Hey ClubLexus!
i recently purchased a 2015 GX460!
but it started to have intermittent overheating problem

It started with slight jerk of the temperature needle. Car temp calms down and doesnt overheat in traffic but only when underload it starts to creep up and down and up

here are what was found and replaced:
- upper rad hose was swollen and leaked, had it replaced.
- replaced thermostat and waterpump.
(weird that thermostat retaining plastic was broken off clean, found the part)

still overheated, (i thought the lower rad hose was too cold on the car) suspected clogged radiator.

- replaced radiator and fan clutch assembly.

Still overheats!
the temp doesn't touch the red zone but still overheating.

while burping the car, it took awhile to get heat in the cabin even at operating temp for awhile. Front started to blow hot after couple revs.
BuT the rear is still blowing room temp!!

i dont see the coolant being “lost” and also did exhaust gas detector test on the radiator too and it didnt trigger.

i am totally lost, the GX is my dream car and got this rough one that fit my budget but i didnt expect it to be this

any guidances?
thank you in advance!

 

head scratcher,I was thinking head gasket but U did that test. Maybe persistent air bubble trapped somewhere.

 

1) Low coolant
2) Trapped air bubble
3) Restriction via debris
4) Head gasket exhaust leak into coolant
5) Sensor/wiring issue
6) Cracked head or block
7) Grill mods?
Agreed, weird.

 

Congrats on the GX! It’s such a solid truck, but man, those cooling gremlins can be a headache when you first pick one up. Since you've already thrown the parts at the radiator, water pump, and fan clutch, that cold rear heat is actually your biggest clue. In these trucks, the cooling loop for the rear HVAC is huge, and it’s notorious for trapping air pockets that just won't budge during a standard bleed. If your front is hot but the rear is blowing room temp, you definitely still have air trapped in that rear line, which is likely causing those weird temp spikes under load. Try burping it again but get the nose of the GX way up on a steep driveway or some ramps and make sure the rear heat is cranked to max. Also, since you mentioned that broken plastic bit from the old thermostat, there's a chance a small piece of it is still floating around and intermittently blocking flow to the heater core or the bypass. If you haven't already, plug in an OBDII scanner with live data or use TS to see if the actual coolant temp matches what the needle is doing; if the needle is "jerking" but the digital temp is steady, it might just be a funky coolant temp sensor. Stick with it, once you get that air out of the rear loop, these engines are usually rock solid!

 

Define overheating. Are you judging this by the gauge or is the radiator boiling over? If it is boiling over a blown head gasket would also be a culprit. There are test kits to check for this.

I would start by hooking up a scanner to look at OBD live data.

 

many thanks! Coming from an FJ to this, such a big upgrade especially with a baby boy. Praying to get this resolved soon and be a reliable family cruiser.

that’s interesting! I will have to try to incline the front even higher to burp the air out of rear heater core lines. And if not, might try to reverse-flush the rear heater core in and out lines..

i was basing overheating off the temp needle on the dash, didn’t check the actuals. But that’s also a good point.

i am really hoping it’s not the engine internals like head gasket or debris i did test the exhaust gas detector thing off the radiator neck with engine running for good 10 minutes and it did’t turn color .

we will see if this car can make sense for us haha really hope it will. I will burp the system sometime this week and report back!

thanks everyone!

 

Let us know! The rear lines are a bit of a hike. Basically, they split off near the firewall on the passenger side and then dive down along the frame rail.

Something else you can do is, If you follow that rail back on the passenger side, you'll see two metal lines that eventually turn into rubber hoses as they head up into the rear quarter panel where the heater core is tucked away. Since you're getting no heat in the back, you should get under there while the car is warm and feel those metal lines near the frame if one is hot and the other is cold (or if both are just lukewarm), you've definitely found your blockage. Those plastic T fittings where the lines split off are notorious for getting brittle and clogged with bits of old gasket or even that broken thermostat plastic you found earlier. If you find a cold spot in the lines before they hit the rear unit, that’s your target.

