Compression and Vacuum
For an engine to run, it must have enough compression to raise the temperature of the air fuel mixture high enough so that a spark can easily start an efficient combustion process.  If there is not enough compression, the engine will fail to start.
Engine compression is most accurately checked with a compression gauge. This test can take considerable time, especially when the spark plugs are difficult to remove. A quicker test is a manifold vacuum test. If you have over 3" of cranking vacuum, there should be enough compression to run the engine.
There are three types of engine vacuum in gasoline engines. The one we use for testing engine compression is manifold vacuum. This is the vacuum created between the throttle plate and the intake valves. Normal manifold vacuum on an engine at idle speed is around 18 to 20 inches. If you have an engine that is running at idle speed, and your vacuum gauge reads more than 8 or 10 inches of vacuum, you are hooked into manifold vacuum. Not all vacuum lines are hooked directly to manifold vacuum. As you gain experience working with engines you will be able to quickly identify vacuum lines and vacuum ports that will allow you to hook up a vacuum gauge for testing purposes.
When you are testing manifold vacuum you must be able to easily read small changes in vacuum. The larger the vacuum gauge, the more accurate your vacuum testing will be. I recommend using a vacuum gauge that is at least 2" in diameter. An inexpensive gauge will work fine as long as it is easy to read small changes in vacuum.
You will find that the length and condition of your vacuum hose will affect the accuracy of your vacuum gauge. Old - used vacuum hose can easily collapse and provide a false reading to the vacuum gauge. Longer lengths of vacuum hose will slow down or eliminate quick changes in vacuum. It is important to be able to see quick changes in vacuum as they indicate problems like burned or sticking valves.
Vacuum is traditionally measured in inches of mercury and is really a measurement of pressure. For manifold vacuum you are measuring the difference in air pressure from the inside of the intake manifold, to the air pressure on the outside of the engine. A difference of one pound per square inch pressure will register as two inches of mercury.  1psi = 2"hg on the vacuum gauge. If your engine is reading 20 inches of vacuum it is measuring a difference of 10 pounds per square inch between the inside of the intake manifold and outside air pressure.
Vacuum is created when each piston travels down the cylinder with the intake valve open. It will change drastically as the engine is running at different RPM and load conditions. It will also change if the compression in a cylinder is lost, or there are air leaks between the throttle plate and the cylinders.
A primary factor in manifold vacuum is throttle plate position. When the throttle is closed, manifold vacuum should be higher than when the throttle is open. With an open throttle it is easy for the outside air to get into the engine and there will be less difference in pressure between the outside air and the air found in the intake manifold. As the throttle plate closes, a restriction in airflow is created and manifold vacuum increases.
A second factor in manifold vacuum is engine RPM. A slow turning engine will produce less manifold vacuum than a fast turning engine. Manifold vacuum will be at its highest when the engine RPM is high, and the throttle plate is closed. This happens when the engine is decelerating. Manifold vacuum will be lower when the engine is at starter motor cranking speeds than it will be at idle speeds.
A third factor in manifold vacuum is engine compression. If an engine has low compression in all cylinders, it creates a steady, but low, manifold vacuum. If an engine has low compression in one cylinder, manifold vacuum will be low only during that cylinders intake stroke. This will be observed as a gauge that fluctuates or quickly changes reading. Low compression in a cylinder can be caused by leaking valves or rings, leaking head gaskets, or other ways that keep the cylinder from sealing during the compression stroke.
Engine compression is directly affected by how easily the engine can breathe. If a cylinder can not pull in fresh air it will have low compression. An engine with a restricted exhaust will lose its ability to bring in fresh air.
Defects in the opening or sealing of an intake valve, or exhaust valve will lower the compression for that cylinder. This will be evident in a vacuum gauge that fluctuates or bounces as the engine is running. Clogged catalytic converters or other restrictions in the exhaust system will cause intake manifold vacuum to be lower, especially as the engine load increases.
A final factor affecting manifold vacuum is leaks in the air intake between the throttle plate, and the intake valves. Leaking vacuum lines, vacuum operated devices, or intake manifold gaskets are common causes for low vacuum.
Vacuum testing is quick, easy, and provides excellent diagnostic information IF you know what you are doing. The more you use the vacuum gauge, the more valuable it will become. The Vacuum Testing Lab will begin to provide the practical experience you need to understand what a vacuum gauge tells you about the engine and exhaust system. For this lab you will collect data on a few different engines. I encourage you to keep collecting this data outside of class. The more often you use a vacuum gauge, the more valuable a tool it will become.
There are a few general guidelines that you can follow when using a vacuum gauge on gasoline engines.
1) At idle, the more vacuum the better.
2) The highest manifold vacuum will be on deceleration.
3) A closed throttle will create more vacuum than an open throttle.
As the engine load increases, the throttle will open wider. This will allow for more air and fuel, create more power, and lower the manifold vacuum reading. Caution! The only time you can safely run an engine with an open throttle is when the engine is under a load. This can be done using a dynamometer, or on a road test. If you attempt to leave the throttle wide open on a running engine that is in Park or Neutral, the engine RPM will quickly climb too high and damage the engine. For most engines that are running in neutral, the maximum safe engine speed is 2,500 RPM.
The "Throttle Snap Test" can allow a technician to quickly observe engine vacuum when the throttle is open and the vehicle is not under a load however this test must be done with care to avoid damage to the engine. We will practice the throttle snap test in the lab.
Any time you use a vacuum gauge, begin by paying attention to how much vacuum is present in an engine that is cranking. A vacuum gauge is one of the best ways to test for sufficient compression on an engine that will not start. If you understand what "Good" cranking vacuum looks like, you will be able to quickly spot "Bad" cranking vacuum.
Cranking vacuum is a good indication there is enough compression to run the engine. Finding no cranking vacuum on a gasoline engine does NOT mean the engine has poor compression. Many vehicles will use an idle air control valve that opens wide during cranking. The idle air control valve opens an air passage that bypasses the throttle plate. This lets in more air and lowers the cranking vacuum. Below is an illustration of a typical throttle body injection idle air control.
Notice the idle air bypass opens during cranking. This allows more air in (while the injector stays on longer adding more fuel)
So how can you tell if a no-start engine, with no cranking vacuum, has good compression?
A) Listen to the engine. An engine with no vacuum will crank over faster and sound different than an engine with normal compression
B) Place your hand over the throttle body. An engine with good compression will have a strong suction at the throttle opening