Why is My Car Burning Oil?

You’ve probably seen vehicles driving down the road emitting smoke from the tailpipe that’s blue in color, and if you’re behind one of those vehicles, it also doesn’t smell good. An engine burns oil when oil enters the combustion chamber of the engine (where it shouldn’t be) and gets burned along with the fuel/air mixture. It is important to note that a car that is burning oil may not always have smoke coming out of the tailpipe. In lower amounts, oil can be consumed and not show. Owners will notice this when they check oil and frequently have to add upwards of a quart of oil every time they check, yet there are no signs of any oil leak.

What Causes Oil Burning?

An engine is designed so that oil is circulated to lubricate all the moving parts of the engine, but not enter into the combustion chamber area and be burned. Naturally, a very small amount of oil can and will be present in the chamber, but in a normally operating engine, this amount is so minute that it is not noticeable. There are several key parts on an engine that can fail and cause oil consumption, which are listed below.

1. Blocked Positive Crankcase Ventilation (PCV)

When an engine operates, the “crankcase” portion of the engine (the oil pan, crankshaft, bottom of pistons, and oil galley leading up to the valve covers) becomes pressurized by blow-by from the pistons. To relieve this pressure, a crankcase ventilation system recirculates this air/oil vapor pressure into the intake tract as a one-way valve. If this system clogs, the blow-by sits in the crankcase and builds pressure, which can blow out gaskets, or force oil past the valves and into the combustion chamber.

Most cars historically have used a replicable PCV valve in this system, which should be changed every few oil changes in an interval, as they are inexpensive and easy to replace. Many manufacturers today have made this system more complicated and have a larger crankcase valve, some of which can even be electric. Many of these valves suffer clogs from oil vapor, and eventually completely block the system, leading to oil consumption.

2. Damaged Valve Seals or Guides

Valve seals serve one purpose – to keep oil that is flowing around the valves from leaking down into the combustion chamber. Valve guides also keep the valve’s path perfectly straight and square as they open and close.

Once a valve seal wears out, oil can leak past the seal. If the valve guides are also worn, this leak can be even worse. As this oil leaks down into the valves, it burns off. When valve seals degrade, oil that pools in the cylinder head after shut-down also can also leak down into the combustion chamber slowly.

This is evident on startup of the vehicle by a puff of blue smoke being emitted only at start up, as this oil that slowly leaked into the combustion chamber is suddenly burned off.

3. Broken or Worn Piston Rings

Piston rings do 2 things. For one, the compression rings allow for the piston to properly compress the air/fuel mixture without any leaks or blow-by. Second, the oil control rings scrape the lubricating oil from the cylinder walls so that no oil enters the combustion chamber.

As these rings wear, 2 things can happen. As compression rings deteriorate, blow-by gasses can escape, enter the crankcase, gather oil vapor, and be pushed into the intake tract via the PCV system. Secondly, and more commonly, the oil control rings can wear or become gunked up with carbon, leaving oil on the cylinder walls and combustion chamber, which is then burned. Worse yet, a ring can fracture or break, leading to an even bigger leak.

4. Oil Entering the Turbocharger

While this only applies to turbocharged vehicles, it is a frequent problem. Turbochargers use oil to lubricate the turbine bearings inside the unit. Seals are also in place to keep this oil from leaking past the bearings and either into the cold side of the turbo (the compressor side leading to the intake) or the hot side (the exhaust side leading to the rest of the exhaust system). Either leak will result in burned oil. Usually once the oil seal goes, the bearing follows, resulting in complete turbo failure.

Oil Burning Outside the Engine

If your engine isn’t burning oil internally, there is a chance that oil could be burning externally. This happens when oil leaks down onto the exhaust manifold, turbo, or exhaust system.

1. Leaking Valve Cover Gasket

For oil to leak and immediately be burned, it usually must start somewhere high up in the engine. One of the prime locations is a valve cover gasket that slowly seeps oil onto the exhaust manifold. In this case, you may never see an actual leak underneath as all of this oil is being vaporized by the hot exhaust manifold.

2. Broken PCV Tract

We talked before about the PCV system. This system is usually composed of several pipes or hoses that can also leak, and drip / force oil vapor down onto the exhaust.

3. Damaged Turbo Oil Line

Most turbo oil lines are either steel or braided stainless hoses. They can fail, leading to an oil leak directly onto the hot turbo or exhaust manifold.

4. Loose Oil Filler Cap

he oil filler cap is the cover to the opening through which you fill the engine. If the cap is missing, loose, or worn out, the engine oil might flow onto the surface of the engine and burn.

How Can You Fix a Car That is Burning Oil?

If you have an issue with a car burning oil, the most important thing is to check your oil often and carry extra oil. While it’s not a good idea to continue to operate your vehicle consuming oil, sometimes it’s also difficult to correctly diagnose where it’s coming from.

The first order of repair is diagnosing exactly the cause of the oil consumption. For this, if you need a mechanic to help, search through our Preferred Shops in your area to help you identify someone who can help diagnose what’s going on.

The cost of repairs will depend on the cause of the leak. A clogged PCV system could be as simple as a $10.00 replacement PCV valve. Worn piston rings will involve an entire engine overhaul or replacement, which will be thousands of dollars.

The best method of stopping oil consumption is proper prevention. Change your oil at proper intervals and consider using synthetic oil. Understand the parts of your engine’s PCV system, so the valve can either be changed, or cleaned in some cases. For all of these parts, and great advice, come see your AutoZone store today!

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