A vacuum gauge is a device that measures the amount of air pressure in a vehicle's engine. Air pressure in a properly functioning idle engine should be lower than the outside air because the movement of the pistons pushes the air out. Vacuum gauges measure in inches of mercury: The higher the reading, the less pressure there is. When it reads zero inches, the pressure inside the engine is equal to the normal air pressure outside. You can use the vacuum gauge when the engine is running to diagnose a number of potential problems.
Read the gauge when the engine is idling. On a normal engine, it should read between about 15 and 22 inches, and the needle should hold steady.
Read the gauge when the engine is accelerating. The needle should quickly move down to zero because when the throttle plate is open, it temporarily lets the outside air in. The needle should move back up once you are at a steady speed.
Read the gauge when the engine is decelerating. The needle should quickly move up to about five or ten inches of mercury and then return to its normal position. This happens because when the car decelerates, the throttle plate is closed and does not let in any air, but the cylinders continue to suck air out.
Troubleshoot for problems if the needle seems to indicate engine issues. For instance, there may be a problem if the needle jumps or flickers in any abnormal way and does not hold steady in idle, or if the reading is steadily below 14 inches or above 22 inches. Potential problems include vacuum leaks, a timing issue, worn rings, excessive exhaust back pressure or sticking, burnt or broken valves.
