Many parts of the vehicle rely on the intake manifold vacuum for operation. Depending on the vehicle, everything from the brake booster to the fuel pressure regulator may require intake manifold vacuum. An engine vacuum leak can also cause problems by allowing unmetered air to enter the engine. This disrupts the air/fuel ratio. On old engines, vacuum is also required to pull fuel into the carburetor.
First, How Does a Car Engine Create a Vacuum?
Vacuum functions as a fundamental dynamic air flow of the internal combustion engine. Without the proper vacuum, a car would starve of the air and fuel mixture required to produce combustion. Vacuum is the difference in pressure, usually measured in inches of mercury, between the interior of the intake manifold and the outside air pressure.
Piston Draw
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.
Vacuum Efficiency and Engine Performance
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.
Vacuum Auxiliary Components
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 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.
Proper Vacuum Conditions
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.
Vacuum Readings and What They Mean
A healthy engine should produce a steady 17 to 21 in. Hg of vacuum at idle. This measurement is typical of a vehicle at sea level. Readings will decrease by about 1 in. Hg for every 1000 ft. increase in altitude.
Vacuum can be measure by connecting a vacuum gauge to a source of manifold vacuum. An intake vacuum reading that is out of specification can indicate anything from an internal engine problem to a leaking exhaust gas recirculation valve (EGR). Here are a few potential causes of different vacuum gauge readings.
- Low but steady gauge reading: late timing, low engine compression, throttle body, or intake gasket leaks
- Needle fluctuates as RPMs increase: weak valve springs, worn valve guides, clogged catalytic converter, or leaking head gasket
- Gradual drop in reading at idle, reading decreases with engine speed: Clogged catalytic converter or restricted exhaust system
- Intermittent fluctuation at idle: sticking valves or ignition misfire
- Drifting needle: leaking intake manifold or bad fuel injector o-rings
- Needle that drops regularly: burn valve or improper valve clearance
- Needle that fluctuates at idle but steadies as engine speed increases: worn valve guides
Symptoms Of an Engine Vacuum Leak
A vacuum leak will cause the engine to run lean. In other words, there will be more air entering the engine than is needed to balance out the air/fuel ratio. This can result in the following:
- Engine stalling
- A rough running engine
- Too fast of an idle
- Hesitation
- Increased emissions
- Poor fuel economy
- An illuminated check engine light
Diagnosing An Engine Vacuum Leak
If you have access to a scan tool, you will want to use that first during diagnosis. Select the data parameters short term fuel trim (STFT) and long-term fuel trim (LTFT). On most vehicles, fuel trim readings should be between -10 and +10 with the vehicle at operating temperature. A reading greater than +10 indicates the powertrain control module (PCM) is trying to add fuel to compensate for a lean condition. To determine if the lean condition is the result of an engine vacuum leak, bring the engine up to about 2,000 RPM. If the readings return to normal, there is a vacuum leak.
If you do not have a scan tool, you can visually inspect under the hood for leaks. You can also listen for leaks, which can sometimes be heard making a hissing noise. Then, follow up with one of the location methods listed below.
Locating An Engine Vacuum Leak
Engine Vacuum leaks can originate from various sources. Here are some of the most common:
- Hoses and lines
- The Intake manifold and/or gaskets
- The throttle body and/or gasket
- The EGR valve
- The brake booster
There are a few different ways to locate an engine vacuum leak. The best method is to use a professional smoke machine. These devices send smoke into the intake manifold. If there is a leak, the smoke will be seen billowing out. Unfortunately, these devices are quite costly and outside the budget of most DIYers. However, some “old school” mechanics connect a large vacuum line to the intake and blow cigar smoke into the system, and then look for any smoke exiting the vacuum system.
A second method is to spray brake or carburetor cleaner around the area of a potential leak. If there is a leak, the cleaner will artificially enrich the air/fuel mixture. This will cause the engine RPMS to increase when the cleaner is sprayed near the source. This method is the most common, and easy to conduct for an engine vacuum leak.
How To Repair Vacuum Leaks
In theory, repairing an engine vacuum leak is simple: you replace the leaking component. This could mean anything from replacing a broken vacuum line (easy) to replacing a cracked intake manifold (difficult). Whatever the case may be, you will need to repair the leak to get your vehicle running right.
Finally
Now you can see why mechanics often joke that an internal combustion engine is just a gigantic air pump. This is because the pistons move up and down in the cylinders, sucking air in and compressing the air/fuel mixture. Intake vacuum is created because of the pistons pumping and the restriction of the throttle plate. Proper vacuum is required for an engine to run properly. Hopefully, you now have a better understanding of an intake vacuum and the significant role it plays in engine performance.
