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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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Any code/CEL? Thermostat (may have been installed incorrectly), dirty/contaminated coolant, clogged radiator, engine coolant temp sensors (there are two), internal air temp sensor, water pump, radiator/coolant leak, even air in the system could be the culprit. Thanks.
 

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Thank you for the reply. No codes/CEL. The issue happened both before and after I replaced the thermostat so I don't think it is related unless the new thermostat is faulty. I was certain to install it properly following multiple video tutorials online and I've installed thermostats in other vehicles previously (but won't rule it out completely and will re-replace it with another unit if necessary). I considered air in the system being a possibility but haven't found a great bleed tutorial for the pilot (my M3 has a bleeder screw on top of the radiator).

What would be the best way to narrow down which one of those components it could be? What's the most likely to cause such an intermittent issue? Sometimes it appears to overheat under almost no load so I don't think it's completely load dependent although it's definitely the worst up steep highway hills.

Let me know if there is any more information I can provide regarding the symptoms

Any code/CEL? Thermostat (may have been installed incorrectly), engine coolant temp sensors (there are two), internal air temp sensor, water pump, radiator/coolant leak, even air in the radiator could be the culprit. Thanks.
 

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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

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