Power Plant Diagnosis With Scope Analyzer; Compression Test; Cylinder Leakage Test - Jeep 1979 CJ-5 Technical & Service Manual

GENERAL SERVICE AND DIAGNOSIS 1A-11
Service Diagnosis—Performance (Continued)
Condition
Possible Cause
Correction
(4)
Improper PCV valve air flow.
(4) Test PCV valve and replace
as necessary.
(5)
Vacuum leak.
(5)
Check manifold vacuum and
repair as necessary.
(6)
Clogged main jet(s).
(6) Clean main jet(s).
(7) Undersize main jet(s).
(7)
Replace main jet(s).
(8) Blocked air bleeds.
(8) Clean air bleeds.
(9)
Clogged fuel filter screen.
(9)
Replace fuel filter.
(10)
Restricted air cleaner. (10)
Clean or replace air cleaner.
DIAGNOSIS WITH SCOPE ANALYZER
The scope analyzer is an ignition tester that provides
quick and accurate diagnosis of ignition system per-
formance. All phases of the ignition cycle are shown
graphically on an oscilloscope (cathode ray tube) as they
occur in engine operation.
The manufacturers of scope analyzer equipment pro-
vide descriptions of test procedures possible with their
equipment. This section is not intended to cover all uses
of scope equipment, but to point out differences in scope
pattern between the conventional point system and the
SSI (Solid State Ignition) system used on Jeep engines
(fig.lA-1).
The upper section shows a typical scope pattern of a
conventional point system from firing line to firing line
and areas of the pattern significant to diagnosis. The
scope pattern shows time duration horizontally and volt-
age vertically.
Compare the scope pattern of the point system with
the typical pattern of the SSI system.
Note the somewhat longer duration of the spark line
shown on the SSI pattern. This longer spark provides
superior combustion with the leaner air-fuel mixtures
now used.
The SSI waveform pattern is below the zero line in the
coil section but otherwise is similar to that of the point
system in this area.
Other than the differences described, scope ignition
diagnosis procedures for point and SSI systems are es-
sentially the same.
COMPRESSION TEST
(1) Clean spark plug recesses with compressed air.
(2) Remove spark plugs.
(3) Remove coil wire from distributor and connect
to ground.
(4) Block throttle in wide open position.
70344F
(5) Insert compression gauge and crank engine for
three revolutions. Record reading on third revolution.
CYLINDER LEAKAGE TEST
Satisfactory engine performance depends upon a me-
chanically sound engine. In many cases, unsatisfactory
performance or rough idle is caused by combustion
chamber leakage. A compression test alone may not
reveal this fault. The cylinder leakage test provides an
accurate means of testing engine condition. Cylinder
leakage testing will point out exhaust and intake valve
leaks, leaks between cylinders, leaks into the water
jacket or any causes of compression loss.
(1) Check coolant level and add as required. Do not
install radiator cap.
(2) Start and run engine until it reaches normal
operating temperature, then turn ignition Off.
(3) Remove spark plugs.
(4) Remove oil filler cap.
(5) Remove air cleaner.
(6) Position carburetor fast idle speed screw on top
step of fast idle cam.
(7) Calibrate tester according to manufacturer's
instructions.
NOTE: Shop air source for testing should maintain
70
psi minimum and 200 psi maximum (80 psi
recommended).
(8) Perform test procedures on each cylinder ac-
cording to tester manufacturer's instructions.
NOTE: While testing, listen for air esearring through
carburetor, tailpipe or oil filler cap opening. Check for
bubbles in radiator coolant.
(9) All gauge indications should be even, with no
more than 25% leakage. For example, at 80 psi input
pressure, a minimum of 60 psi should be maintained in
the cylinder. Refer to Cylinder Leakage Test Diagnosis.