sounds like there is too much pressure being created, eg running hot?. probably needs a good flushing of the system as the rad might be blocked and/or the thermostat could be stuck shut, its just like us normal pressure it can cope with, but excess pressure its the safety valve releasing the pressure, eg we cuss etc when hot and bothered?:cheesy:

 

Perfectly normal for the cap to leak or hiss slightly - there is a pressure relief valve in there. If the engine is running hot all the time you should investigate; thermostat, radiator, ignition timing, mixture etc.

 

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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Are you sure it's not something simple like a leaking pressure cap?

Are the radiator hoses getting very firm with pressure, or is the hiss evident when there's not much pressure in the system?

 

You might have a light h/g leak over-pressurising the coolant - but more than likely it's just a dodgy cap..

 

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.

 

The coolant does cool, but not before its temperature has risen due to no radiator effect.

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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