What causes an engine to burn oil?

Question

What are the different problems that can cause an engine to burn oil and in general how difficult / expensive is each one to repair? How do you diagnose each potential cause?

I found this article which describes some of the problems:

Why Does My Engine Use Oil

I've seen three things so far:

• Bad Piston Rings
• Bad PCV
• Valve Guide Seals

Regarding diagnosing oil consumption and smoke the above article says the following:

With older vehicles this was typically accompanied with a puff of smoke from the exhaust. With modern vehicles, the catalytic converter usually prevents smoke. Smoke in the exhaust is vaporized by the converter. Unfortunately, this may drastically raise the temperature and damage the converter, over time.

Answer 1

Towards the diagnosis end of things, there are some general guidelines to follow:

If you are seeing smoke coming from your exhaust, what color is the smoke?

• If it's blue, then it's oil
• If it's black, it means you are running rich (too much fuel).
• If it's white, the car may be burning antifreeze or (quite rare) auto-trans fluid.

Since it's blue smoke you are seeing, you can know that oil is the problem. What can cause oil smoke?

• If the smoke only comes at start-up and quickly goes away, it can be valve seals and/or valve guides. This is because while the car sits for extended periods of time, the oil has time to seep past the valve seal and collect on top of the valve (or if the valve is in the open position, it could run past and on top of the piston). When you go to start your car, the oil is then burned, giving the tell tale puff of blue smoke. COST TO FIX: There is moderate cost involved with this, depending on the vehicle/engine. If just the seals, this can be accomplished most of the time with keeping the engine mostly together and replacing the seals. Most of the cost here is labor. If the valve guides, this requires an engine tear down. Your heads will have to be taken apart and new guides installed. There is a lot more labor here and a bit more in parts.

• If you see smoke as you are decelerating, this too is a possible sign of valve seals and/or valve guides. This is because as you decelerate, there is a large vacuum build up within the intake tract. There is enough vacuum to pull oil past the valve seal if it's worn. COST TO FIX: Same as above.

• If you see smoke while accelerating, this is a sign your oil control rings are worn. These are the bottom rings used in the ring pack. When they are worn (or the cylinders are worn past tolerance), oil can flow past the rings as the piston travels down the cylinder. The oil control rings normally will scrape the cylinder of the oil, pushing it back down into the crankcase. COST TO FIX: Their fix is quite expensive, as it requires a complete engine rebuild to fix.

• If a PCV is bad, you normally won't see burning oil. What you'll see is seals and gaskets failing. This can cause a loss of oil (and a very dirty engine bay). It's one of those things which can sneak up on you if you aren't paying attention. While a bad PCV is not a good thing, don't look here for a reason you're seeing oil smoke coming out the tail pipe. COST TO FIX: If you catch it before it becomes an issue, it's actually quite cheap to fix ... just replace the valve. If you see leaks at seals or gaskets, your expense goes way up, but it depends on which seal or gasket is leaking. Easy to get to seals or gaskets will cost much less, because the labor is much less. The converse holds true - Labor is a determining factor here.

• If you are seeing blue smoke which goes away after an oil change, but slowly comes back as you get closer to your scheduled maintenance, this could be a sign you have an internal fuel leak which is thinning the oil. As the oil gets thinner it passes by the oil control rings easier, causing your vehicle to smoke. An easy way to check this is by pulling the dipstick and smelling the oil. If you smell fuel, this may be the issue. This can be caused by an injector which is stuck open, or possibly an internal failure of a fuel pressure valve which would allow the gas to escape to where it doesn't belong. COST TO FIX: Diagnosis of what is going on will take a little bit of money. This could be a very low cost fix, to a moderate cost, depending on the actual cause.

