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We discussed earlier several of the troubles that may cause an engine to stall or stop. Such troubles as air in the fuel system, clogged fuel filters, unsatisfactory operation of fuel injection equipment, and incorrect governor action not only cause starting failures or stalling but also cause other troubles as well. For example, clogged fuel oil filters and strainers may lead to a loss of power, to misfires or erratic firing, or to low fuel oil pressure. Unfortunately, a single engine trouble does not always manifest itself as a single difficulty but may be the cause of several major difficulties.

Factors that may cause an engine to stall include misfiring, low cooling water temperature, improper application of load, improper timing, obstruction in the combustion space or in the exhaust system, insufficient intake air, piston seizure, and defective auxiliary drive mechanisms.


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 dropslower than recommended for a diesel engine,

Figure 3-38.–Engine cylinder pressure indicator application.

ignition lag is increased, causing detonation, which results in rough operation. This may cause the engine to stall.

If the water temperature is higher than normal, the engine may not cool properly and may suffer heat damage. Water temperature is controlled primarily by a thermostatic valve (thermostat). The thermostat normally operates with a minimum of trouble. High or low cooling water temperature may indicate a malfunctioning thermostat. But before you remove the thermostat to check it, check to see whether the improper temperature may be caused by an insufficient engine load or an inaccurate temperature gauge. When you suspect that the thermostat is not operating properly, remove it from the engine and test it. Use the following procedure to test the thermostat:

1. Obtain an open-topped container such as a bucket or a pot.

2. Heat the water to the temperature at which the thermostat is supposed to start opening. This temperature is usually specified in the appropriate technical manual. Use an accurate thermometer to check the water temperature. Use a hot plate or a burner as a source of heat. Stir the water frequently to ensure uniform distribution of the heat.

3. Suspend the thermostat by a string or a wire so that operation of the bellows will not be restricted.

4. Immerse the thermostat and observe its action. Check the thermometer readings carefully to see whether the thermostat begins to open at the recommended temperature. (The thermostat and thermometer must NOT touch the container.)

5. Increase the temperature of the water until the specified FULL OPEN temperature is reached. The immersed thermostat should be fully open at this temperature. Replace the thermostat if it does not open when you test it, or if the temperatures at which the thermostat opens and closes vary more than allowed from the manufacturer’s specifications.

The Fulton-Sylphon automatic temperature regulator is relatively trouble-free. The unit controls temperatures by a valve that bypasses some water around the cooler. This system provides a full flow of the water, although only a portion may be cooled. In other words, the full volume of cooling water is circulated at the proper velocity, which eliminates the possibility of steam pockets in the system.

Usually, if the automatic temperature regulator fails to maintain cooling water at the proper temperature, it simply needs to be readjusted. However, the element of the valve may be leaking or some part of the valve may be defective. Failure to follow the proper adjustment procedure is the only cause for improper adjustment of an automatic temperature regulator. Check and follow the proper procedure in the manufacturer’s technical manual issued for the specific equipment.

Adjust the regulator by changing the tension of the spring (which opposes the action of the thermostatic bellows) with a special tool that turns the adjusting stem knob or wheel. Increasing the spring tension raises the temperature range of the regulator, and decreasing it lowers the temperature range.

When you place a new valve of this type into service, you must take a number of steps to ensure that the valve stem is the proper length and that all scale pointers make accurate indications. Make all adjustments according to the valve manufacturer’s technical manual.


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