How Often Should I Service My Car?
Last Updated 22/03/2016
I’ve had discussions and read about service intervals a fair bit over the years. There’s a few different points of view. Some people service more frequently than the manufacturer’s recommendation. Some people service less frequently than the manufacturer’s recommendation. And of course some people service according to the book. Which is best? This article explains my point of view.
Reduced Service Intervals
Reduced service intervals means servicing more often than what is recommended by the manufacturer. The time between services is less. Some reasons to support this idea include:
– Extra servicing is a small incremental cost that can help extend the life of your vehicle. The cost is small compared to the cost of the repairs that you may be avoiding through frequent servicing. The old adage “cheap insurance” is often used to explain this idea.
– Modern vehicles are precision machines and need to be serviced regularly to maintain their precision. All the sophisticated electronics, sensors, instruments, mechanical components and emission controls are sensitive to less than ideal lubrication, dirty fuel, poor filtering, etc.
– Manufacturers don’t specify frequent enough servicing as they do not want to be perceived as being expensive to service. They want to be perceived as having low running costs. Fuel efficiency is obviously most important in terms of running costs, but so is reduced servicing costs.
Extended Service Intervals
Extended service intervals means servicing less often than what is recommended by the manufacturer. Some reasons to support this idea include:
– Manufacturers are conservative with their recommended service intervals. They want to over specifiy servicing to encourage indifferent people to service their cars frequently enough. If someone has an attitude of “I’ll stretch it a little bit, for as long as I think I can get away with it” then it’s better to stretch from an over-specified schedule to mininize damage. Manufacturers also want an easy way out of warranty claims if a vehicle hasn’t been serviced according to over specified intervals. They want to protect their reputation as being a reliable manufacturer so they recommend more frequent than necessary servicing. For the manufacturer, it’s cheap insurance, but to the end user it may be unnecessary insurance.
– Manufacturers make money through servicing, so they recommend more servicing than required.
– Modern cars burn clean. They need to burn clean to comply with emission standards. Modern fuel is clean. It needs to be to comply with fuel standards. So contaminants are much lower in new vehicles, which means lubricants and filters last much longer. Manufacturers have kept service intervals about the same without considering the changes in the modern era of motor transport. This means there’s room to extend service intervals on modern vehicles.
You may have noticed I mentioned “cheap insurance” for both reducing service intervals and extending service intervals. That’s because I am making a point – the “cheap insurance” argument is not valid. Cheap insurance means small incremental costs are ok even if there is possibly no benefit, because the cost is small and there is still a chance of benefit. Consider how this applies to service intervals. If the manufacturer recommends an oil change every 10,000km, then an oil change every 8,000km would comply to the cheap insurance argument. Servicing only slightly more often doesn’t cost much more. What about, once you decide to change the oil every 8,000km, you discover that it’s now only a small incremental price increase to change the oil every 6,000km. What about 4,000km? What about 2,000km? Do you see a problem here? Applying the “cheap insurance” argument as a general philosophy means, through many small increments, you now need to change your oil every day. If the “cheap insurance” argument applies from 10,000km to 8,000km then it similarly applies to every other incremental service interval. Through cheap insurance, the accumulation of many small cost increases can continue indefinitely. Your servicing costs will approach infinity. How does one applying the cheap insurance argument pick 8,000km over 6,000km? How do you decide at what point to stop servicing more? The point where you don’t think it provides any extra benefit? In that case there is no cheap insurance arguement at all, the point is chosen based on benefits. If choosing a service interval based on benefits, then it invalidates the initial cheap insurance argument that a small increment in price is ok regardless of a potential lack of benefits.
The argument for servicing a vehicle more often, from say 10,000km to 8,000km, is exactly the same as the argument for extending the service interval from 10,000km to 12,000km. It’s a trade off between servicing costs and maximizing vehicle life. The trade-off works in both directions. “Saving money by extending vehicle life through more frequent servicing” is, in terms of logic, applying the same principle as “saving money by avoiding unnecessary servicing when it yields no benefit.” You could say servicing the vehicle less often is cheap insurance against unnecessary wasting of money on servicing costs. Even if applying the cheap insurance arguement in one direction (erroneously), you cannot apply it in a consistent fashion without servicing costs approaching infinity. It ends up being a decision based on perceived benefits. Any point can be deviated from in both directions using similar arguments. A philosophy that works, is to use all the information and experience at your disposal to pick a point that gives the lowest cost of ownership. This philosophy can be applied consistently and will give the best results. The same idea applies when considering to buy cheap no-brand stuff. The blanket cheap insurance argument isn’t valid. You need some sort of evidence or reasoning to make a decision with. Without that, cheap insurance eventually means you need to change your oil every day.
