By Jim Kerr
Some engines use more oil than others, but how much is too much? Leaf through the repair manuals of several auto manufacturers and you will find a variety of specifications. One states that the engine should use no more than 1 quart (.946 litres) of oil in 3200 km. Another states a litre in 500 km. This wide range of specifications challenges anyone to determine what is excessive. A lot has to do with the design of engine, the application, the oil viscosity and how it is operated.
When the oil level goes down, it has either been burned inside the engine or it has leaked past a gasket or seal. Even a small leak will cause the oil level to drop rapidly, but this problem is easily noticeable because of the oil stains on the bottom of the car and the
Determining how the oil disappears when there are no external leaks may seem complicated but it is really very simple. There are only a few ways where oil can enter the combustion chamber and be burned in the engine: past the valve guides, past the piston rings, through the engine ventilation system, or on some engines, through a leaking intake manifold gasket. Let’s look at each in a little more detail.
The engine’s valves move inside valve guides that are part of the cylinder head. A little oil inside the guide is necessary to provide lubrication for the moving parts and oil seals are used to limit the amount of oil that can enter. Excessive clearances between the valves and the guides or worn seals can allow too much oil to run down the guides and be drawn into the engine.
The only time you will see oil smoke from the tailpipe of a vehicle with worn valve guides or seals is when you first start up the engine after it has sat for several hours. The oil has had time to run down the guides and collect behind the valves. As the engine is cranked, the oil runs into the combustion chamber and is quickly burned. Stop and go driving will tend to pull more oil down the intake guides because of higher intake manifold vacuum but the oil is burned as it is pulled in and the problem has to be very bad before the oil smoke can be seen behind the vehicle.
The first step in repairing this problem is to replace the valve seals. Often the mechanic can estimate the wear in the valve guides while the seals are replaced. If guide clearance is too large, the cylinder heads must be removed for repairs.
Worn piston rings and cylinder walls allow oil past the piston into the combustion chamber. Oil is thrown onto the cylinder walls from the crankshaft bearings so large bearing clearances can also cause oil consumption because there is too much oil on the cylinder wall for the rings to control. High rpm driving will also throw more oil on the cylinder walls, so oil consumption can occur even with good piston rings.
Oil that goes past the piston rings may leave a smoke haze behind the vehicle while driving but won’t cause a cloud of smoke at engine start-up. Engine compression test results may appear to be fine but this is because the large amounts of oil on the cylinder walls help seal the rings at cranking speeds, so the best way to diagnose bad rings is visually. Remove the spark plugs and use a small light to look at the top of the piston. Normal burning cylinders will have an even brown or black carbon coating on the piston top while a piston with oil coming past the rings will have the carbon washed off around the edge of the piston.
Industrial engines and high performance engines are often built with larger clearances and will use more oil. Operating any engine under heavy loads or high rpm will also cause it to use more oil. Using an oil with a higher viscosity will reduce oil consumption but also uses more horsepower from the engine too.
Finally, other ways oil can be consumed in the engine are not common but a leaking intake manifold gasket on some V6 and V8 engines can allow oil to be pulled into the combustion chamber. Because the leak is on the underside of the intake manifold, it can be difficult to test for, but often this type of problem will cause the spark plug to foul in the one cylinder affected by the leak.
The engine ventilation system’s job is to remove harmful vapours from the crankcase. It does this by pulling the fumes through a PCV valve or orifice into the intake manifold where the fumes are burned. If the wrong PCV valve is installed, the flow rate may be too high and oil mist can be pulled through as well. This can use a lot of oil quickly, so always replace the PCV valve with the correct one.
Fixing excessive oil consumption can be costly but some of the problems can be repaired at relatively low cost. For most drivers, the engine oil shouldn’t drop below the “add” mark between oil changes. If it does, the challenge is to find out how the oil is being consumed.