An automotive vacuum gauge is an important tool,used for readings of air pressure in the engine. The vacuum regulates the ratio of fuel and air intake, in to the engine,in order to allow the engine to operate at peak performance. The vacuum controls the amount of airflow enabled by the throttle into the intake system which, generates power to the engine of the vehicle. By limiting the amount of air into the manifold from the engines pistons and throttle plates the vacuum channels fuel into the engine. The vacuum gauge informs you the driver of any air leaks in the system which results in poor gas mileage, rough idling, and increased emissions.
Vacuum gauges are a good source to determine whether an engine is working properly. On vacuum gauges the readings vary, however, it should read between 17-22 inches when at a steady idle between 800-1200rpm. Vacuum gauges move a lot, compared to the fuel gauge. For example, when the engine is decelerated, the vacuum gauge will momentarily go up between 25-30 inches then drop to normal. It is also important to note that the gauge will vary depending on how high you are above sea level. The rule of thumb is that for every 1000 feet above sea level you will subtract 1 inch from what your vacuum reading would typically give if you were at sea level. If the needle drops several divisions when at an idle between 800-1200rpm this typically means there is a sticking valve or broken valve spring. A steady low reading between 8-14 inches means a small vacuum leak or valve timing is off. This means that the compression/worn rings,are low. A steady low reading less than 8 inches means there is most likely a vacuum leak. In that case the brake booster and the vacuum lines should be checked. High performance engines (those with long duration of acceleration) will show a normal vacuum reading lower around 15 inches than typical stock engines. High performance engines vacuum readings will also have a little needle shake which is to be expected. This is because there is a valve overlap which is where both the intake and exhaust valves, momentarily open together.
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