How To Identify Engine Vacuum Leaks - Sucking Up!
The medical community uses the term differential diagnosis to describe established test procedures to rule out reasons for an illness. Anyone who watches the television show House on the FOX network is familiar with this concept. It could be thought of as finding out what is working properly instead of what is not.
Though Pontiacs of all years are excellent cars that are backed up by sound engineering and high-quality workmanship, there may be a time when the engine doesn't run properly. The art of accurately diagnosing a performance problem using the differential approach is an uncommon skill. To many enthusiasts and professional mechanics, the word replace is a synonym for diagnose. They fix cars by throwing parts at them, the logic being that if enough components are changed, eventually you will stumble across what is wrong, albeit through default.
Putting the equivalent of an entire auto-parts store's inventory under the hood of your Pontiac may not be the most cost-effective manner to approach a problem, and in some instances, it's completely useless. One scenario is an engine with a vacuum leak. When unmetered air enters the engine, you can change the carburetor, fuel injectors, distributor, plug wires, and ignition module along with a host of other parts forever without solving the problem. If the proper diagnostic steps are not taken, a vacuum leak can be a very elusive issue.
What Is Vacuum?
The standard to identify pressure is relative to the force of the atmosphere on the Earth, better known as atmospheric pressure. By definition, a vacuum is any pressure less than atmospheric. The many ways this is measured often lead to confusion: It can be represented in pounds per square inch, inches of mercury, or inches of water. According to McGraw-Hill's Dictionary of Engineering, atmospheric pressure is measured at sea level and is considered to be 14.70 psi, which is 1 atmosphere.
A typical vacuum test gauge reads in inches of mercury. If a vacuum gauge is attached to an engine and reads 15 inches of mercury, it means the low-pressure region created in the engine is sufficient for atmospheric pressure to push a test column of mercury 15 inches up into a tube. Expanding upon this, atmospheric pressure of 14.70 psi has the ability to push the same column of mercury 29.92 inches. In near terms, due to the difference in specific gravity, a manometer would read 406.78 inches of water at atmospheric pressure. So as you can see, what we accept as either pressure or vacuum is actually relative to the atmosphere.
Vacuum Types</STRONG>
When discussing a gasoline-fueled engine, there are two types of vacuum: ported (timed) or manifold. A ported-vacuum signal is one that is active only when the throttle plates are moved enough to expose the passage in the carburetor to the intake-manifold depression. For this reason, it is sometimes referred to as timed vacuum since it becomes activated at a defined throttle angle. Manifold vacuum is also called full-time vacuum since it's derived from below the throttle plate and directly from the intake manifold.
Over the years, Pontiac has used both ported- and manifold-vacuum signals to operate certain devices. As an example, early models had a ported signal to the distributor vacuum-advance canister, while some late-'70s engines used manifold vacuum for the same device. The heater control door, PCV valve, and power-brake booster all operate through manifold vacuum.
What Impacts Vacuum?
Throttle angle and engine rpm impact the amount of vacuum in an engine. When the throttle plates are closed, the differential in pressure between the atmosphere and the intake manifold is the greatest, and thus, the vacuum signal is the strongest. As the throttle plate(s) are opened, the differential is minimized and engine vacuum drops. At wide-open throttle, an engine is considered to have 0 vacuum when read in inches of mercury.
Engine speed also impacts the amount of vacuum. The higher the piston speed, the greater the vacuum signal, but only when the throttle plate is closed. An engine produces the most vacuum when coasting since the rpm is high but the throttle plate is closed. This is the trend for any spark-ignition engine but not compression-ignition (diesel). The fuel-metering system on a diesel doesn't use a throttle plate, so there is never a restriction.
Strength of the vacuum signal is greatly influenced by the amount of overlap (the time both valves are open) ground into the camshaft profile. As overlap is increased, engine vacuum decreases. A Pontiac engine equipped with a camshaft with greater overlap will still produce its strongest vacuum signal during coast down, but the value would be substantially less than a stock engine under the same conditions.
As an engine wears, the piston rings and valves don't seal as well, impacting the ability to pump air. When this occurs, the manifold-vacuum signal decreases since the engine isn't sealed. Therefore, it's a good idea to check the vacuum-signal strength of your Pontiac engine as part of every tune-up.
Vacuum As A Diagnostic Tool
The amount of vacuum in the engine can also be used as a diagnostic tool since certain problems impact the signal strength. Anything that either impacts the engine speed (the ability to pump air and create a differential) or the sealing of the cylinder bore are easily seen. Other factors such as late valve and ignition timing can also be diagnosed, along with an exhaust restriction.
More important than the signal strength is the steadiness of the needle during the reading. If the pointer shakes violently, it's usually the result of a burned valve that isn't sealing the cylinder. Most of the better vacuum gauges offer diagnostic tips printed on the instrument's face to aid in locating the problem.
A vacuum leak such as a leaky intake gasket or ruptured vacuum hose will also impact the gauge reading but can be harder to detect since it may impact only one cylinder. For this reason, HPP has prepared this primer on locating and diagnosing an engine-vacuum leak. These tips can be applied to any Pontiac, regardless of year or induction-system design.

Photo Gallery: How To Identify Engine Vacuum Leaks - High Performance Pontiac Magazine

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