What is the plastic plug in the back of the block?

What is the plastic plug in the back of the block?
Hey, if you want to begin diagnosing and adjusting you should probably spend the 18-20 bucks at the auto store on a Haynes or Clymer manual that includes your make model and year.
The troubleshooting section toward the front of the book is a quick way to narrow your symptoms, to the situation that sounds the MOST LIKE YOU. The rest of the manual will walk you through many of the money saving jobs (brakes, filters tune-up, etc.).
The tons of info. will pay for itself in a couple of fine tuning sessions, and the understanding of the different variables, (air, fuel, sensors, filters, etc..) will give you the insight into your chariot that will make some "LOGIC" of the engineers "fun little puzzles" Teach yourself and you will be able to carry a few tools/manual under the seat and fix almost any surprise that comes your way, but a cell phone is always a good back up!