How Does a Car Engine Create a Vacuum?

When the intake valve opens, the air-fuel mixture is drawn into the combustion chamber by the downward movement of the piston, which creates a suction or vacuum. When the piston reaches the top of its compression stroke, the air-fuel mixture is ignited by the spark plug, which sends the piston downward on its power stroke. The exhaust stroke follows, where the spent combustion gases are pushed out of the exhaust valve and into the exhaust system. When the piston makes another downward stroke, suction or vacuum is created again. No matter what speed the engine runs, the pistons draw incoming air-fuel into the combustion chamber.

A cranking engine will normally produce about 3 to 5 inches of mercury, or Hg. Vacuum increases and is highest at idle when the throttle position is closed or slightly open. This results from the restriction of air flow that cannot move in great volume from the air intake to the manifold. As the throttle opens, more air enters the intake, causing a decrease in vacuum.

Vacuum efficiency can be measured by a vacuum gauge. Engine vacuum will decrease as the engine undergoes a heavy load at wide open throttle, such as climbing a hill, or during quick acceleration from a stop. The highest vacuum will result when the engine decelerates from a high speed or during coast, and this happens because the throttle is closed but the engine rpm is high. Manifold vacuum specifications for different conditions are outlined in the vehicle manufacturer's service manual.

Many vehicle components use manifold vacuum through ports and hoses. Brake boosters, which have vacuum assist, use vacuum to actuate a diaphragm that increases pressure when applying the brake pedal. Some windshield wipers and door locks use vacuum-assisted servos to operate valves and linkage. Older vehicles with distributors use vacuum as a means to advance spark timing in the ignition system. The EGR and PVC valve use vacuum to function as part of the emission control system. These auxiliary systems use engine rpm or additional switching valves to regulate the proper amount of vacuum needed to operate.

At sea level, atmospheric pressure reads about 14.7 pounds per square inch and holds a column of mercury at 29.92 inches in height, or Hg. As atmospheric pressure and temperature increase or decrease during changes in weather, slight changes occur in engine vacuum. Less dense or hotter air produces less vacuum because of loosely packed air molecules. The higher the altitude (less dense air), the lower the Hg, or inches of mercury in the vacuum reading. For example, an engine idling at 22 Hg at sea level will show approximately 17 Hg at 5,000 feet of altitude. At 10,000 feet altitude, the mercury will read about 12 Hg.

Normal vacuum for an engine at idle runs at 14 to 18 inches of mercury when measured with a gauge. Abnormally high rpm at idle can indicate a vacuum leak somewhere between the throttle and manifold, usually pointing to cracked hoses, base plates leaks, manifold leaks, faulty carburetor or defective ported vacuum-operated switching valves. This also results in a too-lean air mixture. Low vacuum can result from low compression and burnt valves.