have a quiet, stable coolant surface to watch as someone else blips the throttle up and down. If occasional bubbles emanate either continuously or with each throttle blip then you can verify that you have percolation of hot exhaust gases into the coolant from a bad head gasket by sniffing these bubbles with a hydrocarbon sensor. If the gases are just air then you're insufficiently purged (sometimes requires driving on the highway several times!); but if the bubbles have HC content toss the vehicle, as 1t's 13 yrs old and not worth the full proper HG investment ($1.5k+). 2005 was a mongrel year where the emissions system was exceedingly complex, trying to bridge between the old 2000-2004 spec and the new VVT heads 2006-2009 spec. Indeed, 5 O2 sensors is just too baffling when CELs appear for an aging one.

2004 subaru outback I have an interesting situation: About a month ago my subaru started overheating on the highway. I pulled off the exit and opened the hood. The reserve tank was full so I opened the radiator cap. This seemed to "Burp" the air and the coolant sucked back in after I started it back up. I did not have any issues again for three weeks. Now after the engine heats up (ten or so miles) the thermostat will start increase, but only at lower RPM's. If I can maintain speed or higher RPM's (neutral while stopped at lights) it will keep blowing hot air and even temperature. Here is what I have replaced to date: - Head gasket replaced at 80K currently at 140k -replaced radiator/radiator cap/Thermostat Yes its the correct large Bore -timing belt, tensioner, water pump -Heater Core The following tests have been completed: - Head Gasket pressure test (passed several times) - CO2 test on the radiator (Passed several times) - pressure tests on the coolant system (Passed several times) The mechanic is now telling me they have no idea what to do and recommend replacing the motor. Is there something I am missing? There are no leaks, the oil looks good, the coolant looks good....ANY help would be wonderful. At this juncture if it is not a simple fix I need to to run long enough to trade in. HELP!

What year was the head gaskets replacement? If before 2011 or so you might not have the newer 3-ply gaskets, wherein a best guess is that you're suffering percolation AGAIN, which isn't manifested except under load at temperature; thus it won't be found in static pressure tests. At 15 winters old I wouldn't make the investment ($1.5k) unless the trans and body, tires, brakes are good enough to warrant it. For the hell of it check the heater core HOSES for obstruction, as they're inline with the entire coolant flow. I had one where restricted flow from a partially-clogged one provoked percolation-like overheating. Reaming the hose permanently solved the problem!

JWo20004SUB If you have air sucking in I found out the hard way I had done everything you said in this post but I found out that my head gaskets were leaking slightly under head compression from the combustion in cylinder 2 and would shoot a bit of air into my coolant jacket just enough to build up over a small drive then start overflowling my coolant overflow and would not return due to the small leak it passed sll compression tests and headgasket leak down tests it was only when it was fully heated up we eventually got the slightest leak down test results possible I beleive it could be that when your headgaskets went previous it may have over heated to the point that it warped the block ever so slightly if you did not have the deck machined that happened in my case atleast and then eventually pushed on cylinder to leak under combustion into the coolant jacket the only other thought it a hairline crack in your head or warped head or a worn down hose but usually coolant would leak out of the hose aswell

Regardless of whether a head is warped, shaved too-thin, or the gaskets failed, it's likely there's percolation rather than inhalation.

