Chapter A: General Diagnosis
8
Cylinder Leakage Test
A cylinder leakage tester, or leak-down tester, gives
more detailed results than a compression test. Used as
a follow-up to compression testing, a leakage test can
reveal:
• The exact location of a compression leak
• How serious the leak is in terms of a percentage
of total cylinder compression
The tester forces air into the combustion chamber
through the spark plug hole. A gauge installed in the air
line indicates how much pressure leaks out of the com-
bustion chamber. The gauge scale is graduated from 0
to 100 percent.
Calibrate the leakage tester according to the equipment
instructions before testing. To test a cylinder, the piston
must be at TDC of the compression stroke so that both
valves are closed. Install the test adapter in the spark
plug opening, connect the air hose, and pressurize the
cylinder. Note the percentage reading on the scale and
interpret as follows:
0-10 percent Good
10-20 percent Fair
20-30 percent Poor
30-100 percent Failed!
For cylinders with more than 20-percent leakage, pin-
point the cause of the leaks as follows:
• Air escaping through the air intake indicates a leak-
ing intake valve
• Air escaping through the exhaust indicates a leak-
ing exhaust valve
• Air escaping through the crankcase and PCV sys-
tem indicates worn or damaged piston rings, worn
cylinder walls, or a worn or cracked piston
• Air bubbles in the coolant indicates a leaking head
gasket or a crack in the engine block or cylinder
head casting
• High readings on two adjacent cylinders indicates
head gasket leakage or a casting crack between
cylinders
Ignition System Diagnosis
The automotive ignition system consists of a low-volt-
age primary circuit and a high-voltage secondary circuit.
Voltage varies within these circuits during operation. An
oscilloscope displays voltage changes during a period
of time, and is an ideal instrument for testing ignition
system operation.
Traditional automotive oscilloscopes are installed in
multifunction engine analyzer units that also contain
voltmeters, ammeters, ohmmeters, tach-dwell meters,
vacuum and pressure gauges, timing lights, and ex-
haust analyzers. Small hand-held oscilloscopes that can
also monitor the low-voltage signals of the engine man-
agement system are gaining in popularity.
All oscilloscopes work on the same principle; a voltage
trace of the system being tested is displayed on a view-
ing screen. Voltage traces are displayed as a graph of
voltage over time. The vertical scale on the screen rep-
resents voltage and the horizontal scale indicates time.
The voltage range of the scope is generally adjustable.
Primary circuits are measured in volts and secondary
circuits in kilovolts (kV), or thousands of volts. Time is
measured either as a percentage of one complete en-
gine cycle or in milliseconds (mS), thousandths of a sec-
ond.
An ignition system voltage trace, both primary and sec-
ondary, is divided into three sections, firing, intermedi-
ate, and dwell, figure 1-4. Deviations from a normal pat-
tern indicate a problem. In addition, most scopes will
display ignition traces in three different patterns. Each
pattern is best used to isolate and identify particular
kinds of malfunctions. The three basic patterns are:
• Superimposed pattern
• Parade pattern
• Stacked or raster pattern
In a superimposed pattern, voltage traces for all cylin-
ders are displayed one on top of another to form a sin-
gle pattern, figure 1-5. This display provides a quick
overall view of ignition system operation and can also
reveal certain major problems.
The parade pattern displays voltage traces for all cylin-
ders one after another across the screen from left to
Fig. 1-4.
Primary and secondary ignition oscilloscope patterns.
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