The engine in your car, truck, or SUV relies on oil to keep its internal parts lubricated. Without a thin film of engine oil coating those components, friction from metal to metal contact would produce excessive heat and extreme wear leading to catastrophic damage. So, vehicle manufacturers make sure to include the proper oil type and weight to protect your engine, and they recommend changing the oil regularly to ensure many miles of service.
Sometimes an engine can lose oil due to a leak. An engine has many seams along the edges of its connected components. Those seams or gaps are sealed tight to prevent fluid leaks. But if those seals deteriorate, engine oil (also called motor oil) oil can escape. Leaks can come from around the oil pan, valve covers, intake manifold, or a number of other components that rely on seals to hold the oil inside the engine. A leak could also be caused by a faulty part, or even an oil filter or oil pan plug that is not tightened properly.
But my engine is not leaking
You know, if you scrape yourself on a sharp object or cut yourself while shaving, you might start to bleed. You would certainly notice at some point that blood has escaped. And what do you do? You treat the wound. Oil is often referred to as the “lifeblood” of an engine because of its importance to proper engine performance. So, if the oil is leaking, the issue needs to be addressed. How will you know if oil is leaking out of your engine? You will see signs, telltale symptoms that there is a problem: oil will show up somewhere in the engine bay and eventually make an appearance on the ground.
But oil can disappear in more ways than one. You have to be on the lookout for both external leaks and internal leaks.
It is one thing to bleed on the outside, and another to bleed on the inside. A bruise under the skin is an example of internal bleeding that, as with an external abrasion or laceration, provides a visible sign to alert you to the problem. Unfortunately, internal bleeding is not always so easy to notice, and it can become life threatening.
Likewise, with your car. An external oil leak might show itself as a puddle under your vehicle, but an internal leak, that is a different story.
That is why it is important to check your oil frequently, every one to two weeks. You are not necessarily looking at the color of engine oil, nor are you trying to discern whether or not the oil is dirty, but rather checking the level. Does the level fall in the normal range or is it low?
Naturally, if your engine has an oil leak, you would expect the level to drop over time. But what if you check the oil and the level is low on the dipstick, or you get a low oil alert from the oil life monitoring system, and yet you never see a puddle of oil underneath your car? In that case, your engine likely has some sort of internal oil leak.
Causes of an internal engine oil leak
The problem with an internal oil leak, because they are more difficult to detect, is that they might continue untreated until there is a larger problem. The cause of an internal leak, well, there are a few
While the cause of an external leak is usually a bad seal or gasket, an internal leak is a little different. That is not to say that a bad seal could not be the culprit; indeed it can. For instance, if the gasket between the engine block and the cylinder head fails (the head gasket), engine coolant (antifreeze) can leak into the crankcase and mix with the oil or vise versa, oil could leak into the coolant passageways and make its way to the radiator or overflow tank. Each of these conditions is due to internal bleeding. Clues to a bad head gasket (or possibly a bad intake manifold gasket) include discoloration of the oil (milky appearance) or of the antifreeze (oily brown appearance).
Barring any discoloration of the oil or antifreeze, here are a few other causes of internal bleeding of your engine oil:
- PCV Valve problem. The positive crankcase ventilation (PCV) system helps to eliminate unwanted gases produced during engine combustion. When combustion gases leak past the piston rings and into the crankcase, they are passed through the PCV valve and back into the combustion chamber to be burned off. When the PCV valve is defective or plugged, it can cause oil blowback, where the oil gets sucked into the engine through the air intake. Excessive oil consumption, a drop in fuel economy, engine misfire, and oil sludge can all be results of a bad PCV valve.
- Worn piston rings or cylinder walls. Burning oil is another internal bleeding condition. Sealing the sides of each piston from the cylinder in which it travels is a set of metal seals, or “rings”. These piston rings press against the cylinder wall and prevent combustion gases from escaping. Unfortunately, worn out rings or cylinders can lead to gaps through which gases can escape readily. More than that, oil can leak into the combustion chamber and burn off with the fuel. When oil becomes part of the combustion process, you might notice blue-tinted smoke coming from the exhaust.
- Faulty valve seals. The valves in an engine control when the air/fuel mixture can enter the combustion chamber and when exhaust gases can exit. An engine can have anywhere from 8 to 32 valves or more, all moving frantically inside the head of the engine. If, for instance, a valve stem or its seal are worn or damaged, oil can leak into the cylinders and burn off, just like with worn piston rings.
Likelihood of an internal oil leak
Interestingly, some of these conditions are considered, if not normal, at least common or even expected. That is the very reason your engine comes with an oil dipstick and/or an oil life monitoring system.
Engine component wear occurs naturally over time. It happens eventually to all engines. So, vehicle manufacturers are constantly trying to improve engine performance. That is one of the reasons that the automotive industry is moving to synthetic motor oil. The problem gets worse with age, and it is exacerbated by poor vehicle maintenance.
You might expect an older car to burn oil. But high-mileage vehicles are not the only ones that do. In fact, oil consumption is a relatively common problem, even for newer cars. A move toward lighter-weight oils to help improve fuel economy means that even a small amount of component wear can allow oil to leak into the combustion chamber.
How much oil can you expect an engine to burn? Well, vehicle manufacturers do not provide uniform guidance on what is “normal” oil consumption. But many suggest that it is normal. What is considered excessive for one company could be considered normal for another. An engine could actually burn as much as a quart of oil or more between oil changes and be considered “normal”.
That said, if your car is relatively new, say under 50K miles or so, it should not use more than a quart of oil between oil changes as a rule of thumb. If it does, you should have it checked out. Older engines might see more oil consumption. In any case, if there is not an external leak, the oil consumption is due to one of these internal leak issues.
If you are concerned about how much oil your engine is consuming, have it inspected by a qualified professional. Some shops offer a complimentary vehicle inspection with each oil change service.