If you are not seeing smoke, but you're seeing the oil drop lower and lower as time moves on, this may be a "sort of" normal oil usage in your vehicle. Take for instance my '06 Chevrolet Silverado. Its engine is an LS variant. As these engines get up in age, it is common for them to use more oil. You don't see it from the exhaust, but it uses it up, none the less. The engine still runs great and the gas mileage has stayed about the same.

Your vehicle could also be losing oil via leakage (or seepage). As engines get up in mileage, this is a very common thing. If you see spots on the driveway where you park the car, this could very well be the issue. Be mindful that vehicles will loose more oil during operation than they will just sitting in your driveway.

EDIT: In deference to your edit - I can see where an up-to-temperature catalytic convertor might make some difference in smoke from the tail pipe. This does not hold true, though, for start-up smoke (puff of blue smoke), as the cat is not hot enough to make a difference. It also won't make any difference once the catalyst is covered in oil suit or if there is too much oil for the cat to deal with. Sooner or later when oil gets burnt in this manner, you'll see the blue smoke. You probably have to put new cats on at that point, as well.

Answer 2

An engine that burns oil not only wastes oil but can damage spark plugs, cause the ignition to misfire and eventually affect the catalytic converter. The burning oil can also cause higher emissions, resulting in failed emissions tests due to excess hydrocarbon production

REASONS FOR ENGINE OIL BURNS

1.Worn Valve Guides

Over time, the valves wear down the cylindrical chambers, or valve guides, that keep them on track and create a gap in the chambers, this gap allows oil to flow into the combustion chamber, where it then burns. Once the gap becomes too big, the valve seal cannot prevent the oil from making it into the combustion chamber.

2.Bad Valve Seals

The valve seals prevent the flow of oil into the engine. If the valve seals fail or are broken, cracked, worn down or improperly installed, the oil will be sucked into the engine and cylinders,the compression may not be affected by the leaking oil but the engine will use a lot more oil than necessary.

3.Pressurized Oil Pan

If carbon, a byproduct of the engine, builds up in the positive crankcase ventilation (PCV) system, it can clog the system. Generally, the PCV system acts as an exhaust or breathing passage for the engine, but build-up pressurizes the oil pan. This pressure pushes oil into the engine through the fuel delivery system, and the oil burns.

4.Worn Piston Rings

If the piston rings that seal the engine's combustion chamber wear out, the pressure is sent back down to the oil pan, creating the same end result as when carbon builds up in the PCV system.If the rings are installed upside down, twisted onto the pistons or not staggered correctly, the result will be the same as if they had worn out.

Diagnosing the problem

-Check the oil. Open your car's hood and pull out the oil dipstick. Wipe the dipstick clean with a clean rag and insert it back into its tube. Pull the dipstick back out and check the level of the oil. Add oil until the dipstick reads full. Repeat this every 500 miles. If it reads a quart low in 500 miles, you have an oil burning problem.

-Check the exhaust. Blue smoke coming from a car's exhaust pipe while it is running is a sign of oil burning. Smell the exhaust. An engine that is burning oil produces higher emissions. It will also fail to pass an emissions test due to elevated hydrocarbon emission.

-Monitor the engine to see if it is misfiring or running rough. An engine that is burning oil will foul the spark plugs, causing it to run rough.

-Inspect the spark plugs. Pull the spark plug wires off one spark plug. Use a spark plug wrench to remove the spark plug. Examine the spark plug. An oily, wet or sooty spark plug terminal is a sign of oil burning. Replace the spark plug and wire. Repeat for each spark plug, working on one spark plug at a time.

DIAGNOSING INDIVIDUAL CAUSES

PCV system

Remove the PCV valve with the engine running. There should be a strong vacuum pulling on the valve. If there is no vacuum, the system is clogged with sludge and carbon. It should be cleaned and the valve replaced.

