When a road-going naturally-aspirated gasoline engine is idling, there is vacuum in the intake manifold. Vacuum is any pressure that is less than the ambient atmospheric pressure. Atmospheric pressure is expressed in different units and it varies with altitude. Therefore we usually reference it at sea level. At room temperature (20-25°C), atmospheric pressure is 14.7 psi or 760 mm Hg or 29.9 Inches of Mercury or 760 torr etc. Vacuum at engine idle exists due to the fact that the throttle is fully closed and the little air that gets past the throttle is sucked in by the cylinders. When the throttle is gradually opened, the pressure inside the intake manifold increases and nears the ambient atmospheric value.
In the start of the above paragraph, I used the term road-going vehicle because in some race engines, there may not be enough vacuum even at idle. The race engines use long-duration camshafts and the amount of vacuum in an engine depends on the design details of the camshaft(s) like the valve overlap angle.
An engine can have both vacuum and pressure leaks. Pressure leaks can happen from the cylinder valves (intake or exhaust) or sometimes from spark plug hole. Turbocharged- or supercharged-engines may not have vacuum in the intake manifold depending upon the engine programming (or boost threshold) and hence are not concerned here (They have problems of pressure or boost leak instead).
In the start of the above paragraph, I used the term road-going vehicle because in some race engines, there may not be enough vacuum even at idle. The race engines use long-duration camshafts and the amount of vacuum in an engine depends on the design details of the camshaft(s) like the valve overlap angle.
An engine can have both vacuum and pressure leaks. Pressure leaks can happen from the cylinder valves (intake or exhaust) or sometimes from spark plug hole. Turbocharged- or supercharged-engines may not have vacuum in the intake manifold depending upon the engine programming (or boost threshold) and hence are not concerned here (They have problems of pressure or boost leak instead).
It is easy to detect a vacuum leak in a MAP-only engine (also called the speed-density engine). MAP stands for Manifold-Absolute Pressure. The engine rpm would increase at idle. For this one should know the design or default idle rpm.
For a MAF (Mass Air Flow)-equipped engine, a vacuum leak would result in higher than usual fuel trims (both short term and long term) and can be seen with a scan tool for OBD-II equipped vehicles. Also the computer may show lean combustion codes.
It is also possible to actually measure the vacuum in the intake manifold with the help of a vacuum gauge when there is no leak and note that value. When there is a doubt or possibility of a leak, take a fresh reading of vacuum with the gauge and it would confirm whether or not there is a leak. In the case of a vacuum leak, the intake manifold would have a higher absolute pressure or a lower vacuum reading. For example, if the normal reading was 20 inches of Mercury, then in case of a vacuum leak, the new reading may be 25 inches of Mercury (near to the ambient air pressure of 29.9 inches of Mercury). There are some good tutorials present online on how to detect these vacuum leaks. Here the objective is to show the common places where such leaks may occur (this would help in locating them). The common places where the vacuum leaks can take place are:
For a MAF (Mass Air Flow)-equipped engine, a vacuum leak would result in higher than usual fuel trims (both short term and long term) and can be seen with a scan tool for OBD-II equipped vehicles. Also the computer may show lean combustion codes.
It is also possible to actually measure the vacuum in the intake manifold with the help of a vacuum gauge when there is no leak and note that value. When there is a doubt or possibility of a leak, take a fresh reading of vacuum with the gauge and it would confirm whether or not there is a leak. In the case of a vacuum leak, the intake manifold would have a higher absolute pressure or a lower vacuum reading. For example, if the normal reading was 20 inches of Mercury, then in case of a vacuum leak, the new reading may be 25 inches of Mercury (near to the ambient air pressure of 29.9 inches of Mercury). There are some good tutorials present online on how to detect these vacuum leaks. Here the objective is to show the common places where such leaks may occur (this would help in locating them). The common places where the vacuum leaks can take place are:
1- Intake Manifold Gasket(s)
2- Brake Booster
3- Manifold Absolute Pressure (MAP) Sensor
4- EVAP Solenoid
5- EVAP Canister Connections
6- Injector O-rings
7- Throttle Position Sensor (TPS) O-ring
8- Throttle Body O-ring
9- Idle Air Control Valve (IACV)
10- Rocker Cover Gasket
11- Rocker Cover Nuts
12- PCV (Positive Crankcase Ventilation) hose connections
13- Oil Filler Cap
14- Camshaft Seals
15- Crankshaft Seals
16- Oil Dipstick
Oil dipstick can suck in outside unfiltered air. This unfiltered air shall go to the engine through the PCV system. It is better to put an O-ring on the dipstick so that when the dipstick is pushed in, the O-ring shall help seal the hole.
17- Exhaust Manifold Gasket
Although the exhaust manifold does not have the same vacuum as in the intake manifold, momentarily there exists vacuum (when the exhaust gas rushes out of the cylinder). A leak in the exhaust manifold gasket can falsify the readings by the Oxygen sensor. The sensor would read excess Oxygen and would result in less fuel supply to the engine and thus reduced engine power.
18- Distributor Cap
Mostly ignition distributors are attached to the back-end of the camshafts (in the distributor type ignition system). As there is vacuum inside the rocker cover at idle, distributor caps are usually sealed with an O-ring. If distributor has been dismantled or removed from vehicle, the O-ring should also be renewed to ensure that no leak would happen here.
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