Hey Tubaguy,
  The gauge will read the pressure of whatever is connected to it.  If
there are no leaks in the gauge or the connections to it, the air that's in
there will compress somewhat, but the reading will reflect the pressure of
the coolant.  I don't think that coolant would damage the gauge if some did
happen to get that far.   Go for it.
   You should watch the gauge all the time that the engine is running.  If
the temperature gets high enough the pressure may go above normal levels
since there will be no relief as provided normally by a properly working
cap.

If you remove the rubber from an old valve stem, the brass tube inside
gives you a fitting to attach your tester to an air compressor or a bicycle
pump if you want to pressurize the system yourself.

Have fun,
Larry A.

On Sun, May 21, 2017 at 3:31 PM, Neil N <musomuso@gmail.com> wrote:

> Hi all
>
> I've been reading up on DIY type cooling system pressure testers. I
> plan to re-use a broken WBX blue coolant tank cap to build my tester.
> I probably have a dial type tire pressure gauge around but....
>
> Can a fuel pressure gauge be used to test the cooling system pressure
> with vehicle running?
>
> Does the gauge read resulting air pressure only? In other words, does
> the fluid ever contact the gauge itself?
>
> Thanks
>
> Neil.
>
> --
> Neil n
>
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