Vacuum Gauge

While it's already obvious, the engine is practically just a large air pump. Once combined with heat and pressure, the air your engine intakes will get sent out as exhaust. While it's a rather complicated process, you basically need to understand that the engine works as an air pump to get a good idea of how the vacuum gauge works.

The engine vacuum operates at a pressure that's lower than atmospheric pressure. You can begin to monitor the manifold vacuum at the inlet manifold. The vacuum gauge can be connected to the intake manifold by attaching it to a tap on the intake. The location of the intake manifold tap will vary by design but shouldn't be hard to locate.

Purpose of the Vacuum
The engine vacuum is one of many parts that work to keep your vehicle's exhaust system operating properly. The vacuum gauge is responsible for keeping vacuum motors in your air conditioning, power brake boosters, and certain emissions settings working at all times. A poor performing vacuum could ultimately lead to many serious issues with your vehicle, so it's important to watch for signs of a malfunctioning engine vacuum. A vacuum gauge is a great tool to use in order to do this.

How the Vacuum Gauge Works
The vacuum gauge shows a comparison of what's in the inlet manifold and the atmospheric pressure from outside of the manifold. Your engine will perform differently depending on where the needle is pointing to on the vacuum gauge. It's important to know what the ideal reading on your vacuum gauge should be so your vehicle's engine can perform respectably. Keep in mind that the vacuum reading alone won't mean a corresponding strong performance from your engine, but rather the movement of the needle in the vacuum gauge is important as well.

If your vehicle's engine is idling, the needle in the vacuum gauge will quickly shoot down, especially if there's a vacuum leak. The reading on the vacuum gauge could drop from an average reading down to 10-12" Hg (mercury) or lower. If there are multiple cylinders leaking then you should notice a much worse drop in the vacuum gauge's reading. There are diagnosis tests (such as the compression test) that can be done to determine where the leak is coming from.

Using the Vacuum Gauge
The vacuum gauge is mostly just used to test your vehicle for any possible engine problems. Just connect the vacuum gauge to the inlet manifold and watch the needle in the vacuum gauge. The information on the vacuum gauge is usually shown with intervals of inches/millimeters and is a display of the measurement of mercury.

Since the engine vacuum is being compared to atmospheric pressure, the readings will vary from altitude. A higher altitude would mean a lower amount of engine vacuum – expect roughly an inch decrease in engine vacuum for every thousand feet about sea level. The preferred vacuum gauge reading is 18-22 inches (Hg). With older vehicles, you could find the recommended manufacturer specifications in the driver manuals. However, most new vehicles don't include this information.

Even though you likely won't have any manufacturer specs to rely on when using a vacuum gauge with your vehicle, the preferred range mentioned above is a good basis for your judgment. This may not be an exact range to make sure your vacuum gauge readings are within though. After using a vacuum gauge on multiple vehicles, you’ll notice that different engines can produce different results. This is because some engines are designed with low vacuum motors, but some are the exact opposite. After you become experienced with using the vacuum gauge you can make a good judgment on if the reading you get is a healthy one or not.

There are many tests that can be done in cooperation with the vacuum gauge. You can find various tutorials to run these tests. These are great ways to diagnosis possible engine problems and determine how well your engine is performing. The vacuum gauge is definitely an invaluable tool for any auto shop, and is also beneficial for home use as well.