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I've searched around, but not found a conclusive post. After the car has been turned off, should the overflow reservoir make a hissing noise? Or should it hold a vacuum/air tight? I'm trying to track down minor coolant leakage (takes about 3 weeks for the low warning to chime).
 

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The reservoir cap contains 2 valves, one to release air to maintain the correct operating pressure,
and the other allows air to enter the reservoir as the pressure drops during cooling, this could hiss.
Look for pinkish white powdery residue near cooling system connections, on the 1.8 the CTS flange is common.
 

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Hmm I don't believe I've ever heard my cap hiss. It does seem normal, though--as tomvw mentioned, the system bleeds off pressure as the system cools down, so that is more than likely your hissing noise. Maybe try picking up a spare cap and see if the noise goes away?
 

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The cooling system should hold pressure building up from cool/hot coolant differences (when cold, volume of the coolant/air is lower then when hot) and the coolant cap should not hiss, IMO. Either you are building too much pressure (head gasket) or the tank cap doesn't hold like it should. The cap has a "safety" valve that I think releases when the pressure is too high (I am not sure as I have never seen it in action but that's what people say) but that is for a higher pressure than "normal".
My 0.02, I know Tom will be upset with me :)
 

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QTF
The cooling system should hold pressure building up from cool/hot coolant differences (when cold, volume of the coolant/air is lower then when hot) and the coolant cap should not hiss, IMO. Either you are building too much pressure (head gasket) or the tank cap doesn't hold like it should. The cap has a "safety" valve that I think releases when the pressure is too high (I am not sure as I have never seen it in action but that's what people say) but that is for a higher pressure than "normal".
My 0.02, I know Tom will be upset with me :)

Not upset gigi123 but it is boring and frustrating when you continually contradict what I say with your false guesses.
Do some research before contradicting anyone.
If you stuck to what you know and understand, you would be a good help on this forum.
 
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