Expansion tank cap hissing?

[ian_uk1975]

Embarrassingly basic question, but is it normal for the expansion tank cap to hiss (ie. allow pressure to escape) with an engine at operating temperature? My coolant temperature was around 95C when I shut the engine down after a drive and heard th cap hissing slightly. No steam or anything, just a slight hiss... if I pushed down lightly on the cap, it stopped. Normal???

PS. I'm running with anti-freeze in at about 60/40 mix ratio.

[Steve_D]

Sounds to me like it needs replacing.
Yes it is designed to release pressure when things get too hot but should hold pressure under normal conditions. The fact it changes when touched makes me think it is worn.

Steve

[ian_uk1975]

Will get a new cap then. Thanks.

[adyady1066]

Depending on the make and model but most expansion tanks have a pressure vent in the cap which gets stuck. Look on the under side of it and you'll see a copper like disc in it. Chuck it and see if it sorts it.