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Coolant pressure cap slight hiss?

32K views 8 replies 8 participants last post by  900t  
#1 ·
My coolant pressure cap makes a slight hiss when the car is hot, but shut down. I notice it when I am under the bonnet for some reason.

If I squeeze the hose, I hear air escaping. The cap looks fine, but I am assuming that it is NOT supposed to ever hiss. I am not leaking coolant, there are no other signs that I am aware of, except...she runs hot.

Time for a new cap? I would think so, but I am just checking if anyone has the same deal, or knows the answer before I plunk down the $.

regards,
erns
 
#4 · (Edited)
I'm more concerned with the hot running.

Antifreeze contains an additive which raises the boiling point of the water. If the coolant's old (you should change it at least every two years) then it may be boiling, causing excess pressure. That pressure will be vented through the cap. Unfortunately, that release of pressure lower the water's boiling point some more (water under pressure has a higher boiling point).

Is the radiator original? When was the thermostat last changed?

Read this:
http://townsendimports.com/Web/cooling_system_folder/coolingsys.htm

On the other hand, the expansion tank's cap may be faulty and leaking. When that happens, the coolant boils and excessive pressure will build up in the cooling system. I had that on my 900, and a replacement cap solved the problem.

However, you should thoroughly inspect and, if necessary, service the cooling system.

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#7 ·
You mentioned that the hissing occurs when the car is hot, but shut down. When the engine is no longer running, the coolant cools down and contracts. In order not to cause an underpressure in the system, the vent valve in the cap allows air to enter the system as the coolant contracts and may cause the hissing sound, but check if there is any obvious leaks or loss of coolant.
 
#9 ·
Matthew said:
I'm more concerned with the hot running.

Antifreeze contains an additive which raises the boiling point of the water. If the coolant's old (you should change it at least every two years) then it may be boiling, causing excess pressure. That pressure will be vented through the cap. Unfortunately, that release of pressure lower the water's boiling point some more (water under pressure has a higher boiling point).
The ethylene glycol doesn't break down in and of itself. The boiling point of the coolant should remain relatively constant. In fact it should rise as crud in the engine is suspended in the coolant, thus reducing it's vapour pressure :)