Chapter A: General Diagnosis
7
Cylinder Power Balance Test
The Powertrain Control Module (PCM) in most latemod-
el vehicles incorporates the ability to conduct a power
or cylinder balance test utilizing a diagnostic scan tool.
The scan tool commands the PCM to run the test either
automatically or manually at the operator's discretion.
Each cylinder is disabled by shutting off the fuel and in
some cases the spark to the cylinder being tested. The
average RPM drop is displayed on the scan tool screen
and can be related directly to the contribution of that
cylinder.
Most older engine analyzers have the capability of per-
forming a power balance test, figure 1-2. The balance
test shows if an individual cylinder or a group of cylin-
ders is not producing its share of power. During the test,
the spark plug is shorted so there are no power strokes
from the cylinder being tested. Results are measured in
terms of engine RPM drop, manifold vacuum drop, or a
combination of these factors.
If the changes in engine RPM and manifold vacuum are
about the same for each cylinder, the engine is in good
mechanical condition. If the changes for one or more
cylinders are noticeably different, the engine has a prob-
lem. The fault may be mechanical, or it may be in the ig-
nition or fuel systems.
A power balance test is not conclusive; further tests are
needed to pinpoint the problem. The procedure can be
a time saver because it is performed on a running en-
gine. An engine that passes a cylinder balance test will
have fairly even compression so there is usually no need
to perform compression and cylinder leakage tests.
Cylinder Compression Tests
The compression test reveals how well each cylinder is
sealed by the piston rings, valves, cylinder head gasket,
and the spark plug. Compression pressure is measured
in pounds per square inch (psi), kilopascal (kPa), or bars.
The following quantities are equal to each other: 14.5
psi, 100 kPa, and 1 bar. A compression gauge measures
the amount of air pressure that a cylinder is capable of
producing.
Compression test specifications and procedures for an
engine are normally provided in the vehicle Service
Manual. Follow the steps in the Service Manual to pre-
pare the engine for a compression test, then install the
compression tester, figure 1-3, and run the test.
In general, a compression test is performed with the en-
gine at normal operating temperature, all spark plugs re-
moved, the ignition disabled, the battery fully charged,
and the throttle held in wide-open position. Interpret
compression gauge readings as follows:
• Compression is normal when the gauge shows a
steady rise to the specified value with each com-
pression stroke
• If the compression is low on the first stroke and
builds up gradually with each succeeding stroke,
but not to specifications, the piston rings or cylinder
walls are probably worn
• A low compression reading on the first stroke that
builds up only slightly on the following strokes indi-
cates sticking or burned valves
• Two adjacent cylinders with equally low compres-
sion indicates a head gasket leak between them
• A higher than normal compression reading usually
means excessive carbon deposits have formed on
the piston top or in the combustion chamber. Fluid,
such as oil, coolant, or fuel in a cylinder also pro-
duces high compression pressure
Fig. 1-3.
Compression tester gauge installed in the spark plug hole.
Fig. 1-2.
Typical engine analyzer power balance panel.
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