Wondering if anyone can help - I decided to check the tire pressure on my 06 MCS, tires have these green plastic caps with "36" written on top (nitrogen).

When I try to unscrew them, I can hear the air hissing out, should be unscrewing just the plastic top and not the whole metal thing? Please see pic attached, thanks!

 

That "36" is the pressure those caps are set to. If your tire pressure is below 36 PSI, they will turn red. In order to take the reading, they need to be in contact with the air in the tire, so the cap itself presses the valve stem open. If you remove it all the way, the valve should close and you won't hear any hissing.

See here for details: http://www.accupressurecaps.com

 

Makes sense, thanks!