Readings of the compression and firing pressures must be taken every 200 hours for the trend analysis graphs. They may also be taken at other times when engine operating conditions require additional monitoring such as when an engine misfires, fires erratically, or when any one cylinder misfires regularly. There can be many reasons for an engine to misfire, some of these are a clogged air cleaner/filter, an engaged fuel cutout mechanism, or a loss of compression. If, after checking the air cleaner, the filter, and the fuel cutout mechanism, you determine that the problem is due to loss of compression, then you must perform a compression check with a cylinder pressure indicator.

There are several different types of indicators that may be used. Most indicators used with diesel-cylinder engines are either of the spring balanced type or the trapped pressure type. They are manufactured by various companies such as Kiene, Bacharach, and Kent-Moore. Some of these indicators measure only compression pressure, others measure both compression and firing pressures.

Spring Balanced Indicator

A spring balanced indicator, such as the one manufactured by Bacharach (figure 3-1), employs a spherical ball piston, which is held on its seat by the force of a helical spring actuated by the cylinder pressure which acts against the bottom of the ball piston to oppose the spring tension. Before the indicator is attached to the engine, the vulcanized handle must be rotated clockwise until the reading on the counter is greater than the maximum cylinder pressure expected. The amount of this pressure is listed in the engine manufacturers technical manual. When the indicator is installed, the operator must make sure that it is placed as near the cylinder as possible and position it so that it can be read easily. After the indicator is installed the engine is operated at the specified rpm, then the fuel to the cylinder

Figure 3-1.-Spring balanced Pressure Indicator.

being tested is cut out, the cylinder test cock is opened, and the spring tension on the indicator is adjusted. The tension of the spring is reduced by rotating the vulcanized handle counterclockwise until the maximum cylinder pressure barely offsets the spring pressure. At this point, the latch mechanism of the indicator trips and locks the handle firmly in position, giving a direct and exact reading of the pressure in pounds per square inch (psi). To reset the lock mechanism for a new reading, the handle must be rotated counterclockwise one-fourth turn. When this in-dicator is stowed for future use, the indicator spring must be unloaded by rotating the handle counterclockwise until a zero pressure reading is obtained.