Who Knows Best
If you’re after advice on how often to service your vehicle, you need to watch out for people who have been swayed by isolated experiences. People who had something happen to them who then assume the same applies to all vehicles of that make and model. You also need to try to filter out people who don’t really understand the vehicle or how often it should be serviced or even the basic laws of physics. You want an expert who’s had exposure to as many of your vehicle’s make and model as possible. When looking for a source to guide you in your decision on service intervals, look for a source with the following characteristics:
– Has intimate knowledge of the vehicle’s design.
– Has experience with at least several thousand vehicles of your exact make and model.
– Has experience through the entire life cycle of your vehicle’s make and model, including design, manufacturing, servicing, repairs and warranty claims.
– Has thousands of accumulated man-hours of experience with your vehicle’s exact make and model.
Who complies with all these criteria? Not me! As far as I know, only one entity complies – the vehicle manufacturer. They are in the best position to gauge service intervals, so in my opinion you should service exactly according to your vehicle manufacturer’s recommendations. Not only is the manufacturer in the best position, but it is a perfect compromise between extended service intervals and reduced service intervals.
The problem with recommendations from mechanics, vehicle owners, or any other individuals, is that they are emotional and they are naturally inclined to take an experience and assume it applies to everything. You’ll hear things like “if you don’t service more often you’ll get sludge build up in the engine like I’ve seen in many vehicles in the workshop” or “if you don’t change your fuel filter often you’ll damage your injectors like what happened to me” or “when I change my oil it still looks clean so you can extend the service interval” or “I change my oil every 20,000km and my vehicle has lasted as long as anyone elses.” An individual’s sample set simply isn’t large enough to really know if the experience is isolated or common, even if the experience has been seen on multiple vehicles. How many of that vehicle have been sold in total? What proportion of vehicles are experiencing this problem? Will extra servicing actually resolve the problem? Is there any evidence that extra servicing definitely yields reduced probability of experiencing the problem? Is there a common source of mistreatment that is skewing the experience? Are there any local environmental factors that are skewing the experience? Does the mechanic providing advice have a personal preference for another brand of vehicle? Did the individual giving the advice have a bad experience with a warranty claim and now holds a grudge against the brand?
That’s not to say that the experiences, advice and recommendations from individuals don’t have any merit. A judgment needs to be made, but erring on the side of the manufacturer’s recommendation is usually pretty safe.
An Example – Diesel Fuel Filters
Modern diesel engines, like the one in my Hilux, have sensitive fuel delivery systems. They operate at high pressure, have many sensors and instruments and are metered with exacting precision. This means they are sensitive to fuel contamination. Many owners of diesel vehicles are replacing their fuel filters much more often than recommended by the manufacturer. Not just slightly more often, but in the order of 10 times more often. Ridiculously often. Is this justified or have they just fallen into the cheap insurance trap, incrementally reducing service interval times due to fear of fuel system damage without any evidence that it provides any benefits? Will it actually help protect sensitive diesel fuel delivery systems?
As filters age, they don’t filter less. They continue to filter adequately as they age. In fact they filter more as they age. New filters do not filter any better than old filters. A brand new filter will filter less than an old filter. A new filter is not going to help protect a vehicle’s fuel system from contamination.
As filters age, they do become blocked. They restrict flow. This manifests as a pressure drop across the filter. This flow restriction needs to be kept within certain limits. The manufacturer achieves this through either scheduled replacement of the filter, or through instruments which measure the condition of the filter. Usually the instrument will be some type of pressure sensor, measuring how much pressure drop occurs across the filter. When the pressure exceeds a certain setpoint level, a warning light comes on and the filter needs to be changed. The instruments measure pressure rather than flow. Flow will be maintained according to the requirements of the fuel delivery system. So to detect a flow restriction early, you cannot use flow. Pressure drop across the filter (back-pressure) is used to monitor the filter’s condition. It gives an early warning for flow restriction whilst the flow rate is still at correct levels.
You would have heard of filters that can be washed and re-used. These filters don’t lose filtering ability with age, they become blocked and need to be washed. Once washed, they are no longer blocked and will continue to filter adequately. This is a good example demonstrating the physics of filters. A filter that can be washed operates on exactly the same principle as a filter that is replaced, except the washable filter is mechanically designed so that the filter element can be properly accessed for washing and the filter element is designed to be strong enough to withstand the washing action. Apart from that, it’s just some sort of filtering medium like paper, the same as a disposable filter.
