LO 10-9 Identify potential causes of high oil consumption, the purpose and function of oil level sensors, and maintenance procedures to correct oil consumption

High Oil Consumption

Engines can experience high oil consumption for a multitude of reasons, but ultimately oil is either being burned in the combustion chamber or leaking from the engine. When engine oil is leaking, the common procedure to identify an external leak location is to steam clean or power wash the engine. Then ultraviolet (UV) dye is added to the engine oil, and the engine is run at operating temperature for at least 20 minutes. A black light is then used to pinpoint the source of the leak. Gaskets, rear main seals, and turbochargers are common external leak paths for engine oil. Air compressors are frequently known to pump oil into the engine’s intake when unloaded, which produces unexplained high oil consumption. The air intake pipe to the compressor should be checked as well as the air reservoirs and air dryer for passing excessive amounts of oil.

Worn rings and valve guides are another point where oil can disappear into the engine. Worn rings allow oil into the combustion chamber. Piston rings that are coated in carbon, stuck, or seized can provide little to no oil control as well. These conditions are accompanied by excessive crankcase blowby, and if a road draft tube ventilates the crankcase, the area below and in the tube airstream will show heavy oil contamination. High crankcase pressure can also push more oil from a closed crankcase ventilation system into a turbocharger or engine intake system. Thin oil or oil that does not have the correct viscosity will more easily pass between tight clearances between valve guides, the piston, and the cylinder walls.

Related to high oil consumption is a high oil level. In this situation, the engine appears to “make” oil. Often, the source of this complaint is fuel leakage into the engine. A misfiring injector allows fuel to wash into the crankcase. A low-pressure, fuel-system jumper tube beneath the valve cover or a high- pressure tube for a common rail injector can quickly leak fuel into the engine. Leaking injector O-rings, which are caused by loose injectors or cut injector O-rings, cause fuel to flow from around the injectors and onto the deck of the cylinder head. UV dyes can be used to inspect for fuel system leaks. However, visual inspection of fuel lines when the engine is running is the most common technique used to identify internal fuel system leaks. Remember, a technician should never use their hand to check for leaks because fuel under high pressure can easily penetrate skin and cause blood poisonin

When checking the engine oil level, the acceptable oil level range is stamped on the oil dipstick Figure 61. An excessive oil level will cause the oil to foam when it contacts the crankshaft or simply leak out of the engine through the rear main oil seal located at the end of the crankshaft Figure 62. High oil consumption complaints need to be verified by an actual measurement of oil used for a given distance Figure 63. Usually the technician diagnosing the complaint, and not the customer, will add oil to the engine in order to provide a verifiable and accurate record for oil consumption.

FIGURE 10-61: The oil dipstick measures the engine’s oil level. The acceptable level is between the MAX and MIN markings.

FIGURE 10-62: The rear main seal of the engine prevents oil from leaking out of the crankcase from around the end of the crankshaft.

FIGURE 10-63: High oil consumption diagnosis process.