How to Troubleshoot a Vacuum Leak in a Carby Chris Stevenson
The internal combustion engine functions as a sealed container of controlled explosions. The engine requires air (oxygen) for combustion, drawing it in through air an filtering source. Sometimes extra air enters into the system through a failed seal, belonging either to a component, or vacuum line that has broken or disconnected. These are called vacuum leaks, and they can originate from several sources. Vacuum leaks upset the air-to-fuel ratio and can cause lean misfire, poor engine performance and changes in fuel economy. An observant vehicle owner can find these leaks by using a few tools and learned techniques.
Troubleshooting Vacuum Leaks
Place the vehicle in park or neutral with the emergency brake engaged. Start the engine and raise the hood. While the engine idles, listen for any hissing or whistling noises coming from the base of the throttle body, carburetor or intake manifold. The noise might accompany a stumbling engine or rough idle. The sucking air noise will indicate a vacuum leak at a gasket seal. Notice if the engine idle shows a higher than normal rpm reading, or if the carburetor refuses to adjust down for speed or mixture setting. A small vacuum leak will increase the idle speed and will not adjust down.
Check all vacuum lines and hoses for cracks and disconnections while the engine idles. Look at the vacuum line connections at the throttle body base, or carburetor base gasket (if you have one). Examine the large PCV valve hose that enters the valve cover for a proper seal and good condition. Examine the large booster hose for disconnection at the brake booster diaphragm. Check the condition of the grommet seal--a vacuum leak at the brake booster will cause your brake pedal to feel "hard." Check all vacuum lines at sensor ports for a correct seal and line condition.
Use a can of carburetor cleaner to spray around the area of the intake manifold, throttle bottle base or carburetor base gasket. Look for any change in engine rpm which will rise when the carburetor spray hits near the source of the vacuum leak. Be extremely careful not to spray the carburetor cleaner near the spark plug locations or plugs wires.
Hook up a vacuum gauge to any vacuum source line or hose near the intake manifold. Refer to your owner's manual for the proper vacuum reading in inches of mercury (Hg). If the gauge shows a too low reading, it indicates a vacuum leak. Generally, for most cars, any reading that does not stay within 16 to 22 inches of mercury and falls below that number will point to a vacuum leak.
Items you will need
- photo_camera Auto Engine image by Andrew Breeden from Fotolia.com