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All Info About Auto Repair: Engine Testing With A Vacuum Gauge Part 1
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Engine Testing With A Vacuum Gauge Part 1

Vacuum gauges have been a valuable tool to mechanics for years. Even with modern computer controlled engines a vacuum gauge is still a valuable tool for diagnosing engine and transmission problems.
» Engine/Transmission
» Valve Timing

Engine/Transmission Relations

An important part of transmission diagnosis is to make certain the engine operates properly. If the engine performance is incorrect, the transmission will receive the wrong information. Many times what is perceived as a transmission problem is in actuality an engine problem.

The engine sends signals to the transmission through a vacuum line, throttle cable or both. These signals basically synchronize torque with transmission line pressure, shift feel and shift timing.

Malfunctions in items like the air filter; spark plugs, EGR valves and other parts of the fuel, electrical and emission systems could result in improper transmission performance.

Vacuum Gauge Engine Performance Testing

A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:

  • Piston rings
  • Valves
  • Ignition system
  • Fuel control system
  • Other parts affecting the combustion process (emission devices, etc.).


Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading. Gauge readings that may be found are as follows:

Normal Engine Operation

At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more. See figure 1.

General Ignition Troubles Or Sticking Valves

With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves. See figure 2.


» Part 1       » Part 2
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