A vehicle that has an idle speed that is too high, idles roughly or stalls and dies out has a vacuum leak in its engine. Indicators of engine vacuum leaks are hesitation or misfiring when accelerating and an idle that cannot be adjusted. The quickest way to detect a vacuum leak is to attach a vacuum gauge to the intake manifold of the engine to measure the vacuum's strength. If the vacuum is lower than the 16 to 22-inch Hg or mercury reading, or the gauge needle does not remain steady, the engine has a vacuum leak.
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Things you need
- Vacuum gauge
Warm up your vehicle by letting it run for 10 minutes. Shut off the engine.
Find a vacuum hose that connects directly into the vehicle's intake manifold, or you can use an unused port on the intake manifold. Consult the emission sticker, which is located either on the underside of the hood or the firewall. It will have a hose diagram that will help you figure out which hoses are the vacuum hoses.
Disconnect a vacuum hose. Connect the vacuum gauge to the port from which the hose came. Start the vehicle and take a reading.
Tips and warnings
- Another way to take a reading is to disconnect a PCV hose attached to a valve cover on the intake manifold. Connect the vacuum gauge to the PCV hose with a T-connector and start the vehicle. The engine will idle slowly, but the intake vacuum reading will not be affected.
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