Oil Leaking, Blow-By, and Excessive Oil Consumption

How to troubleshoot extreme engine oil loss

You check the oil level in your car every week. It’s routine, and part of your automotive repair ritual. But one of those checking days, you notice the oil level is down. No big deal, but every week it just seems to be getting worse. What’s up with that? The truth is, engine oil that’s missing in action spells trouble and it can usually be attributed to three things: oil leaking, blow-by, or oil consumption (burning oil). The easiest to fix (but perhaps not the easiest to troubleshoot) are oil leaks. We’ll begin there:

Oil Leak

“Missing” engine oil can often be the result of good old-fashioned oil leaks out of tears in a gasket or at any point where two components of the engine are joined (a good example and common culprit is the valve cover gasket on the cylinder head). Other locations where oil leaks can be expected tend to be around oil seals and around moving parts that exit the internals of the engine. These can include the crankshaft seals around the nose and back ends of the crank (typically where the crankshaft protrudes from the timing chain cover and the rear main seal between the cylinder block and the crankshaft flange (the flange is where the flywheel or flex plate attaches to the crankshaft).

An even more common location for oil leaks is the area around the oil filter and around the edges of the oil pan and the pan drain plug. Keep in mind that a small amount of oil loss will occur at these points when the engine oil is changed, but if you climb under your car and see evidence of large amounts of oil on the underside of the engine or near the transmission tunnel, you know there’s a major leak somewhere.

When searching for oil leaks, keep in mind that fresh oil dripping from the inside of the engine will certainly prove cleaner than oil that has been outside the engine for a long period of time (remember that oil is a magnet for dirt). If the oil loss proves considerable and has coated a large area of the engine, it may be necessary to pressure wash the engine and engine compartment before attempting to track down the oil leak. In many instances, once you find the leak, it’s often just a case of tightening the component that is leaking. Case-in-point are the valve covers. They use relatively small fasteners and they can vibrate loose. If you find that the gasket is the culprit then it has to be replaced (they’ll leak even if the fasteners are tight). Main seals are another matter too. They often require considerable engine disassembly to enable replacement, and that’s beyond our scope for this article.


One cause of oil leaks few ever consider is blow-by. Blow-by is usually created when piston rings become so worn they allow pressure from the combustion process to pressurize the oil pan area. The combustion process blow-by is sufficiently strong to force oil out around the pan gasket and even past oil main seals in severe situations.

But there is another cause of blow-by. It can occur when the crankcase ventilating system is plugged up and cannot relieve internal pressures. Modern cars and light trucks are equipped with a positive crankcase ventilation system (PCV). This is a simple arrangement of hoses coupled to a check valve that directs crankcase vapor and pressures back into the intake manifold. This allows the vapors to be burned again within the engine. Many older cars (typically pre-1968) were not equipped with a PCV system; they merely exhausted crankcase vapors into the atmosphere by way of a draft tube setup. The draft tube arrangement was revised to meet emission (smog) requirements.

The check valve found within most PCV systems is a basic one-way valve used to prevent the reverse flow of air (and oil) through the system. The valve is designed to be serviced and, over time, it can become plugged with sludge and dirt. If it becomes plugged, it is impossible for the system to relieve crankcase pressure. The result is oil leaking (much like those caused by blow-by).

In order to test the operation of the PCV system, track down the hose leading from the valve cover to the base of the carburetor or fuel injection intake manifold. At some point in this line you will find the PCV valve. Fire up the engine and allow it to reach normal operating temperature. Using a set of smooth jaw pliers, pinch the rubber PCV line closed. If the PCV system is functioning properly, the engine's idle speed will drop almost 100 rpm (that's an audible drop). If the car doesn't experience an rpm, there's a good chance the PCV valve is plugged.

Over the years, a few PCV valves could be disassembled and cleaned, but for the most part the valves are throwaway items. For older cars, the PCV valve should be replaced at each tune up. Modern cars have different service intervals. Your owner's manual should provide information on when (usually mileage based) the PCV is serviced.

There’s one more external culprit (and it’s rather rare): On select cars equipped with a vacuum booster pump, excessive oil consumption can occur if the internal diaphragm is torn or ruptured. If the diaphragm is damaged, the vacuum pump can suck oil vapor from the crankcase and direct it to the intake manifold. If that occurs, it will appear as if the car has worn piston rings.

Worn Components

If the engine is burning oil, it simply means that oil is internally allowed to enter the combustion chamber. A major sign of an engine burning oil is a smoky blue exhaust. It’s most prevalent once the engine has been idling for a few minutes and is then accelerated. Another telltale symptom of burning oil is fouled spark plugs. Here the spark plugs will be coated in a black, gummy (wet) substance. Now, if you scroll back up the page, keep in mind that the PCV system (and to a lesser degree, a car fitted with a vacuum booster pump) can show similar symptoms. The key though is the volume: An engine with worn internals will have more pronounced symptoms than a faulty PCV.

There are two principal locations where oil can enter the combustion chamber: the valve guides and the piston rings. There are often seals on the valve guides to prevent oil from entering the combustion chamber. Those seals along with piston rings wear over time and they begin to allow oil into the combustion chamber. Once oil is in the combustion chamber, it burns. Valve stem seals are easy enough for a mechanic to replace, however if the oil consumption becomes serve, it's likely time for the engine to be rebuilt.

For a closer look at how to troubleshoot oil leaks, blow-by, and excessive oil consumption, check out the accompanying photos.

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