Is the fuel rail test port a Schraeder (sp) fitting?
 

I need to test my fuel pressure on my 2000 S and I bought an Actron gauge from Amazon that has a connection for a schraeder fitting. I haven't taken off the cap yet because the big supply house in AZ hasn't sent my new replacement cap yet. Is the Actron gauge going to work?

And secondly can I temporarily use a metal tire stem cap as a fuel rail test port cap?

Only if you have also purchased the correct adaptor, the gauge will not connect directly as the end fitting is wrong size...........

And no, you cannot substitute a valve stem cap; you need a new one in the correct metric size with a new seal inside.