Oil consumption has been an issue for engineers since the inception of the internal combustion engine. The ability to keep oil in the crankcase and out of the combustion chamber is critical to engine operation and lifespan. Motor oil is the life blood of the internal combustion engine and a lack of oil can create problems as minor as a check engine light to severe conditions that result in complete engine overhaul or replacement. While most modern engines don’t use oil between oil changes, there are some egregious offenders that use oil like it’s going out of style. The questions this raises are: Where is the oil going, what can be done about it and what service interval should be used?
Consumer Reports performed a study of 2010-2014 model year engines that need to have the oil level adjusted in between oil changes in their 2014 annual auto survey. While 98 percent of the vehicles they studied showed no signs of oil consumption, the remaining 2 percent burned oil at a rate of as much as a quart per 600 miles. While 2 percent of vehicles built after 2010 seems to be a small statistic, you might reconsider when you realize that the 2 percent is over 1.5 million vehicles. So who are the 2 percent, you might ask? The largest offenders were Audi, BMW and Subaru, according to the report. However, even the manufacturers known for engine reliability such as Toyota and Honda have had their share of oil users.
In order to understand where the oil is going, I went to one of the best sources for technical information our industry has to offer: The Society of Automotive Engineers. A simple search of the technical paper database at SAE.org using the keywords “oil” and “consumption” revealed 1,297 technical papers. This number of papers seems to be evidence to support the idea that oil consumption is indeed a challenge for engineers. Another great source for in-depth information is the Sloan Automotive Laboratory at Massachusetts Institute of Technology (MIT). Many of the papers found on the subject have been authored by the distinguished professors at MIT such as John Heywood, who literally wrote the book on internal combustion engines.
Pinpointing the exact source of oil consumption can be a tricky science. There are several ways that oil can make it into the combustion chamber. Oil either has to make it past the three piston rings, commonly known as the “ring pack,” or it has to enter through somewhere else, such as the valve guides or an intake air source such as the PCV or turbocharger that is contaminated with oil. According to a 2012 SAE paper (2012-01-1617) by Arnault, N., Bonne, S., sources of oil consumption can be classified as follows:
1. Entrainment in blowby flow – In layman’s terms, blowby gasses from the combustion chamber can get past the piston rings due to the piston ring end gaps and cylinder distortion at high engine temperatures. Another byproduct of blowby combustion gasses is the heating of the oil, which can lead to evaporation. In both cases a vaporized form of oil can be transferred over to the combustion chamber via the PCV system. Turbocharged vehicles can exhibit this at even greater levels. The contribution of this phenomena towards total oil consumption can vary greatly from engine to engine and can be anywhere from 10 percent to 55 percent of an engine’s total consumption. While combustion gasses can lead to blowby, the pressure they create causes a force that holds the piston rings tightly in place against the cylinder walls. If there is a lack of cylinder pressure, such as in events like cylinder deactivation, oil may find an easier path to the combustion chamber.
2. Evaporation from the cylinder liner and piston – At high engine load and temperatures, oil can evaporate from the cylinder wall. Evaporation can account for as much as 40 percent of total oil consumption at full load.
3. Valve guide leakage – While this was an issue in early spark ignition engines, later engines have much more dependable sealing systems. As such, the contribution of valve seal leakage to overall oil consumption is minimal.
4. Leakage from turbocharger oil seals – Depending on the condition of the seals, total consumption due to turbocharger seals can be anywhere from minimal to significant.
5. Oil loss via piston ring – There are two ways oil can get past the ring pack. The first is that through normal engine operation, oil can migrate past piston rings through mechanical transfer. The second is that there are times where the pressure on the second ring land is greater than combustion pressure. This can cause the top ring to distort and allow oil into the combustion chamber. Stuck rings also present a significant problem. If you have ever had an engine apart that was poorly maintained, you know just how stuck the piston rings can get.
Dealing with oil users
Knowing the source of oil consumption falls under the “nice to know” category. However, what you need to know is how to deal with oil consumption problems in your bays. Here is a list of steps to help you identify and deal with oil users:
1.Documentation — Implement a plan to document oil levels at every service BEFORE performing service. Documenting oil level before service begins is the first step in identifying a vehicle that consumes oil. Failure to document before an oil change will lead to big problems if it is not implemented into your service bays.
