Most of us know why many new models are fitted with oil life monitors, but more to the point; if a customer should ask you, the expert technician, if he could trust the oil life monitor in his new car, what would you tell him? There is no simple “yes” or “no” answers to this question, so in this article, we will take a closer look at oil life monitors, and under what conditions you may or may not want to trust these systems to assess or calculate the condition of the engine oil in a customer’s vehicle accurately. Let us start with this question-
As experienced technicians, we know that improvements and advances in both lubrication and engine technologies have obviated the need to replace engine oil at regular 5000 km intervals, but that is only half of the story. The other half involves the fact that car manufacturers the world over are coming under increasing pressure from governments and environmental pressure groups to increase oil change intervals as a means to reduce the several millions of litres of used engine oil that end up in the environment each year.
However, even the best engine oil formulations have finite service lives, so to both satisfy the demands of environmentalists and to maintain proper lubrication in engines, car manufacturers have developed oil life monitors. In theory, an oil life monitoring system has the function of monitoring the quality of the engine oil in order to extract the maximum useful life out of the oil in order to prevent replacing oil that is still serviceable, but the practice is often a whole lot different.
Essentially, the problem of how to make oil life monitors accurate revolves around four issues, these being accounting for all the factors that contribute to oil degradation, oil quality, how people drive and/or use their vehicles, and the fact that different car manufacturers use different oil monitoring systems, each of which depends on different parameters to work. Each of the above points influence the accuracy of even the most advanced oil life monitoring systems, so let us look at these points in some detail, starting with-
The single biggest factor that causes engine oil to degrade is heat, with oxidation and the formation of various corrosive acids following closely behind in second and third place, respectively.
Although modern engine oils contain additives that resist excessive changes in viscosity, the biggest issue that comes with high temperatures is air that can become entrapped in the oil in various ways. In practice, air can exist in oil in one of four ways; the air can be dissolved in the oil, air can be entrained in the oil, or the oil can contain air in the form of air bubbles (foam), or free atmospheric car air can exist in the oil. From the perspective of an oil life monitoring system, the biggest problem is entrained air, which is microscopic air bubbles that become trapped in the oil as a result of the normal operation of the oil pump.
According to Boyle’s Law, when trapped air is compressed, its temperature rises but in engine oil, these bubbles can be heated to the point where the oil that surrounds the bubbles reach boiling point, which in turn, causes the formation of various gums, varnishes, and waxes on a microscopic scale. Eventually, when enough of these harmful substances are created, they are deposited onto metal surfaces where they interfere with, or prevent the operation of almost all beneficial oil additives.
There are many other factors that contribute to oil degradation such as particle scrubbing, water washing, surface adsorption, rubbing contact, condensation settling, and evaporation of additives, among others. We do not have to delve into these mechanisms here, but suffice to say that since oil life monitors are not created equal, it could happen that some oil life monitoring systems might under or over report the oil’s remaining useful life as a result of the combination of premature degradation of oil, and an oil life monitoring system’s inability to detect oil degradation.
In all cases, the accuracy of oil life monitoring systems is predicated on the notion that the oil life monitoring system on the vehicle is monitoring oil that is recommended for that particular engine. However, we know that many vehicle owners use the best oil they can afford, as opposed to the best that is available or recommended for their vehicles, which means that in such cases, the monitoring system will almost inevitably over or under report the oil’s remaining useful life.
The issue here is not a failure of the oil life monitoring system, but the fact that for instance, the unsuitable oil’s viscosity may increase or decrease beyond the limits that some oil life monitoring systems are programmed to accept as a normal, or to recognise as acceptable given other inputs from various engine sensors.
Many, if not most car owners (as well as many mechanics and technicians) fail to recognise that recommended oil change intervals on a given vehicle are based on that vehicle being operated under ideal conditions.
However, asking what constitutes “ideal” conditions is akin to asking how long a piece of string is. As a rule of thumb though, “ideal conditions” could be described as not operating a vehicle in extremely dusty conditions for long periods, not doing only very short trips, not operating the vehicle at high engine speeds for extended periods, not accelerating aggressively, and perhaps most importantly, not operating the vehicle without keeping to its prescribed service schedule(s).
In practice though, many, if not most car owners are under the impression that their vehicles are always operated under ideal conditions when in fact, the opposite is true. In rural areas, most vehicles are almost always operated in dusty conditions, while in urban and city environments, many vehicles are subjected to stop-and-go traffic almost daily, while others are used almost exclusively on short trips to schools, shops, and the like. Under such conditions, oil will degrade much faster than it would in vehicles that see long(ish) periods of highway driving at constant speeds, which is the most effective way to allow water to evaporate from the oil. This point is particularly important, since the presence of water in oil is the mechanism that triggers the formation of various harmful acids in engine oil.
Therefore, depending on vehicle use, and the oil life monitoring system in use on the vehicle, an oil monitoring system might alert a driver to the fact that he needs an oil change only a few thousand kilometres since the last oil change, or an oil change might only be recommended several thousand kilometres beyond the interval stated in the manual.
