In general, the troubles that prevent an engine from starting are (1) the engine can neither be cranked nor barred over, (2) the engine cannot be cranked, but it can be barred over, and (3) the engine can be cranked, but it still fails to start. Figure 3-36 illustrates various conditions that commonly cause difficulties in cranking, jacking over, or starting the engine.

Engine Cannot Be Cranked nor Barred Over

Most prestarting instructions for large engines require you to turn the crankshaft one or more revolutions before applying starting power. If you cannot turn the crankshaft over, check the turning gear to be sure it is properly engaged. If the turning gear is properly engaged and the crankshaft still fails to turn over, check to see whether the cylinder test valves or indicator valves are closed and are holding water or oil in the cylinder. When the turning gear operates properly and the cylinder test valves are open but the engine still cannot be cranked or barred over, check for a serious problem. A piston or other part may be seized or a bearing may be fitting too tightly. Sometimes you may need to remove a part of an assembly to remedy the difficulty.

Figure 3-36.Troubles that may prevent a diesel engine from starting.

Some engines have ports through which pistons can be inspected. If inspection reveals that the piston is defective, remove the piston assembly. Figure 3-37 illustrates testing for stuck piston rings through the scavenging-air port.

If the condition of an engine without cylinder ports indicates that a piston inspection is required, you must take the whole piston assembly out of the cylinder. Engine bearings must be carefully fitted or installed according to the manufacturers instructions. When an engine cannot be jacked over because of an improperly fitted bearing, someone probably failed to follow instructions when the unit was being reassembled.

Engine Cannot Be Cranked but Can Be Barred Over

You can trace most of the troubles that prevent an engine from cranking, but not serious enough to prevent barring over, to the starting system. Although other factors may prevent an engine from cranking, only troubles related to starting systems are identified in this chapter.

If an engine fails to crank when you apply starting power, first check the turning or jacking gear to be sure it is disengaged. If this gear is not the source of the trouble, the trouble is probably with the starting system.

Figure 3-37.Checking the condition of the piston rings.

Engine Can Be Cranked, but Fails to Start

Although the design of air starting systems may vary, the function remains the same. In general, such systems must have a source of air, such as the compressor or the ships air system; a storage tank; air flask(s); an air timing mechanism; and a valve in the engine cylinder to admit the air during starting and to seal the cylinder while the engine is running. All air starting systems have a unit that admits starting air to the proper cylinder at the proper time. The type of unit as well as its name-timer, distributor, air starting pilot valve, air starting distributor, or air distributor-may vary from one system to another. The types of air timing mechanisms are the direct mechanical lift, the rotary distributor, and the plunger-type distributor valve. The timing mechanism of an air starting system is relatively trouble-free except as noted in the following situations.