1997 legacy GT 2.5 What if I’m not receiving any percolation in the coolent and my fans work fine still overheating I’ve got a “new motor” with 80k on it new water pump etc,new OE t stat, rad cap, radiator flushed several times I’m not convinced it’s my head gaskets because it would do the same with my old motor is that anyway it could potentially be a bad sensor or like guru said possibly a closed heater hose maybe I need a new radiator or if it’s really HG why am I not receiving any bubbles forming in my coolent. Also was driving one night with my buddy’s out here in Texas, made it to work and home then made from home to meet up for food, then drove 10 miles up road to meet at a parking lot as I’m getting onto HW going home not even 1 mile later temp peaks again first time all night. I pull over as most of us do we check under the hood what I find most interesting is then coolent res was over flowing but as I looked elsewhere I noticed my upper and lower rad hoses were completely sucked shut like if you were to suck on a straw while holding the other end tried everything (without opening the radiator) to get the hoses to open again (once it cooled down) I was able to just lucky make it back home 15 miles with 0 problems now I know this is due to the high pressure and negative pressure (vacuum) created by the cooling system itself but if it was my head gaskets failing wouldn’t it be sucking in air from the separation between the block and gaskets or is it only when it’s on and hot that the pressure is so great that it takes in air from those areas been dealing with this problem forever and still can’t seam to fix it even after a new motor I think this HG thing will be the death of me I know my cars probably not worth it but I’ve alre spent 8k on it in the last 2 years and money to do what I need to is not the problem but what is, is that taking it to 20 different mechanics around town and getting different opinions from everyone isn’t helping I’m not gonna blow 6 k on it to find that I was just one clogged hose or if a sensor was bad any way the overheating could be connected to the transmission I also get a “AT OIL TEMP” light that flashes 16 times on my dash (after car over heats and I turn it back on to continue driveing) I’ve read somewhere in a forum that that light has to do with a few things but they all deal with my trans could my trans be low on ATF or could it be a trans sensor plz let me know how this all my be connected if at all just don’t wanna throw away my beloved hunk of junk because I can’t for the life of me figure find the source of my problem

Also this used to not be a problem when I had the new motor put in drove all the wa to Arkansas and back in her over a 6 hour drive there and back plus around town maybe they were already bad when then put it in I asked my mechanic (thinking he would have checked them and replaced them wile it was out to begin with) but he never checked them or even compression tested it........ don’t worry I took it upon my self to do so I’m fourtunate enough to go to a aviation school when we have several of them but still it used to not be a problem maybe cause if the Winter cold yo there it wasn’t hot enough to overheat but even now (January in Texas some morning it’s like 30 + degrees and I can’t even make it 9 minutes up the road but if I get on the highway to go to school (30 minuets drive) it’s fine untill I get off the highway is there a pressure related item I’m missing or a cloge somewhere after since my last incident I bought a new rad cap (the old one had corrosion on the vacuum bleeder valve that’s fixed half the problem now I can drive smaller distances with ease but I’m still scared to go long distances and it still overheats mysteriously maybe it is my HG but I need unreasonable confirmation that this is a suspect it only had 65 k on the motor when I bought it I’ve up 20k on it in the last year (I used to live and hour from my job) so could that and the Texas heat (105degrees) possibly warped the head or is it more beyond that after 2 years of working in boxer airplane engine I know my way quite well around a boxer engine but of could airplanes are air cooled now days and cars are water cooled so that’s why I’m stumped there no reference I can pull from for my cooling situation

Phew! Decaf!!! "New" motor means used? Then it's HGs. If "new" is new, then it may be something else. Let's place bets....

What “else” could it be I’m going to try flushing the heater core and then checking my coolent temp sensor if the rad cap had corrosion on it then the temp sensor could too right?!

And sorry I know it’s a lot of questions but I’ve been messing with this for so long and I have all the parts to do it but I don’t wanna waste 1k on labor to do the HG if that’s not the problem

The coolant temp sensor mounted under the rear of the throttle body simply fails electrically internally, not from corrosion of its copper housing. As such, this sensor mainly controlls cold start richness through the ECU, so isn't likely involved. When you installed the "new" motor, had its HGs been replaced? At that time were the heads checked? I would add another experienced local brain to this mess to see if there's another clue. There must by now be a few such wrenches down south? I'm still worrying about a cracked head or breeched gasket.

Hi 2009 Tribeca - 2 year ago had HG done, waterpump, rad, Tstat, and heater core flush. Been fine for 2 years, now 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 that 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 heat from the cabin (some heat at lowest fan level but dies quickly if I turn the fans up). Temp stays in the 90's C while driving. Just had the system flushed and Tstat replaced (was an oem Stant - I know don't shoot me...). Thoughts on next steps - thinking of bypassing heater core to rule out that? Possible air lock - but why before and still after flush would that still happen - and wouldn't the car self bleed the air out after a bunch of warm/cold cycles? Thanks in advance for any help/advice

We have a 2011 Subaru Outback 2.5L. Overheating on highway- we replaced the Torque Converter, then replaced the transmission. All lights came off, was fine. Took it on the highway, AT OIL TEMP light starts flashing, break light on, etc. Buddy of mine sugguested exterior tranny cooler, and replacing ground strap. Just tell me this isn't a head gasket, I'm already out so much money.