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Discussion Starter · #1 ·
2006 pilot, 240k miles on it. Coolant overflow bottle is filling up on it's own after about 300 miles. Not boiling up and over, just filling up. When the car is off, I can hear is sort of bubbling, but it's not steaming and boiling, it's just sort of...bubbling I guess is the best way to describe it. I take the coolant out with a syringe from the overflow, put it back in the radiator, it fills the coolant overflow bottle back up slowly. After about 150 miles it's half full, 300 miles it's up way past the max mark. Something is causing the pressure and it's not creating the reverse suction it should to fill the radiator back up.

The thermostat IS NOT new, I haven't made it that far yet, it was replaced at around 180k miles by the previous owner but I'm not convinced the thermostat is the problem here? What do you guys think?

What I've done:

New denso radiator, new oem cap, oem antifreeze, new oem water pump, coolant overflow tank hose is in good shape, no leaks in it or anywhere (tested it with pressure). Coolant bottle is in good shape. Coolant overflow cap is in good shape and has no visible cracks. I checked the radiator with the blue chemical headgasket test kit and it passed, so I know there isn't exhaust gases from a blown head gasket coming into the radiator causing the pressure. No symptoms of a blown head gasket either, car runs perfect and seems to have good power, no white smoke, doesn't overheat and never has been overheated as far as I know, temp gauge is always in the middle like it should be. I've burped the system at least 5 times with the cap off, revving to 2k rpm with the heat on.

Could this REALLY be a thermostat problem? Stumped here. I know the thermostat is easy to replace, but don't want to throw money at it.
 

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2006 pilot, 240k miles on it. Coolant overflow bottle is filling up on it's own after about 300 miles. Not boiling up and over, just filling up. When the car is off, I can hear is sort of bubbling, but it's not steaming and boiling, it's just sort of...bubbling I guess is the best way to describe it. I take the coolant out with a syringe from the overflow, put it back in the radiator, it fills the coolant overflow bottle back up slowly. After about 150 miles it's half full, 300 miles it's up way past the max mark. Something is causing the pressure and it's not creating the reverse suction it should to fill the radiator back up.

The thermostat IS NOT new, I haven't made it that far yet, it was replaced at around 180k miles by the previous owner but I'm not convinced the thermostat is the problem here? What do you guys think?

What I've done:

New denso radiator, new oem cap, oem antifreeze, new oem water pump, coolant overflow tank hose is in good shape, no leaks in it or anywhere (tested it with pressure). Coolant bottle is in good shape. Coolant overflow cap is in good shape and has no visible cracks. I checked the radiator with the blue chemical headgasket test kit and it passed, so I know there isn't exhaust gases from a blown head gasket coming into the radiator causing the pressure. No symptoms of a blown head gasket either, car runs perfect and seems to have good power, no white smoke, doesn't overheat and never has been overheated as far as I know, temp gauge is always in the middle like it should be. I've burped the system at least 5 times with the cap off, revving to 2k rpm with the heat on.

Could this REALLY be a thermostat problem? Stumped here. I know the thermostat is easy to replace, but don't want to throw money at it.
Have you plugged in a scanner to see what the engine temperature is when it's warmed up. I'd want to verify it isn't going over 190°F.
 

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Have you checked after driving for a whe to make sure the it is not getting drawn in by the radiator after heating up. Also try running your heater on high. The closed valve might have trapped air in the heater core.
 

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Discussion Starter · #5 ·
Took it for a drive watching the OBD2 live data...176 degrees, drove around up the interstate came back, temps rose to 190 degrees at idle with the AC on revving it up to 2k rpm...
 

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Discussion Starter · #6 ·
Have you checked after driving for a whe to make sure the it is not getting drawn in by the radiator after heating up. Also try running your heater on high. The closed valve might have trapped air in the heater core.
Coolant bottle is completely full at 300 miles, it's not getting drawn back in.
 

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Coolant bottle is completely full at 300 miles, it's not getting drawn back in.
Coolant bottle is completely full at 300 miles, it's not getting drawn back in.
It seems you may be getting air drawn into the system if its not pulling in the coolant from the bottle. My suspicion would be a bad radiator cap, but you said you replaced it. What is the condition of the rubber hose to the bottle? If it's old, tie strap it to the radiator port and the bottle. It could be pulling air in instead of fluid. Just a guess.
 