Valve guides and valve seals

Run the engine for several minutes at idle. Turn the engine off and let it sit for about 15 minutes. Restart the engine and immediately increase the engine speed while observing the exhaust. If a heavy billow of bluish smoke is exhausted then disappears and the exhaust remains relatively clean, the most likely cause is excessive valve guide wear. In this case, the valve guides or valve guide seals require service. If the above test only produces mild smoke and the smoke remains at the same level during all operating conditions, the piston rings will have to be tested.

Worn out Piston rings

remove the spark plugs and test each cylinder individually for the total PSI of compression. If you find that one cylinder is low, then wet test it. To wet test the cylinder, remove the gauge, squirt oil into the cylinder, and then retest it. If the compression in that cylinder comes up, then that cylinder has worn piston rings. How do we know this? Because when oil was squirted into the cylinder, it filled the gap between the worn rings and cylinder wall, sealing the ring gap and thus increasing compression. In this is the case, consider rebuilding or replacing the engine.

NOTE:Compression testers are available at most auto parts stores. Refer to the manufacturer’s instructions for proper usage. When performing a compression test it is critical that the ignition system be disabled to prevent the engine from starting. This is usually done by connecting a wire to each of the spark plug wires to a good engine ground away from your work area

Hope this helps you .Cheers!

Answer 3

My car typically consumes about 3 liters per service interval (10000 km). My car now has 330000 on the odometer. You didn't indicate which type of engine: diesel or petrol. Also it is important to know whether the engine is turbocharged. To begin, you must figure out normal oil consumption for your specific engine.

Here is some info from Volvo VADIS for its 2.5L B5254T engine with low pressure turbo:

Normal oil consumption

Running-in period

The oil consumption can be as much as 1 liter per 1,000 km during a running-in period of approx. 5,000 km if the engine is new or replaced (replacement of piston ring).

Driving between 5,000 and 150,000 km

• Normal driving: up to 0.25 liters per 1,000 km is considered normal oil consumption.
• Driving at high speed/with heavy trailer: up to 0.40 liters per 1,000 km is considered normal oil consumption.
• Mountain/alpine driving: up to 0.50 liters per 1,000 km is considered normal oil consumption.

The above stated values for oil consumption can be used as guidelines if there is a suspicion that oil consumption is abnormally high.

So during the first 150,000 km the oil consumption can be as 0.25 - 0.55 liters per 1,000 km under all conditions.

To determine the exact oil consumption a test drive of at least 1,000 km must be performed under controlled conditions. The oil must be weighed before and after the test drive. This way the exact oil consumption can be calculated.

General causes of high oil consumption

Again from VADIS.

Overfilling

If the oil has been filled above the recommended maximum level, oil is forced up against the cylinder walls and is ejected through the crankcase ventilation. The oil level must not be filled above the maximum mark on the dip stick!

Oil Grade

Using a non Volvo recommended oil grade can cause increased oil consumption. The oil contains a larger amount of volatile molecules if the oil is too thin. Thin oil has difficulty in maintaining a comprehensive oil film on the cylinder walls at higher temperatures. This results in increased engine wear and increased oil consumption

Hard Driving

Long and continuous driving at high engine speed (RPM) causes high engine temperature. The oil thins and consumption increases. Hard cornering at high engine speed (RPM) causes the oil to be forced against the side of the cylinder block and against the cylinder walls, resulting in increased oil consumption.

Mountain Driving

Numerous and long periods of regular engine braking also causes high engine temperature. More oil is sucked into the cylinders due to an increased negative pressure in the intake pipe.

Idling

Driving the car in urban traffic conditions, where the engine may be running for long periods but only covering short distances, does not necessarily increase oil consumption but the ratio of oil consumption to distance travelled may be misleading.

Climate

In hot climates, the engine is required to work harder at a higher temperature. This increases oil consumption.

Answer 4

Since it costs less than 10 dollars to change a PCV you should just go ahead and replace it. I've seen them fail enough times to know its safer that way. Especially if your having head gasket changes or tune ups done to your vehicle. A lot of the time the PCV is in the same area as these jobs and can become worn in the process.