Ford 2003 Mustang Workshop Manual page 848

Intake Manifold Vacuum Test
Bring the engine to normal operating temperature. Connect the Vacuum/Pressure Tester to the intake
manifold. Run the engine at the specified idle speed.
The vacuum gauge should read between 51-74 kPa (15-22 in-Hg) depending upon the engine
condition and the altitude at which the test is performed. Subtract 4.0193 kPa (1 in-Hg) from the
specified reading for every 304.8 m (1,000 feet) of elevation above sea level.
The reading should be steady. If necessary, adjust the gauge damper control (where used) if the
needle is fluttering rapidly. Adjust the damper until the needle moves easily without excessive flutter.
Intake Manifold Vacuum Test—Interpreting Vacuum Gauge Readings
A careful study of the vacuum gauge reading while the engine is idling will help pinpoint trouble areas.
Always conduct other appropriate tests before arriving at a final diagnostic decision. Vacuum gauge
readings, although helpful, must be interpreted carefully.
Most vacuum gauges have a normal band indicated on the gauge face.
The following are potential gauge readings. Some are normal; others should be investigated further.
1. NORMAL READING: Needle between 51-74 kPa (15-22 in-Hg) and holding steady.
2. NORMAL READING DURING RAPID ACCELERATION AND DECELERATION: When the
engine is rapidly accelerated (dotted needle), the needle will drop to a low reading (not to zero).
When the throttle is suddenly released, the needle will snap back up to a higher than normal
figure.
3. NORMAL FOR HIGH-LIFT CAMSHAFT WITH LARGE OVERLAP: The needle will register as
low as 51 kPa (15 in-Hg) but will be relatively steady. Some oscillation is normal.