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Discussion Starter · #8 ·
It seems you may be getting air drawn into the system if its not pulling in the coolant from the bottle. My suspicion would be a bad radiator cap, but you said you replaced it. What is the condition of the rubber hose to the bottle? If it's old, tie strap it to the radiator port and the bottle. It could be pulling air in instead of fluid. Just a guess.
I pressure tested the hose with a syringe under water...didn't see any bubbles.
 

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I pressure tested the hose with a syringe under water...didn't see any bubbles.
If you have properly blead the air out, and it fills back up with air, then there has to be a leak somewhere in the upper part of the system.
 

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I think Nail Grease is on the right track. They way the system is suppose to work is when the at heats up if there is excess coolant it goes to the overflow tank. When they system gets low and the system cools off it will siphoned the coolant out of the reservoir. So my first guess would be that something Is wrong with the cap-hose-reservoir part of the system. It may not be the hose but the connection between the hose and the radiator.

You said you have a syringe so try leaving the overflow hose connected but put the syringe on the end of the hose that goes into the coolant in the reservoir. Leave it all connected to the radiator. Then try applying both positive pressure and negative pressure to the hose and see if any of the connections are leaking. There are several places for a potential leak.
 

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A similar situation happened to the Pilot of someone I know. The black hose on the inside of the overflow tank had detached from the underside of the screw-on cap and was just sitting there inside the overflow reservoir. Therefore, while overflow from the radiator could travel toward the overflow reservoir, in the opposite direction it was just sucking in air. The result was, like in your case, the overflow reservoir gradually filling up, but the fluid level in the radiator gradually went down.

This was resolved by simply fishing the little black hose out from the overflow reservoir with needle nose pliers and, if memory serves, chopsticks and a barbecue skewer, then reattaching said hose to the underside of the overflow reservoir cap where it belongs.

I'm not saying this is exactly your problem, but first double check the that little black hose is properly attached to the underside of the reservoir cap (there's a kind of nipple there), and the other open end should reach down well below the minimum level mark on the reservoir. Once you're sure it is where it should be, start checking for anywhere along its path, both inside and outside the reservoir cap, where there might be cracks in the hose, looseness in connections or any way it might be possible air is getting sucked back into the radiator rather than, or along with, the fluid.
 

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Discussion Starter · #12 ·
All lines going into the coolant overflow have been removed and checked multiple times, all are good rubber with no cracks or leaks. I bought a coolant bleed kit to see if maybe that’s my problem even though I’ve bled it 5 times with the cap off. Only thing I can think of is that I’m either not removing all the air or I’m adding air when bleeding it without a loaded up funnel to burp out the air and not let any in.
 

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Discussion Starter · #14 ·

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Have you checked after driving for a whe to make sure the it is not getting drawn in by the radiator after heating up. Also try running your heater on high. The closed valve might have trapped air in the heater core.
Good point. Adding to this make sure the rear heater is turned on as well when bleeding/burping air out of the cooling system.
 

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How long are you taking to bleed the air out of the cooling system? When I replaced the radiator, hoses, and thermostat a few years back it took a good 20 minutes to burp all of the air out of the system. It was about 70 degrees outside that day. If you go by the Honda manual I believe the radiator fan is supposed to kick on at least once during the process. Mine never did but I chalked it up to it was too cold outside.
 

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When a head gaskets very first starts to go bad, it will generally only leak into the coolant system at higher rpm. Many times it won't show at idle, it would be worth redoing your combustion gas analysis test. Hold the rpm at 3kish for 15 minutes or so and see if that shows anything. I have also had a combustion test that didn't change color, but had the champange bubbles, confirming the head gasket was leaking.
 

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Just throwing out there, is your heater valve moving as it should?

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Discussion Starter · #19 ·
I burped the cooling system again for at least 45 minutes, it was still bubbling at the end of 45 minutes. Thermostat still needs changed to completely rule out a headgasket problem. I tested the coolant again with the chemical test off and on during that 45 minutes with absolutely no change in the color of the HG test fluid. OEM Thermostat it is next, I'm out of options. Weird part is, it was only bubbling when the thermostat was either closed or open, not sure which it was; when the coolant raised high into the funnel, it was bubbling a lot, but when the thermostat would either open or close (not sure what lowering means), the bubbling wasn't there and looked like it was moving through the radiator more. Not sure what that means tho.
 
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