 

Alright, tried jacking driver side way up and burp
and tried jacking the front way up and burp.

still the temp needle fluctuates and starts to boil in the burping funnel once the thermostat opens up.

double checked thermostat function and it works fine.

i touched the in and out heater core lines and they're both pretty hot all the way to the rear quarter panel. But the rear air was luke warm, so i tried sending some water to back flush and immediately saw coolant come through. Still warm air, got a little warmer but no where as hot as driver side.

I feel like the timing of overheating is to do with thermostat opening. But it’s weird because when I rev it out in neutral, temp seems to calm down. Could it be sucking in air from that area once thermostat coolant starts to flow?

also, when i reved pretty high in neutral, i did hear some hissing gurgle sound from the driver side footwell area. Air lock still?

oil looks pretty clean, no loss in coolant ..

 

Interesting, that "hissing gurgle" you heard in the driver’s side footwell is the answer, that’s the sound of air bubbles rushing through your heater core. It’s a classic sign of a stubborn air lock.The fact that the temp drops when you rev it out also points to air. When you increase the RPM, the water pump spins faster and finally creates enough pressure to shove coolant past that air pocket, which is why the temp calms down for a second. Once you drop back to idle, the air settles back into a high spot (like the heater core or thermostat housing) and starts blocking flow again, causing that "boiling" look in your funnel.

Since you've already tried the nose-up burp and it's still gurgling, you might be at the point where a standard gravity bleed just won't cut it. You should look into using a vacuum coolant filler. It sucks all the air out of the system until the hoses literally collapse, then uses that vacuum to pull the coolant back in so there’s zero room for air pockets. Most shops use these because GX and 4Runner rear loops are notorious for this drama.

If the lines to the back are hot but the air is still lukewarm, you might also have a stuck blend door in the rear HVAC unit, but that wouldn't cause the engine to overheat the air lock in the dash is definitely your main culprit for the temp spikes.

 

^Agreed, it's time to try a vacuum fill

 

Anyone else here perform their own coolant changes and encounter this? Seems like a bear.

 

have dealer do it?

 

I did my own coolant change about 3k miles ago and didn't run into any of these issues. I drained through the radiator petcock and both engine block drains, then refilled with no drama. To burp it, I just cranked the heater to max (low fan speed) and spent some time squeezing the upper and lower hoses to work the air out.

Same procedure with my previous cars, It seems like most GX owners have a smooth experience with a standard drain and fill, so what the OP is dealing with definitely sounds like an unusually stubborn air lock likely in that rear loop. The vacuum fill is probably the way to go at this point just to be 100% sure the air is purged once and for all.

BTW you can go to a shop to perform it or if you are willing to do it yourself its not a hard job but you will need the right equipment, basically a coolant pressure tester and vacuum and an air compressor.

Now, I'm here just thinking if altitude might be playing a role here. im almost at sea level im arounnd 14 meters above.

But if you’re at a high elevation, the lower atmospheric pressure makes air bubbles expand and lowers the boiling point of the coolant. That could explain why you’re seeing boiling in the funnel and why the air pockets are being so stubborn for you compared to my experience. It makes the case for that vacuum filler even stronger since it pulls a total vacuum and ignores the local air pressure entirely.

Out of curiosity, what’s your elevation? sorry to go a bit off, but im curious.

But as the OP mentioned that the upper hose was swollen and the thermostat plastic snapped suggests the system was previously neglected or ran very hot. When internal plastic bits break off, they can travel and get lodged in the narrow passages of the heater core or the rear lines, creating a permanent restriction that traps air, started by the original failure (the broken plastic/leaking hose) and is now being made worse by the difficulty of bleeding such a long, complex system.

 

I will be stopping by the parts store to see if vacuum coolant filler is available for rent. As I have a friend with compressed air.

i’m near or slightly above sea level as well.
the car was in rough shape in and out aesthetically, and while replacing the radiator, saw that it was badly replaced previously, missing bushings and bolts

Not the wisest car buying decision but the condition made it fit my budget haha i just hope it isnt something catastrophic yikes.. lol

i just hope it was badly burped and some bubbles were stuck.

thank you everyone,
I will report back !

 

Did anyone mention if the rear heater has a isolation valve? Curious if that could not be fully opening causing this issue and is the fan belt routed correctly if not then the pump is turning the wrong way.

just a thought.