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WSG-1068 GENERAL INFORMATION
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 DEACCELERATION:
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.
4. WORN RINGS OR DILUTED OIL: When the
engine is accelerated (dotted needle), the
needle drops to 0 kPa (0 in-Hg). Upon
deceleration, the needle runs slightly above 74
kPa (22 in-Hg).
5. STICKING VALVES: When the needle (dotted)
remains steady at a normal vacuum but
occasionally flicks (sharp, fast movement) down
and back about 13 kPa (4 in-Hg), one or more
valves may be sticking.
6. BURNED OR WARPED VALVES: A regular,
evenlyspaced, downscale flicking of the needle
indicates one or more burned or warped valves.
Insufficient hydraulic lash adjuster or hydraulic
lash adjuster (HLA) clearance will also cause this
reaction.
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 over about a 13 kPa (4 in-Hg) range at
idle speed, the valve guides could be worn. As
engine speed increases, the needle will become
steady if guides are responsible.
9. WEAK VALVE SPRINGS: When the needle
oscillation becomes more violent as engine rpm
is increased, weak valve springs 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 somewhat low
reading.
12. INSUFFICIENT SPARK PLUG GAP: When spark
plugs 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 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, kinked tail pipe or other concerns will cause the
needle to slowly drop to 0 kPa (0 in-Hg). 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 idling.
When vacuum leaks are indicated, search out and
correct the cause. Excess air leaking into the system
will upset the fuel mixture and cause concerns such as
rough idle, missing on acceleration or burned valves. If
the leak exists in an accessory unit such as the power
brake booster, the unit will not function correctly. Always
fix vacuum leaks.
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