2. 1,000 mile test – Once consumption is suspected, perform a complete oil change and ask the customer to return in 1,000 miles. Check the oil level to determine consumption over the 1,000-mile interval. If the oil usage is minimal, simply adjust the level accordingly. If usage seems excessive, it may be time to consult service information.
3.Determine accepted usage – Most manufacturers list an accepted usage for their vehicles in the repair manual. This is usually a protection for the manufacturer for replacing an engine under warranty. However, it can also give you an indication of how often you should have the customer return for service. For example, if the accepted use is a quart per 1,000 miles and the pan holds six quarts, do you really want to have the customer wait the manufacturer-suggested 10,000 mile interval?
4.Check TSBs – There are many Technical Service Bulletins (TSBs) across many product lines that highlight oil consumption causes and fixes. These TSBs can range from simple Band-Aid fixes such as software reprogramming to complex repairs such as engine block replacement. For example, many Honda and Toyota vehicles are getting piston rings and in some cases pistons and rings in some recent service campaigns.
Service intervals
Recently I was contacted by a popular women’s magazine to give some advice on oil change intervals. This was a slippery slope. Most consumers are told that when in doubt to check the owner’s manual. However, is the interval the manufacturer is recommending correct? In most cases, probably not. One important piece of information to keep in mind is that oil change service intervals are a good way for manufacturers to make it appear that their cost of service is low. In addition, the long service intervals look good to the EPA and other government agencies that are concerned about waste oil and its impact on the environment. So what do I recommend? I recommend that service is a preventative measure. While the manufacturer may recommend 10,000 miles for synthetic, I perform the service every 5K. Oil is the single most important service item on any automobile. If you can convince your customers of that, they will thank you for it in the long run.
A word about motor oil selection
When selecting a motor oil for a specific engine, the manufacturer has to pay close attention to volatility. A chemical that is volatile evaporates readily at normal temperatures. As such, volatility in relation to oil consumption is extremely important. So how do you know what the volatility of a given oil is and ultimately what type of oil to use for what engine? The answer is simple. Rely on what the manufacturer is using and recommending and use it. This might mean carrying a broader selection of motor oils or ordering on demand, but using manufacturer-recommended oil is a good front line defense for oil consumption problems. If you are looking for a step up in the quality of oil you are using, many European brand techs swear by brands such as Motul as a “go to” for performance and reliability.
The switch to synthetic
You may have noticed that just about every manufacturer now recommends synthetic oil. Toyota, for example, recommends it across their entire product line. The reasons for this are many, but one of the most obvious is the ability of synthetic motor oils to stay liquid and resist vaporization. As mentioned earlier, vapor leads to consumption. With today’s engines using oil viscosities such as 0W-20, the weight of the oil lends itself to easy vaporization. Synthetic is an elegant solution. Again, go with manufacturer-recommended specifications or better. Oil is not the place to cheap out on today’s engines.
It’s a ring thing
Many of the fixes for oil consuming engines involve work with the pistons and piston rings. This topic comes up frequently in both my Toyota and Honda engine classes at the college where I teach. Best practices for piston ring replacement is making sure the ring lands, the areas where the ring resides, are fully cleaned and free of imperfections. In many cases, piston replacement may not be a bad idea if the rings are coked in place to the point where no cleaning will suffice. When replacing rings it is always a good idea to refer back to the suggested alignment of piston ring end gaps, as well as the orientation of the ring itself. It may have been a while since you did a piston ring swap so remember there is a top and bottom to the ring itself and orientation of all three rings will need to be exact.
Low budget connecting rod heater
Many engine designs with oil consumption issues feature interference fit connecting rod/wrist pins. Honda, for example, recommends the use of a heat gun in order to avoid a trip to the local machine shop. Simply heat the connecting rod near the wrist pin to remove and install the piston. While it’s not an ideal set up, it will save you some time and money at the local machine shop.
When I first started as an auto shop teacher, the manufacturers were moving away from a teaching model that included engine rebuilding tasks as they felt that their engine technology had improved to the point of being able to withstand the life of the car. Today engine rebuilding is a large part of high school and college automotive curriculums. Following the manufacturers’ lead and staying up to date on engine repair best practices will help you in the fight to keep oil in the crankcase, as well as repair dollars in your pocket.
