What Is the Difference Between a Car Engine Burning Oil Vs. Leaking Oil?
Newer car engines rarely burn or leak oil. This is because the factory seals, O-rings, gaskets, bearings and components are new and undisturbed. With higher mileage and age, seals and gaskets break down, allowing oil seepage. Oil can leak past seals and gaskets, to enter the combustion chamber where it burns. Oil leaks not associated with the intake system or combustion chamber can show up as visible leaks on the ground's surface. The difference between leaking oil and combustion-burnt oil can be straightforward and uncomplicated. However, oil that enters the combustion chamber can be the direct result of internal leaks and a vehicle owner should know the difference.
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Signs of External Oil Leaks
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The lower part of the engine provides the primary sources of external oil leaks, especially where pressure exists, or gaskets or seals must hold most of the engine oil. The rear main seal at the rear of the crankshaft will often leak oil, due to a faulty seal at the rear bearing location. The oil pan bolts, valve cover gasket, oil drain plug, oil filter and sensor, oil cooler and oil lines -- if so equipped -- will also show visible signs of leaking oil. Any puddle on the ground, or visible oil film or stream on the chassis or bottom of the engine, points to a failed seal, bearing, gasket or O-ring. If the visible marker on the oil dipstick constantly reports low readings, it can indicate a substantial external oil leak.
Signs of Oil Burning
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Oil burning originates in the combustion chamber of the engine. The most notable sign appears as a blue-white smoke exiting the exhaust pipe or pipes. Large clouds can appear just after engine warmup upon acceleration. Combustion-burnt oil can drip from the end of the exhaust pipe, or leak from exhaust pipe fittings and pipe clamps. Excessive oil burning can cause an engine miss or poor performance, unlike an external oil leak. Excessive oil inside the catalytic converter can cause clogging, leading to back-pressure, which can cause hard-starting and poor engine performance. Combustion-burnt oil will show low levels on the oil dipstick.
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Causes for Oil Burning -- Valve Guides and Seals
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Engine valves seat inside valve guides, which are precisely drilled and honed sleeves. If the valve guides become worn, they let oil pass from the head into the combustion area and burn through the exhaust. Valve seals, which consist of small rubber O-rings, also keep oil from passing down the valve stem shaft, and if worn or broken, allow raw oil to seep into the combustion chamber. Oil-fouled plugs will appear wet or contain an oily sheen, and frequently cause the spark plug to misfire.
Causes for Oil Burning -- PCV Valve and Rings
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Positive crankcase ventilation, or PCV, consists of a valve component and tube or hose routing that recirculates engine bypass gases back into the intake. If the PCV valve clogs shut, or sticks periodically, it can cause high pressure in the engine crankcase. High crankcase pressure can cause excessive oil consumption, especially if the valve guides and seals are worn. Worn piston rings allow combustion gas pressure to enter the engine crankcase and cause the same high pressure. Oil diluted by gasoline, in the case of worn rings, will have a higher tendency to leak from worn O-rings, gaskets and seals.
Causes for Oil Burning -- Head Gaskets
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Depending upon the configuration of the vehicle's head gasket, an oil jacket in proximity to the edge of the cylinder can leak oil into the combustion chamber. If the head gasket is blown in this location, the oil can be sucked into the combustion chamber and burnt along with the fuel mixture. This will show as blue-white smoke exiting the exhaust pipe or pipes.
Summary
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Although a difference exists between a common oil leak and that of oil-burning, the difference is a fine line. Internal oil leaks, although different from external oil leaks, can be directly responsible for oil-burning, whereas an external oil leak will not show a decrease in engine performance, or show any visible signs of oil burning. However, they will both shows signs of oil loss with low readings on the oil dipstick.
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References
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