intake air, piston seizure, and defective auxiliary drivemechanisms.MisfiringWhen an engine misfires or fires erratically or whenone cylinder misfires regularly, the possible troubles areusually associated with the fuel or fuel system, wornparts, or the air cleaner or silencer. In determining whatcauses a cylinder to misfire, you should followprescribed procedures in the appropriate technicalmanual. Procedures will vary among engines because ofdifferences in the design of parts and equipment.Many of the troubles caused by fuel contaminationrequire overhaul and repair. However, a cylinder maymisfire regularly in some systems because of the fuelpump cutout mechanism. Some fuel pumps have thistype of mechanism so the fuel supply can be cut off froma cylinder to measure compression pressures. When acylinder is misfiring, check first for an engaged cutoutmechanism (if installed), and disengage it during normalengine operation.LOSS OF COMPRESSION.–Acylinder maymisfire due to loss of compression, which may be causedby a leaking cylinder head gasket, leaking or stickingcylinder valves, worn pistons, liners or rings, or acracked cylinder head or block If loss of compressionpressure causes an engine to misfire, check thecompression pressure of each cylinder. Some indicatorsmeasure compression as well as firing pressure whilethe engine is running at full speed. Others check onlythe compression pressures with the engine running at arelatively slow speed. Figure 3-38 illustrates theapplication of some different types of pressureindicators.After you install an indicator, operate the engine atthe specified rpm and record the cylinder compressionpressure. Follow this procedure on each cylinder in turn.The pressure in any one cylinder should not be lowerthan the specified psi, nor should the pressure for anyone cylinder be excessively lower than the pressures inthe other cylinders. The maximum pressure variationpermitted between cylinders is given on engine datasheets or in the manufacturer’s technical manual. Acompression leak is indicated when the pressure in onecylinder is considerably lower than that in the othercylinders.If a test indicates a compression leak, you will haveto do some disassembly, inspection, and repair. Checkthe valve seats and cylinder head gaskets for leaks, andinspect the valve stems for sticking. A cylinder head orblock may be cracked. If these parts are not the sourceof trouble, compression is probably leaking past thepiston because of insufficient sealing of the piston rings.Improper Cooling Water TemperatureIf an engine is to operate properly, the cooling watertemperature must be maintained within specifiedtemperature limits. When cooling water temperaturedrops lower than recommended for a diesel engine,Figure 3-38.–Engine cylinder pressure indicator application.3-31
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business