None of the above necessarily indicates failures in, or malfunctions of oil life monitoring systems, but given the fact that all oil life-monitoring systems have inherent limitations, it is important to understand that these limitations bear directly on how oil change intervals are calculated, which raises this question-
When the question of whether or not oil life monitoring systems are accurate, it should be remembered that a), there are four types of systems in common use today, and b), that some systems are several orders of magnitude more complicated than others. Therefore, it is important to understand how the four main types of oil life monitoring systems that are in common use today work. Let us start with -
These systems are the most basic of all, and only measure the distance travelled since the last oil change. This type of system is in common use on some Toyota, Hyundai, and Honda applications, and at their most complex, they merely illuminate a warning light, and/or display a warning message when a pre-programmed distance had been covered since the last oil change.
In practice, when these types of systems are reset they count backwards from a pre-programmed value, which value represents the recommended oil change interval on that vehicle. When the counter reaches zero, it may record a negative distance, and a warning message might then read something like “Oil change is 600 km overdue”.
More importantly though, this type of system takes no account of operating conditions such as the number and duration of short trips, engine coolant temperature, engine speed and/or load, ambient temperatures, or any other factor that contributes to engine oil degradation. As a practical matter, this means that vehicles that are never driven hard may have their oil changed prematurely, while vehicles that are driven hard while towing trailers or boats may not get an oil change when they really need it.
This type of software-based system was introduced by General Motors as long ago as 1998, and while it is not perfect, other manufacturers such as Ford and Fiat/Chrysler have made significant improvements to the algorithms that control the system.
These types of systems use four basic parameters, or vehicle operating conditions, these being normal driving at steady highway speeds, high engine temperatures/loads, short trips in city driving conditions, and cold starts combined with frequent, but very short trips. These conditions are based on original research by GM that showed that there is a direct correlation between the rate of oil degradation and oil temperatures.
Note though that further research revealed that low oil temperatures combined with frequent short trips created conditions where water and other contaminants that are related to fuel delivery during cold starts, degraded oil at a similar rate than high temperatures do. Thus, the rate at which oil degrades as a function of both high and low temperatures is the basis of the algorithms that determine oil quality in applications that use this type of oil life monitoring system.
In practice, systems such as these also use input data from a variety of engine sensors, including IAT, ECT, and the TPS to make routine adjustments to the calculated oil change interval, based on changing driving and operating conditions. Note though that the version of this system used on Ford applications also incorporates a timer that illuminates a warning light one year after the last oil change- regardless of the distance travelled. Moreover, the version of this system used on Fiat/Chrysler applications can take account of the cumulative concentration of ethanol in the fuel used since the last oil change.
Having said the above though, the oil life monitoring systems used on GM, Ford, and Fiat/Chrysler applications all assume that the correct, and recommended oil for these applications is present, and that all required reset procedures had been carried out after the last oil change.
While tracking only operating conditions is a fairly accurate way of determining the remaining useful life of engine oil, VW/Audi had gone a step further with their system, which also continuously measures the oil level and temperature when the engine is in operation.
In addition, this system also uses a time/distance-travelled algorithm that is especially useful in cases where the oil level had dropped to below the “FULL” mark. The practical advantage of this is that the system can calculate both accelerated wear rates in the engine and the depletion rate of additives in the oil with complex algorithms that are based on the temperature of the oil, which tends to be higher than normal when the oil level is lower than normal.
It should be noted that when VW/Audi adopted this oil life monitoring system, they also switched to using only synthetic oil as factory fills, and to recommending that only synthetic oil be used to replace the oil in their engines during routine servicing.
This is the most advanced type of oil life monitoring system, which apart from monitoring operating conditions, also uses a highly advanced sensor that measures the electrical conductivity of the oil.
Since water, metal wear particles and some types of combustion products conduct electricity, this system monitors the electrical properties of the oil on a continuous basis. Based on the changing properties of the oil, the system will recommend an oil change when the electrical conductivity of the oil approaches or exceeds a maximum allowable limit.
Moreover, some systems, such as the FSS (Flexible Service System) used by Mercedes-Benz also have the ability to both measure the rate of additive depletion in the oil as a function of the build-up of contaminants, and particularly of acids, and to detect the presence of fuel and/or coolant in the oil.
BMW uses a similar system with similar capabilities, and it is therefore not unusual to see oil change intervals of 20 000 km or two years, or sometimes more on these vehicles. However, for this system to work as designed, it is imperative that the recommended oil be used, and that the correct oil level is maintained at all times.
The short answer is a cautious yes, provided of course that a), the correct oil is being used in the engine in question, and b), that the oil in the engine in question is not contaminated or diluted with oil types and formulations that are not recommended for use in that engine.
Unfortunately, though, we as technicians do not always have the ability to test oil for the presence of unsuitable oil or other contaminants. Thus, this writer’s advice would be to err on the side of caution, and to replace engine oil in strict accordance with maintenance and service schedules when there is any doubt about the condition of the oil in any engine.
