Intake Vacuum

• The intake vacuum caused by the pumping of the pistons and the throttle restricting air flow into the engine sucks fuel into the engine. The intake vacuum typically remains steady at between 16 and 22 inches Hg on the vacuum gauge.
  
  

Lower Readings

• A reading lower than 16 inches Hg, combined with symptoms like rough idling, stalling and misfiring usually mean there is a vacuum leak. A reading that oscillates can indicate that a valve or worn valve guides are leaking vacuum.
  
  

Finding Leak

• After using a vacuum gauge to determine there is a vacuum leak, a mechanic must trace the site of the leak. This can be done by visual inspection, a smoke machine or an exhaust analyzer .