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Old 03-14-19, 09:59 AM   #1
Grandprix
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Default 2 coolant caps

What is the second engine coolant cap and small reservoir on top of the engine center right?
What does it do?
Thanks for indulging my ignorance.
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Old 03-14-19, 10:06 AM   #2
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Default Re: 2 coolant caps

Coolant expansion tank.
when coolant heats up, it expands, so it needs to go somewhere, when the car cools down, it goes back.
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Old 03-14-19, 10:13 AM   #3
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Default

What, it doesnt do that in other cars?

Pretty sure thats exactly what expansion tanks are for, and all cars have em. They’re not sealed/pressurized.

I think it’s just a way to fill from the highest point. The expansion tank doesn’t need to be the highest, and isn't. The top tank, whatever we cal it is sealed, and in fact, if it is filled with coolant, there is no space for expansion
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Old 03-14-19, 10:44 AM   #4
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Default Re: 2 coolant caps

Like mheyman said I believe its the highest point and helps with filling/bleeding the coolant system.
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Old 03-14-19, 10:44 AM   #5
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Default Re: 2 coolant caps

No, the expansion tank is to the left of the main radiator cap. This upper one is a pressure cap as well.
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Old 03-14-19, 10:49 AM   #6
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Default Re: 2 coolant caps

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Originally Posted by Grandprix View Post
No, the expansion tank is to the left of the main radiator cap. This upper one is a pressure cap as well.
Like i said, its an expansion tank, sti has two of them, the one next to radiator and the upper expansion tank that you are asking about.

Last edited by RussellX; 03-14-19 at 10:53 AM.
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Old 03-14-19, 11:36 AM   #7
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Default Re: 2 coolant caps

Thanks. Told you I was ignorant.
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Old 03-14-19, 12:01 PM   #8
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Default Re: 2 coolant caps

That makes the entire coolant system an "expansion tank" since it is all under pressure. An Expansion tank will be VTA and allow the coolant expansion to be recovered... That tank is part of the system. I'd say it's more of a thermal siphoning tank for the turbo.
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Old 03-15-19, 04:09 AM   #9
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Default Re: 2 coolant caps

Quote:
Originally Posted by yamahaSHO View Post
That makes the entire coolant system an "expansion tank" since it is all under pressure. An Expansion tank will be VTA and allow the coolant expansion to be recovered... That tank is part of the system.
Yes, and regardless of what parts people call it (and w'ell have to too) if function is not an expansion tank. If you fill it with incomprehensible coolant,there is no space for expansion - Additionally and there is a lower pressure path for that expanding fluid to go, out the radiator cap into the actual expansion tank.


Quote:
Originally Posted by yamahaSHO View Post
I'd say it's more of a thermal siphoning tank for the turbo.
You've written this before, please explain. I don't even know the full coolant system, but I'd be willing to bet all necessary and engineered coolant flow is due to pump pressure.
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Old 03-15-19, 05:34 AM   #10
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Default Re: 2 coolant caps

Quote:
Originally Posted by mheyman View Post
Yes, and regardless of what parts people call it (and w'ell have to too) if function is not an expansion tank. If you fill it with incomprehensible coolant,there is no space for expansion - Additionally and there is a lower pressure path for that expanding fluid to go, out the radiator cap into the actual expansion tank.




You've written this before, please explain. I don't even know the full coolant system, but I'd be willing to bet all necessary and engineered coolant flow is due to pump pressure.
Hot coolant from the turbo goes up to the reservoir, which is placed at the highest point, which draws in "cooler" coolant from the bottom.
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