DSG-423 GENERAL INFORMATION
01-14
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, evenly-
spaced, 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
(swings back and forth) 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 approx.
33-50 kPa (10-15 in-Hg) 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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