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155K miles.

On two different trips, the first of which was about 1.5 hours, and second was 4 hours, I found coolant sprayed around the engine bay, concentrated on the passenger side. Outside temps were in the 90's both times. I inspected the radiator, and found no leaks. There are no leaks leading to a puddle underneath the vehicle or onto the skid pan. I pressure tested the system after the first occurrence, and no leaks were detected. All hoses have been inspected and I cannot find any active leaks or cracks in the hoses. The reservoir is not showing any visible signs of cracks. Normal driving does not cause this issue, and coolant levels do not drop. Engine does not run hot (according to temp gauge in cabin).

I suspect there is a blockage somewhere leading to high pressure in the system, and the coolant is escaping either through the reservoir overflow hose, or, as you see in the pic below, the radiator cap (the cap overflow did not occur the first time this happened). The cooling fan is obviously slinging the coolant to the passenger side. Could this be a sticking thermostat, or a failing water pump?

Hopefully these pics will help explain.
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Did you pressure test the radiator cap? What does the seal on the cap look like? It seems that is the leak point; I would probably start with a new cap.
 

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Did you pressure test the radiator cap? What does the seal on the cap look like? It seems that is the leak point; I would probably start with a new cap.
I did not test the cap itself since there was no leak from that area the first time it happened, although I did the pressure test through the radiator fill. However, after seeing this from Monday, I need to inspect the cap a bit more closely.
 

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Update

Thermostat and lower hose replaced. Pressure test revealed no leakage in system.

When inspecting the radiator cap, I discovered the "button" on the cap was missing (see photos below). I had an extra cap on hand from another vehicle that is spec'd to fit the Sequoia, and replaced it. Test drive went well, with exception of a slight increase in engine temp at 65mph. I turned the heat on full blast, and temp returned to normal. Possibly air still in system.

This morning I decided to burp the radiator, and when removing the cap, I noticed the button was missing from the replacement cap! I'm not sure about the first one as far as where it may be, but it's probable that the second one is in the system somewhere. Now I have one, possibly two obstructions floating around in the system.

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Looking down into the radiator while burping, I saw this sitting just to the left, lodged in the grooves inside. It wasn't moving around with flow of coolant. Just sitting there.
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I don't think you have anything to worry about. The parts from the cap can't get out of the radiator. The flow is top to bottom.
 

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I don't think you have anything to worry about. The parts from the cap can't get out of the radiator. The flow is top to bottom.
No risk of those pieces clogging up coolant flow throughout radiator, headed towards thermostat? I would think if they made it to the thermostat they would be stopped there.
 

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The only coolant flow through the radiator is through those little tubes that you can see from neck. As long as the pieces are bigger than the tubes, they aren't going anywhere. If it's something that's going to keep you up at night, you could remove the radiator, turn it upside down and shake the pieces out.
 

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Looking down into the radiator while burping, I saw this sitting just to the left, lodged in the grooves inside. It wasn't moving around with flow of coolant. Just sitting there.
View attachment 152608
155K miles.

On two different trips, the first of which was about 1.5 hours, and second was 4 hours, I found coolant sprayed around the engine bay, concentrated on the passenger side. Outside temps were in the 90's both times. I inspected the radiator, and found no leaks. There are no leaks leading to a puddle underneath the vehicle or onto the skid pan. I pressure tested the system after the first occurrence, and no leaks were detected. All hoses have been inspected and I cannot find any active leaks or cracks in the hoses. The reservoir is not showing any visible signs of cracks. Normal driving does not cause this issue, and coolant levels do not drop. Engine does not run hot (according to temp gauge in cabin).

I suspect there is a blockage somewhere leading to high pressure in the system, and the coolant is escaping either through the reservoir overflow hose, or, as you see in the pic below, the radiator cap (the cap overflow did not occur the first time this happened). The cooling fan is obviously slinging the coolant to the passenger side. Could this be a sticking thermostat, or a failing water pump?

Hopefully these pics will help explain.
View attachment 152579 View attachment 152580 View attachment 152581 View attachment 152582 View attachment 152583
This is a crazy coincidence. I just went on a trip with my truck camping. I noticed the smell of coolant when I got out after driving. Looked under the hood and Noticed evidence of where coolant had been leaking. Remove the cap and what do you know no button in spring And the coolant level is low well the recovery tank is all the way to the top. My truck was not overheating. I know that one poster was saying there was no problem having some debris floating around at the top of the radiator because it couldn’t make it down through the tubes. But it could float down into the manifold from the upper radiator hose. I’m looking into it now and will get back to you.
 

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This is a crazy coincidence. I just went on a trip with my truck camping. I noticed the smell of coolant when I got out after driving. Looked under the hood and Noticed evidence of where coolant had been leaking. Remove the cap and what do you know no button in spring And the coolant level is low well the recovery tank is all the way to the top. My truck was not overheating. I know that one poster was saying there was no problem having some debris floating around at the top of the radiator because it couldn’t make it down through the tubes. But it could float down into the manifold from the upper radiator hose. I’m looking into it now and will get back to you.
Also my tundra is a 2008 model year also with about 185,000 miles on it. I’m usually a maintenance geek and I don’t know how I overlooked not replacing this cap on a routine maintenance schedule. I think this is the original cap too.
 

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This may not apply as I have the 1st gen Sequoia but a year or so ago noted coolant spray when getting ready for a trip. Found that hose must have softened or something and lower radiator hose had somehow rubbed on serpentine pulley. It did not do it while parked and appeared to have plenty of clearance but found gouge on the hose that matched pulley edge perfectly. Something else to keep eyes on.
 
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