Quick question...

When you open the expansion tank cap, is there supposed to be some pressure even after the car sits outside overnight?

i recently replaced the expansion tank cap. The old cap would release some pressure after opening it (hissing/bubbling sound) in the morning and cause the cold coolant evel to rise slightly. The new cap I put in didn't seem to make any noise or release pressure when I opened the expansion tank this morning.

What is considered 'normal' ?

 

Mine makes the slight hiss when opened after overnight park.

 

I just went out to my car on lunchbreak and opened my cap. Had a very very small hiss without hearing any sort of bubbling when I released the cap.

Wonder if this new cap is doing its thing and holding pressure? Temperature was fine this morning (didnt break 200F going to work from Scanguage). But I swear I did not hear anything at all when opening the cap this morning....

Both caps are rated 1.4bar / 20psi from Rockauto

New one is a GATES Part # 31331
old one is a STANT Part # 10246

Maybe I just have a case of Rover paranoia today....

 

This is something that concerns me. Is this normal?
Every time I check (open) this cap it makes this noise and the level of coolant goes up. Also no matter how much coolant I put in, it always end with the same level.

 

There is two kinds of hisses... By releasing pressure, and by releasing vacuum at opening.
Which is the one you notice is significantly different.

Coolant level slight rise should be because there was pressure before opening the cap and the hoses collapsed somewhat reducing its volume ('deflated'), pushing the extra coolant out of them into the reservoir.

Suction hiss has to be because the cap vented during hot use and when cooled there was less amount of coolant or the overnight temperature dropped more than the previous night.

 

Haha,
When I had bad head gaskets..
I would see the coolant tank was LOW.

Then if I took the cap off when it sat for a few hours or even over night - I would get a lot of pressure out

and there were bubbles coming into the tank and then the level of the coolant ROSE.

...
If you have just a little suction in the morning then that's OK as the mass of coolant contracted by cooling off.

If you have a whoosh going out - then head gaskets are part of your future plans.