How to Measure Engine Vacuum Pressure
Once a standard tool for engine performance diagnosis, the vacuum gauge has been overlooked in favor of digitized testers. None the less, it remains a cheap and accurate means to tune or test four-stroke piston engines. By definition, vacuum is the absence of matter or atmosphere in a space. The amount of vacuum is measured in inches of mercury, with absolute vacuum being 29.92 inHG at sea level. The actions of pistons and valves provide a steady vacuum source while the throttle plate is at idle position. Vacuum gauge readings reflect operation at or near sea level.
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Instructions
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Locate a vacuum port below or behind the throttle plate. Add a tee fitting into an existing vacuum hose and connect the gauge hose to the fitting or an unoccupied manifold port. Start the engine and let it reach operating temperature.
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Monitor the gauge with the engine at idle speed. Interpret a steady reading of 17 to 20 inHG as indication of a healthy engine in proper tune. Adjust the ignition timing to recommended specifications if the reading is lower than 17inHG and steady. Adjust the air-fuel mixture or correct injector performance should the gauge needle wander slowly near the optimum reading.
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Adjust or repair the valve train if the gauge needle travels back and forth rapidly across a short span of 2 to 3 inHG. Examine the exhaust system for restrictions if the vacuum falls to zero and stays there while you rev the engine slightly. Regard continuous low readings as indication of vacuum leaks or worn cam lobes.
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Tips & Warnings
Make some allowances for the age of the engine by tolerating readings that are less than ideal. Valve stem clearance faults can be detected with a vacuum gauge, but exhaust emissions of blue-gray smoke usually precede the need for further diagnosis.
Stay well clear of moving parts, like cooling fans, while working on a running engine.
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