What is the function of vacuum tube between valve cover and intake branch pipe of automobile engine?

This is an exhaust gas recovery pipe. It allows the descending airflow of the engine to enter the engine to participate in the secondary combustion, which can effectively reduce the pollution to the ambient air. However, in order to prolong the life of the engine, it is better to hang the pipe directly: the hole in the intake manifold is blocked, and the valve cover hole is hung after passing through a pipe of about 80cm.

If it's a new car, there's no need to hang up. If it's an old car, especially a carburetor car, the idle speed is not good, and you stop at high speed, just hang up.

For reference only.