Vacuum Gauge Readings - Ford LRG-423 Service Manual

01-9
Engine Service
01-9
SERVICE PROCEDURES (Continued)
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.
Vacuum Gauge Readings - Interpretation
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 aauae face.
VACUUM/
PRE
TE
059
A23453-A
Following are potential gauge readings. Some should
be considered as normal; others should be
investigated further.
Al 0428-P
1.
2.
3.
4.
5.
6.
7.
8.
9.
NORMAL READING: Needle between -51 and
-74 kPa (15 and 20 in-Hg) and holding steady.
NORMAL READING DURING RAPID
ACCELERATION AND DECELERATION: When
engine is rapidly accelerated (dotted needle),
needle will drop to a low (not to 0) reading. When
throttle is suddenly released, the needle will snap
back up to a higher than normal figure.
NORMAL FOR HIGH LIFT CAM WITH LARGE
OVERLAP: Needle will register as low as -50
kPa (15 in-Hg) but will be relatively steady. Some
oscillation is normal.
WORN RINGS OR DILUTED OIL: When engine
is accelerated (dotted needle), needle drops to 0
kPa (0 in-Hg). Upon deceleration, needle runs
slightly above 74 kPa (22 in-Hg).
STICKING VALVE(S): 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.
BURNED OR WARPED VALVES: A regular,
evenly spaced, downscale flicking of the needle
indicates one or more burned or warped valves.
lnsuff icient hydraulic lash adjuster clearance will
also cause this action.
POOR VALVE SEATING: A small but regular
downscale flicking can mean one or more valves
are not seating.
WORN VALVE GUIDES: When the needle
oscillates (swings back and forth) over a 13 kPa
(4 in-Hg) range at idle speed, the valve guides
(6510) could be worn. As engine speed is
increased, the needle will become steady if the
guides are responsible.
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 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 carburetor
mounting flange gasket leak.
14. BLOWN HEAD GASKET A regular drop of
approximately 33-50 kPa (1 O-l 5 in-Hg) can be
caused by a blown head gasket or warped head-
to-block mounting surface.