Okay, I know this is gonna be a 'newbie' question, but I just gotta know.

There is a tube that runs behind the engine, connecting the two intake manifolds. I presume that this is to equalize the pressure between the two.

However, at the bottom of the tube, there is a small tube that connects to a hose. My presumption is that this is acting as a vacuum source - but FOR WHAT??

Thanks!!

 

Good Morning. Its a good question, and one I had at the beginning of my journey... That is a vacuum line that runs almost the length of the car and feeds vacuum information to the cars ECU. You'll find where it terminates on the side wall of the ECU in the boot. Its important. Car will run terrible if at all without it connected... I recently did a top end refresh of sorts in mine, buttoned evvvvverthing up, started the car and could NOT figure out my mistake... I forgot to reconnect the vacuum line.

WARNING: under no circumstances allow that hose to drop down in the area behind the engine and transmission... There is nothing fun about retrieving it. Some have said it can become a major job... If you pull vacuum (with mouth or tool) on that small hose, it must hold a complete vacuum.

 

That is the balance tube. It idle control valve is on the left side of the engine, and it provides idle speed air to the right bank through the balance tube. The small port as noted above, provides the vacuum signal to the ECU. The port is on the balance tube to give average vacuum of both sides, in case they are slightly different.

The ECU is a speed density type, which means rather than directly measuring the mass of air coming into the engine, it calculates it indirectly. The vacuum level is the load signal to the ECU.