By Jim Kerr
The materials and finishes used inside today’s automotive engines makes them more dependable than ever, but recently I have had several inquiries about a variety of vehicles that are using oil. The only thing they all have in common is that they are higher mileage engines. Let’s see why they may be consuming more oil.
Oil leaks are the most obvious cause for low engine oil level. If an engine loses only a drop a kilometre, it would be dangerously low on oil after only a couple hundred kilometres. Fortunately, it is relatively easy to spot these leaks, as they will leave stains or wet spots on the driveway or parking spot. Looking beneath your vehicle is an excellent way to be pro-active in keeping your vehicle operating properly.
If there are no oil leaks, the oil must be going out the tail pipe. For this to happen, it must go past the piston rings, the valve seals, or through a vacuum leak on the intake side of the engine. On some V-style engines it is possible for the intake manifold gasket to fail on the bottom side of the manifold and oil is sucked in from the valley area of the engine. This rarely occurs on today’s engines, but it is a possibility. Engines with this type of leak will have one cylinder with fluffy, wet oil deposits on the sparkplug electrodes.
Most oil consumption goes past the piston rings or the valve seals. Pinging or detonation can break piston rings. The engine may operate fine but uses oil. Piston rings can also sometimes become stuck, typically on engines that are not operated for several months.
Engines with broken or stuck piston rings often do not use oil when driving around town because rpm’s are lower and there is less oil splashed on the cylinder walls for the rings to control. On the highway, or during high rpm operation, the engine can use oil quickly as it passes the rings and is burned in the combustion chamber. You likely will not see oil smoke out the tail pipe, although a person in a vehicle following you may be able to notice a slight amount. If the rings are broken, replacement is the only fix. If the rings are stuck, there are oil additives available that may free them up, but it can take some time.
Compression testers are not good diagnostic tools for oil consumption. The oil seals the rings so compression appears normal. One way of diagnosing oil consumption past piston rings is to inspect the tops of the pistons visually. Pistons normally have a coating of black carbon on their tops. Oil going past the rings will remove this black deposit around the edges, leaving clean aluminum. A remote camera with display screen is a commonly available automotive tool that will allow clear images in tight spaces.
Valves and valve guides wear as the engine operates. This creates bigger clearances where oil can flow down the guides into the combustion chamber. Oil seals on most engines limit the amount of oil but these seals can get hard and fail. Badly worn guides can also prevent the seals from working properly.
Oil that gets past the guides occurs mostly during variable throttle driving, such as stop and go city driving or hilly or mountainous highway roads. Every time the throttle is reduced, it creates higher vacuum that will try to suck oil past the intake guides. The engine won’t use as much oil during steady state speeds.
If you notice a puff of oil out the tailpipe when initially starting the engine, or after coasting to a slow speed and then accelerating, it is past the guides that the oil is being consumed. The first step is to have the valve seals replaced. This can be done without removing cylinder heads and is relatively low cost compared to cylinder head overhaul. If the seals are good, you are looking at major engine repair.
It is possible to operate an engine for many thousands of kilometres even though it is using oil. Just be sure to check it often and keep it topped up to the full mark. When the budget allows, then you can look at repairs or another vehicle.