Idle vacuum for most engines is about 18 to 22 in. -Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

Why is vacuum higher at idle?

If vacuum is steady at idle but lower than normal, the ignition or valve timing may be retarded. Low compression, an intake leak or tight valves also can cause low vacuum at idle. Higher-than-normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing.

How do I locate a vacuum leak?

5:3210:41How to Find and Fix Vacuum Leaks - Ultimate Guide - YouTubeYouTube

How can you tell if you have a vacuum leak?

Symptoms of a vacuum leak include the Check Engine light, rough idle, stalling and a hissing sound coming from the engine bay. The engine may run well at higher RPMs, but surges, runs rough and struggles to maintain stable RPMs at idle. Often, the engine stalls when stopping.

Why does engine vacuum decrease with RPM?

Restricting the airflow and creating vacuum helps slow the engine down and limit rpm. The vacuum created by the piston pulls air from outside, through the intake, into the cylinder. As the throttle blades open, restriction and vacuum are reduced. This allows more air/fuel into the cylinder and rpm increases.