Intake Manifold Vacuum Test

Bring the engine to normal operating temperature. Connect Vacuum Gauge 059-00008 or equivalent to the intake manifold (9424) . Run the engine at the specified idle speed.

The vacuum gauge should read between 15 and 22 inches depending upon the engine condition and the altitude at which the test is performed. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.

The reading should be quite steady. It may be necessary to adjust the gauge damper control (where used) if the needle is fluttering rapidly. Adjust damper until needle moves easily without excessive flutter.

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. Remember that vacuum gauge readings, although helpful, must be interpreted with care.

Most vacuum gauges have a NORMAL band indicated on the gauge face.

The following are potential gauge readings. Some should be considered as normal; others should be investigated further.

1. NORMAL READING: Needle between 15 and 22 inches 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 (not to zero) reading. 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: Needle will register as low as 15 inches but will be relatively steady. Some oscillation is normal.

4. WORN PISTON RINGS OR DILUTED OIL: When the engine is accelerated (dotted needle), the needle drops to zero inches. Upon deceleration, the needle runs slightly above 22 inches.

5. STICKING VALVE(S): When the needle (dotted) remains steady at a normal vacuum but occasionally flicks (sharp, fast movement) down and back about four inches, one or more valves may be sticking.

6. BURNED OR WARPED VALVES: A regular, evenly spaced, downscale flicking of the needle indicates one or more burned or warped valves. Insufficient hydraulic valve tappet clearance will also cause this action.

7. POOR VALVE SEATING: A small but regular downscale flicking can mean one or more valves are not seating.

8. WORN VALVE GUIDES: When the needle oscillates (swings back and forth) over a four-inch range at idle speed, the valve guides could be worn. As engine speed is increased, the needle will become steady if the guides are responsible.

9. WEAK VALVE SPRINGS: When the needle oscillation becomes more violent as engine rpm is increased, weak valve springs (6513) are indicated. The reading at idle could be relatively steady.

10. LATE VALVE TIMING: A steady but low reading could be caused by late valve timing.

11. IGNITION TIMING RETARDING: Retarded ignition timing will produce a steady but low reading.

12. INSUFFICIENT SPARK PLUG GAP: When spark plugs (12405) are gapped too close, a regular, small pulsation of the needle can occur.

13. INTAKE LEAK: A low, steady reading can be caused by an intake manifold or throttle body gasket leak.

14. BLOWN HEAD GASKET: A regular drop of fair magnitude can be caused by a blown head gasket (6051) or warped cylinder head-to-cylinder block surface.

15. RESTRICTED EXHAUST SYSTEM: When the engine is first started and is idled, the reading may be normal. But, as the engine rpm is increased, the back-pressure caused by a clogged muffler (5230) , kinked tail pipe, etc., will cause the needle to slowly drop to zero. The needle then may slowly rise. Excessive exhaust clogging will cause the needle to drop to a low point even if the engine is only idled.

    When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, the unit will not function correctly. ALWAYS FIX VACUUM LEAKS.