Four Reasons Why Engines Burn Oil

When gas engines burn oil the telltale sign is bluish smoke from the exhaust. The amount of smoke is directly proportional to how much oil is being burnt. Therefore, an automobile belching out clouds of blue smoke is probably burning large amounts of oil. This can be a major problem for those living in states with emissions testing. When automobiles fail for smoke major repairs may be necessary to obtain a passing sticker.

If an automobile owner discovers the engine is burning oil they might be tempted to panic if they’re not prepared to buy a new car or replace the engine in the old one. When engines burn oil, it’s quite possible, the worst case scenario is responsible. This article walks you through four common reasons why blue smoke might be observed from the exhaust. These examples are arranged from the worst-case to the best possible cause.

Worn Piston Rings

On high mileage engines worn piston rings is a likely cause of excess blue smoke from the tailpipe. Each piston in the engine has three separate rings. Two are used to seal the combustion chamber, and one is used to control oil on the cylinder wall. As the oil control ring wears it starts to leave more oil behind to be burnt in the combustion chamber.

Mechanics can use a wet and dry compression test to get an idea of how worn the piston rings are. First, the dry compression reading is taken and then a small amount of oil is injected into the cylinder for the second test. These two separate results are recorded and compared to manufacturer specifications. Although worn out rings are considered the worst-case scenario, its unlikely the cause on modern engines with less than 100,000 miles.

Leaking Valve Seals

Valve seals act like an umbrella to prevent oil from running down the valve stem into the combustion chamber. When an engine is considered to have leaking valve seals a very specific set of circumstances are often found when the symptoms are most evident. Since they start allowing small amounts of oil to pass when they begin to wear out the symptoms are most viewable after the vehicle has sat for a while.

When an engine is started cold and a puff of blue smoke is observed from the tailpipe and quickly clears up would be a good reason to suspect a worn valve seal. The conditions are often amplified when the vehicle is started after a hot soak. If the vehicle doesn’t emit bluish smoke during normal operation, but belches out a nice cloud after it sits at the grocery store for about twenty minutes, inspection of the valve seals should be made a priority.

Burning Transmission Fluid

Automotive manufacturers use to control the throttle valve that’s responsible for up shifting and downshifting dictated by engine load with a diaphragm operated modulator. The vacuum modulator mounts on the outside of the transmission and has a rod that passes through the case attaching to the throttle valve inside the transmissions valve body. It’s controlled by engine vacuum that runs from the modulator to a manifold vacuum port.

The only thing stopping transmission fluid from being sucked into the intake is the rubber diaphragm itself. If this becomes compromised or falls apart from dry rot, transmission fluid is allowed to flow into the intake and be burned in the combustion chamber. Although this condition was more common through the 70s and 80s car makers did utilize the system into the mid-90s. Chrysler was one of the last companies to use a vacuum modulator on the early 90’s Jeep line up.

Defective PCV Valve

A positive crankcase ventilation valve (PCV) isn’t found on every model of automobile, although some type of crankcase ventilation will be deployed. The PCV valve or system is responsible for removing blow by gases from the crankcase. If this operation is not performed correctly pressure can build up internally and push out oil sealing gaskets.

Blow by will also carry some unburned fuel that can dilute the engine oil, which in turn reduces protection plus the thinner oil is more likely to be burned during the combustion process. Due to the configuration of some PCV systems on specific models it’s possible for the valve to malfunction and oil to be sucked in through the vacuum line. You can verify this condition by removing the valve and looking for signs of fresh oil in the supply hose.

One of the worst pieces of news a motorist can receive at the inspection station is their engine has failed for a blue smoke. Although it’s quite possible for the worst-case scenario of worn piston rings to be the ultimate cause of the excessive smoke, there is a chance that other failures are responsible. Car owners who seek resolutions for excessive smoke emitted from the tailpipe should entertain a second or third opinion when engine replacement is recommended.