Curious whether it's possible and how to do tighten up the throttle cable on my 2004 Toyota Echo manual.
The Toyota Echo (and most cars pre-2011ish) uses a cabled throttle assembly, so you should be able to tighten the cable fairly easily.
Look for the following in your engine bay
You see the nut to the left of the arrow, that's where you can tighten the system.
A good link is http://www.toyotaownersclub.com/forums/topic/112409-a-guide-to-adjusting-the-throttle-cable/ , which is for a Corolla, but covers the basics of cable adjustment for cabled throttles.