John Deere stx38 - Manual (Page 54)

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4/23/97
TESTS AND ADJUSTMENTS ENGINE
CYLINDER COMPRESSION TEST
Reason:
To check pressure capacity of piston rings and cylinder
bore for efficient engine operation.
IMPORTANT: On engines equipped with an
automatic compression release (ACR)
mechanism, a compression test will indicate the
ACR compression, not actual compression. A
compression test does not provide an adequate
method to diagnose the condition of the rings
and valves. A cylinder leakdown test is the best
method to determine the condition of the rings
and valves. By pressurizing the combustion
chamber from an external source, you can
determine if the valves or rings are leaking.
Equipment:
• JT-03502 Cylinder Leakdown Test Kit
—ACR engines only
• JDM-59 Compression Gauge
• JDM-74A-5 Spark Plug Ground Tool
Connections:
1. Move transaxle shift lever to neutral. Engage park
brake.
IMPORTANT: Spark plug wire must be grounded
or electronic ignition could be damaged.
2. Disconnect and ground spark plug wire.
3. Remove spark plug and install JDM-59
Compression Gauge.
Procedure:
1. Move the throttle lever to FAST idle position.
2. Crank engine and record compression reading.
Specifications:
Minimum Compression . . . . . . . . . 345 kPa (50 psi).
Normal Compression. . . . . . . . . . . 550 kPa (80 psi).
Results:
• If compression is low, put clean engine oil on piston
rings through spark plug hole. Repeat cylinder
compression test.
• IF COMPRESSION PRESSURE INCREASES:
check rings, piston and cylinder bore for broken
rings, scoring, wear, or damage. Replace as
necessary.
• IF COMPRESSION PRESSURE IS STILL LOW:
check for leaking intake or exhaust valves, valve
seal, or cylinder head gasket. Replace as
necessary.
AUTOMATIC COMPRESSION
RELEASE (ACR) TEST
Reason:
To determine if the automatic compression release is
opening the exhaust valve.
Equipment:
• Dial Indicator
Procedure:
1. Remove valve cover and spark plug. Rotate
crankshaft slowly to observe ACR operation.
2. The exhaust valve (A) must open (depress) briefly
just after the intake valve closes.
3. Use a dial indicator to measure exhaust valve ACR
movement.
4. Install rocker cover, see Remove and Install
Cylinder Head in the this section.
Specifications:
Exhaust valve ACR movement
Minimum . . . . . . . . . . . . . . . . . . 0.25 mm (0.01 in.)
Results:
• If the exhaust valve does not open or depress
properly, the automatic compression release tab is
faulty and camshaft assembly must be replaced.
c
CAUTION
Engine components are HOT. DO NOT touch
with bare skin, especially the exhaust pipe or
muffler while making test. Wear protective eye
glasses and clothing.
M55936
A
M55859