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I recently purchased a 2006 Honda Pilot with 207,000 miles and have put about 7,000 on it in the past couple months. I noticed the car was overheating one day (gauge spiked towards the right/red) while I was going up a semi-steep hill at ~55mph. I turned the heat on full blast which only ran lukewarm and cooled down to almost cold once the motor cooled down. I checked the coolant and it was just below the min line on the reservoir so I added blue Honda approved coolant up to the max line. The same problem continued to occur occassionally and I used the Car Scanner app to monitor the temps at all times which fluctuated between ~190f and ~235f. I replaced the thermostat and thought the issue may had been resolved but tonight on a long drive home up a steep highway the car begin significantly overheating, reaching as high as ~255f according to the Car Scanner app. I pulled over twice when it started spiking to these temps and it came down and settled to ~228f where it would remain unless I started driving again. The first time I stopped the coolant was at the bottom of the reservoir so I added coolant up to the max level but the second time I pulled over the coolant was far above the max line. The lower radiator hose by the thermostat did not feel that warm as if the thermostat was not opening completely or there was blockage somewhere in the system. Once I got back to a downhill area the motor finally cooled way down to ~181f and then stabilized at ~200f for the rest of the drive home, even when going up other steep hills at speed. The problem seems completely intermittent although much worse under load.

Is the newly replaced thermostat also faulty? Could it be the water pump? Radiator? A clog somewhere in the system? Head gasket? There is also only lukewarm heat coming out of the vents at full blast that only warms up a bit when the car overheats. Otherwise the car runs great with no other issues. I just changed the oil a few weeks ago with Castrol Edge 5w20. Can't wrap my head around why this problem is happening so intermittently. The engine fans kick on when my Car Scanner app reads about 230f and usually help to bring the car back down close to 200f but since this problem occurs while driving with heavy airflow through the engine bay then I don't believe the fans have any relation to the issue. The 255f spikes below were both after a long incline at ~60mph and both subsequent drops in temp were from cresting the hill and then coasting down the other side.

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I bled the system this morning according to a tutorial I found online that recommends starting the car with the radiator cap off and idling at 1500rpm with the heat on full blast until the engine fans kick on twice. A significant amount of bubbling came from the radiator and a fair bit of coolant spewed out. I was also able to get my heat working again by tightening the heater core valve screw near the firewall. But the intermittent overheating problem persists and the temp spiked again today every time I went up any sort of incline on the highway. When I arrived home I checked the reservoir and it was down at the very bottom (essentially empty). I looked everywhere for leaks but didn't find anything except for an almost imperceptible amount of what appears to be wetness on the drivers side of the radiator halfway down. I haven't noticed any drips or puddles under the car and check frequently

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My first thought was you need to burp the air out of the cooling system but you already did that. Were you on an incline when you did that? Were both the front ad rear heaters set to max?

Do you have sweet smelling white smoke coming out of the exhaust? That would signify coolant getting into the combustion chamber.

Does your tranny fluid look normal? I’m guessing it does otherwise you’d notice slippage if tranny fluid and coolant were getting mixed.

I’m just wondering where all that coolant you’re adding is going.
 

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Any chalky blue residue at the bottom of the radiator where metal fins meet the plastic base? That is where coolant ended up on my radiator when it started leaking.
 

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My first thought was you need to burp the air out of the cooling system but you already did that. Were you on an incline when you did that? Were both the front ad rear heaters set to max?

Do you have sweet smelling white smoke coming out of the exhaust? That would signify coolant getting into the combustion chamber.

Does your tranny fluid look normal? I’m guessing it does otherwise you’d notice slippage if tranny fluid and coolant were getting mixed.

I’m just wondering where all that coolant you’re adding is going.
I was on flat ground when I did the bleed. Front heater was set to max but I'm not sure about rear. I haven't noticed any sweet smelling white smoke but will check more closely today. Haven't noticed any water in the oil or sweetness in the oil either. I'll check tranny fluid today but haven't notice any slippage except on two strange occasions around 20-30mph where there was a slight "buck" and that hasn't happened in over a month. The car runs perfect and shifts perfect.

Yesterday I noticed the reservoir was overflowing causing coolant to drip down below the passengers side headlight. After a few minutes the coolant level went down in the reservoir to the normal level (between min/max) then once the car has shut off and cooled it goes down to the very bottom of the tank (essentially empty). This happened last night so I added more coolant to get it above the min then this morning it was back empty at the bottom of the reservoir


Any chalky blue residue at the bottom of the radiator where metal fins meet the plastic base? That is where coolant ended up on my radiator when it started leaking.
I've been monitoring the bottom of the radiator and saw others have that issue online but I haven't noticed any wetness or chalky residue there. The only spot I've seen coolant come out is overflowing from the reservoir when running but then it's below the min line once the car cools down.
 

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You might also check under the Pilot where the rear heater core is plumbed into the center console area. Perhaps it’s leaking at that location. For the amount of coolant you’ve added it’s gotta be going somewhere.

I’ve always burped the cooling system while parked on a slight incline. If you can’t find a coolant leak you might try that just to rule out a trapped air bubble being the cause of your issues.
 

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You might also check under the Pilot where the rear heater core is plumbed into the center console area. Perhaps it’s leaking at that location. For the amount of coolant you’ve added it’s gotta be going somewhere.

I’ve always burped the cooling system while parked on a slight incline. If you can’t find a coolant leak you might try that just to rule out a trapped air bubble being the cause of your issues.
I bled the car again today, this time with a filter kit to keep coolant from getting everywhere. I noticed I didn't have any rear heat or air blowing regardless of settings on the dash (tried turning on and off the rear manual A/C and had the dial in the backseat all the way to the right). I only filled the coolant to just above the min line in the reservoir since it had previously overflowed. The coolant bubbled a lot in the funnel on the top of the radiator. I ran it for several minutes with the heat on full blast revving to 2000rpm. I drove the car after and the temp still fluctuated between 185f and ~212f but didn't get as hot as I had seen it previously (255f) but I also didn't do any highway hills which is the only spot I've seen it get that hot. It also didn't lose any coolant today with the short 20 minute drive I did and is still just above the min line where I left it. I'll do a highway drive tomorrow or Monday and see if it loses coolant and overheats to 230f+ again. The lower hose also seemed to be hot sooner than it was previously as if the water was flowing properly and thermostat opening properly.

I haven't yet checked under the center console for leaks but will if it loses coolant again
 

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I'm thinking the radiator may be clogged and losing cooling ability. Unless fans are not coming on.
 

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Hmm, aside from a faulty thermostat (placement may also be incorrect?), I did some googling and this what I found. "You could be having a problem with your radiator being clogged up. This will cause the flow of the coolant in the system to be restricted, causing the truck to overheat. A clog in the heater core could also cause this problem. However, if the car is still getting heat, the heater core can easily be ruled out. The most likely cause is a clogged radiator."
 
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