LO 10-8 Describe the purpose of scheduled oil sampling and outline the steps to properly obtain an oil sample

Excessively Low and High Oil Pressure

Virtually all HD, diesel-powered vehicles use a dash-mounted oil pressure gauge. Although engine protection systems shut down the engine, derate, or at least warn the driver of excessive oil temperature or low oil pressure, the presence of a prominent dash gauge underlines for the driver and the technician the importance of monitoring this engine parameter.

Low oil pressure can lead to oil starvation of critical engine components, which can cause seizure or rapid, premature wear. It is important to differentiate between low and abnormally low oil pressure. As engines wear normally, bearing clearances become wider, which results in more oil pump volume and lower oil pressure. This happens because pumps move oil volume while restrictions to volume (e.g., normal bearing clearances) produce pressure. Low oil pressure, or a drop in oil pressure from operating speed, is not necessarily abnormal. When oil becomes hot, it thins and causes a normal drop in pressure. Abnormal pressure is defined by the manufacturer and measured when the engine oil is at operating temperature, preferably with an accurate, direct reading mechanical gauge. The potential for reporting the wrong oil pressure due to a defective sending unit or dash gauge requires another independent measuring device to validate a dash gauge measurement. Before confirming a low oil pressure complaint, the OEM’s specification must be consulted; these numbers may surprise technicians and drivers alike because they are often lower than expected. The oil level, viscosity, and presence of dilution with fuel should be checked when investigating low oil pressure complaints.

Subsequent to preliminary checks, diagnosis of low oil pressure should follow an OEM fault tree to differentiate problems caused by blocked oil filters, plugged oil coolers, internal oil leaks, worn oil pumps, and faulty pressure-regulating valves and thermostats. Worn engine bearings are best checked by removing several main and rod bearing caps to determine whether the top layer of babbitt is worn away. Some engines may use an oil viscosity-regulating valve to correct oil pressure when the oil is cold or thickened. These valves can fail and produce a rapid drop in oil pressure only after the engine begins to warm up.

When investigating any oil pressure complaint, a technician should note where the dash sending unit is located in the lubricating system circuits. Generally, most sending units are located after the oil pump and before the oil filter. Watching little or low oil pressure for many minutes that only slowly rises after an engine starts cold can cause alarm for a driver; locating the sending unit between the oil pump and the filter minimizes occurrences for concern.

Complaints about low oil pressure should be verified when the engine is at operating temperature. Idle oil pressure and high idle oil pressure can be compared against the manufacturer’s specifications Figure 60.

FIGURE 10-60: Low oil pressure diagnosis process.

High oil pressure and abnormally high oil pressure need to be differentiated from normal oil pressure following procedures similar to those used to investigate low oil pressure. The high system pressure must be validated with an independent, accurate pressure gauge. While plugged filters, thick oil, and even tight engine bearings or cam bushings can produce high oil pressure, abnormally high pressure is defined by OEM specifications. Generally, abnormally high oil pressure is caused by a defective oil pressure-regulating valve. Replacing the valve after visually inspecting it is often the only effective diagnostic procedure in this case. Operation of the valves and oil pump are evaluated using a direct reading pressure gauge in the main oil gallery. A high oil level can cause oil to foam because the oil contacts the rotating crankshaft, thereby aerating the oil. As a result, the oil cannot properly pressurize and produces erratic readings as well as rough running, stalling, and other related drivability problems with HEUI fuel systems.