No hissing when coolant cap is opened

[paker]

I don't hear a hissing sound when I open the coolant overflow reservoir cap after engine is cooled. On my other Ford I hear a hissing sound even after a few days of parking. Should I be concerned? I see no sign of coolant and oil mixing. No sign of coolant leak. Thank you.

 

EDIT: Coolant reservoir cap was replaced a few months back because of no hissing. Still no vacuum leak sound. 

Edited July 16, 2019 by paker

[vega]

I really don't think these reservoirs hiss at all.  I have never heard a hiss  from my two vehicles, 2012 and 2015.  But I do make a mark with a magic marker when it is full cold so that I can monitor any leakage.  Every time I lift the hood for any checks, one of them is to give an eyeball to that mark to make sure all is OK.   Cheers.  

[paker]

Thank you for the confirmation. Coolant level has not gone lower over last 2 months (of no hissing). By the way, it hisses when you open the cap while coolant is warm. The vacuum doesn't seem to hold as long as my other Ford (pickup).

Edited July 17, 2019 by paker

[1004ron]

17 hours ago, paker said:

Thank you for the confirmation. Coolant level has not gone lower over last 2 months (of no hissing). By the way, it hisses when you open the cap while coolant is warm. The vacuum doesn't seem to hold as long as my other Ford (pickup).

It has a one-way vent to allow pressure to build when hot, and not allow it to draw vacuum as it cools, ........ to prevent collapsing the coolant hoses, and drawing in dust/dirt into the coolant pump seal.

[paker]

That's smart, really smart. And I am much relieved. I really thought no vacuum meant a leak somewhere. Thank you for the explanation.