LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal

Crankcase Pressure Testing
One of the best practices for evaluating the condition of piston rings in a diesel engine is a crankcase pressure test. This is a measurement of the amount of cylinder blowby, which is proportional to the sealing ability of the piston rings and cylinder walls. Because blowby gas pressure in the crankcase is a function of the effectiveness of the piston ring-to-cylinder wall seal, the quantity of blowby and crankcase pressure is a good measure of piston ring and cylinder wall condition. Excessive blowby and crankcase pressure or blowby volume means there is poor sealing and the cylinders are worn out. Crankcase blowby pressure is usually very low—just a few inches (centimetres) of water column pressure when the engine is under full load and the rings are seated well by high cylinder pressure. When the piston rings and cylinder walls are worn, blowby volume increases, driving up the crankcase pressure. Blowby testing is a fast, simple, and nonintrusive technique to evaluate the condition of piston ring-to-cylinder wall sealing Figure 72. Pressure is measured at high-idle, no-load operation. When piston rings seal well, little blowby gas enters the crankcase. When cylinder pressures are higher, such as during high idle rpm operation, the rings seal much better than they would at low engine rpm. Crankcase pressure is typically higher at idle than at high idle in good engines; however, glazed cylinders are indicated when crankcase pressure is significantly higher at idle and drops as engine speed increases.

FIGURE 07-72: Measuring crankcase blowby pressure involves the use of a restrictor orifice and a Magnahelic gauge measuring inches of water column pressure (or kilopascals). Blowby is measured on a warm engine operated at high-idle rpm.

FIGURE 07-73: A Magnahelic gauge measures pressure in inches of water column (or kilopascals). These gauges are much more sensitive to pressure changes than measuring using with mercury.

FIGURE 07-74: Various types and diameters of restriction orifices for measuring crankcase pressure.

Pressure in the crankcase is measured in inches (centimetres) of water column using a water manometer or Magnehelic gauge Figure 73. A calibrated restriction fitting is placed over the crankcase breather tube Figure 74. If equipped, the crankcase depression regulator, a device that limits how low crankcase pressure can fall, is blocked during the test. After the engine coolant and oil are warmed to operating temperature, the engine is put to full throttle, no load, and crankcase pressure is measured. Usually only a few inches (centimetres) of pressure are measured in an engine in good condition. Small-bore diesels will typically have a maximum limit of 10" (25 cm) of water column pressure indicating 0% service life. Larger engines may be allowed as much as 15–18" (38–46 cm) of water column pressure TABLE 7-3.

To perform a crankcase blowby test, follow the guidelines in Skill Drill 7-3.