intake air, piston seizure, and defective auxiliary drive
mechanisms.
Misfiring
When an engine misfires or fires erratically or when
one cylinder misfires regularly, the possible troubles are
usually associated with the fuel or fuel system, worn
parts, or the air cleaner or silencer. In determining what
causes a cylinder to misfire, you should follow
prescribed procedures in the appropriate technical
manual. Procedures will vary among engines because of
differences in the design of parts and equipment.
Many of the troubles caused by fuel contamination
require overhaul and repair. However, a cylinder may
misfire regularly in some systems because of the fuel
pump cutout mechanism. Some fuel pumps have this
type of mechanism so the fuel supply can be cut off from
a cylinder to measure compression pressures. When a
cylinder is misfiring, check first for an engaged cutout
mechanism (if installed), and disengage it during normal
engine operation.
LOSS OF COMPRESSION.–A cylinder may
misfire due to loss of compression, which may be caused
by a leaking cylinder head gasket, leaking or sticking
cylinder valves, worn pistons, liners or rings, or a
cracked cylinder head or block If loss of compression
pressure causes an engine to misfire, check the
compression pressure of each cylinder. Some indicators
measure compression as well as firing pressure while
the engine is running at full speed. Others check only
the compression pressures with the engine running at a
relatively slow speed. Figure 3-38 illustrates the
application of some different types of pressure
indicators.
After you install an indicator, operate the engine at
the specified rpm and record the cylinder compression
pressure. Follow this procedure on each cylinder in turn.
The pressure in any one cylinder should not be lower
than the specified psi, nor should the pressure for any
one cylinder be excessively lower than the pressures in
the other cylinders. The maximum pressure variation
permitted between cylinders is given on engine data
sheets or in the manufacturer’s technical manual. A
compression leak is indicated when the pressure in one
cylinder is considerably lower than that in the other
cylinders.
If a test indicates a compression leak, you will have
to do some disassembly, inspection, and repair. Check
the valve seats and cylinder head gaskets for leaks, and
inspect the valve stems for sticking. A cylinder head or
block may be cracked. If these parts are not the source
of trouble, compression is probably leaking past the
piston because of insufficient sealing of the piston rings.
Improper Cooling Water Temperature
If an engine is to operate properly, the cooling water
temperature must be maintained within specified
temperature limits. When cooling water temperature
drops lower than recommended for a diesel engine,
Figure 3-38.–Engine cylinder pressure indicator application.
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