In industry, the lube systems of large equipment worth millions of dollars are protected by an instrument that measures pressure across a filter. When the pressure exceeds a setpoint level, a “filter blocked” alarm is raised on the control system for the machine, and the filter is replaced. The owners of these multi-million dollar machines do not bypass this strategy and replace the filter extraordinarily often in the hope that it will extend the service life of the machine. They know it doesn’t work. So both heavy industry and vehicle manufacturers are in agreeance – consistent with the fact that old filters filter adequately but need to be replaced as they become blocked.
Water will damage a modern diesel’s fuel system. Water is not filtered by any filter element, new or old. To separate water from fuel you need something that works on the differing density. Some fuel filters are housed in a bowl that is designed to trap water in the bottom due to water’s greater density than diesel. This process is not dependent on the filter element itself. A fuel filter element can trap suspended contaminants but it can’t filter out molecules of water. So we can disregard the water argument for replacing fuel filters more often. This is true for any other liquid contaminants that may be present in diesel – kerosene, ethanol, etc. A filter element will not filter out molecules of liquid.
So your fuel system is equally protected from fuel contamination and water with either an old or new fuel filter, and your fuel system is protected from excessive flow restriction through manufacturer recommended replacement intervals and / or instruments that measure the condition of the filter. Replacing your fuel filter more often does not improve filtering. If you’ve stuck with the manufacturer’s recommendation and you’ve had a fuel system failure, more frequent filter replacement wouldn’t have prevented it. Replacing your fuel filter more often increases the risk of contaminating the fuel system during the filter replacement process. In fact it’s guaranteed that you will be introducing contaminants into the fuel system when changing the fuel filter. There’s dust in the air, dust on the surfaces of the new fuel filter, dust and dirt covering the filter housing. It’s not possible to introduce exactly zero contaminants when changing a filter. If anything, you are increasing the risk of fuel system damage. At best you are unnecessarily spending money on something that yields no benefits. I worry that people are making it worse – that was one of my motivations for writing this article.
Based on my understanding of filters, the vehicle manufacturer’s service interval for fuel filters is correct. Those that replace more often are in conflict with the physics of filters. In fact always having a new filter may be allowing more contaminants through and increasing the risk of fuel system damage. The designed operating point of a filter is midway between being new and needing replacement. This is a stable area of operation, where the big holes of a brand new filter are plugged up but the resistance to flow is still low. This operating area is the right compromise between filtering ability and pressure drop according to the design.
For my model Hilux, there is no scheduled replacement of the fuel filter. According to the manual, you change it when the instruments that monitor its condition tell you to change it. Mind you the newer model hilux has a recommended replacement interval of 40k. Perhaps in my older model it should be changed according to the new schedule, or maybe Toyota just kept getting asked too many questions about not having a scheduled service interval so they thought it would be easier to put one in. Anyway I changed mine at 100,000km out of interest, to see how bad the filter looked and to not let it get too old. The inlet side was dark, the outlet was clean white. There was no sludge or thick coating on the filter. The inlet side felt clean but the colour was dark grey. The bottom of the canister was clean apart from some metallic specks. Those specks would just stay in the bottom of the canister forever without effecting anything. Looking at a used filter it not really relevant – the condition of a filter should be judged by its resistance to flow, not its appearance. The instruments that monitor my fuel filter were reporting that the filter was ok after 100,000km.
Some people change the filter in their hilux every 5k. That’s 20 times more often than me. For a filter worth about $30 over a vehicle life of say 300k, that’s $1800 worth of filter vs $90 when replacing every 100k – an additional cost of $1710. That much money will buy you a couple of new injectors. What if we include labour? Time isn’t worthless, even if doing it yourself. It means less time to do the things you want to do. It means being closer to your death without having done those things. Let’s put a value of time at $50 per hour. Thats cheap – my time is worth double on weekends because I’d rather be fishing then servicing a car. If it takes half an hour to replace the filter that’s $25 in labour per filter change. Over the life of the vehicle that’s $1500 when replacing every 5k compared to $75 every 100k. Total cost of fuel filters when replacing every 5k is $3300 compared to $165 replacing every 100k – an additional cost of $3135. That will probably do you a complete set of new injectors. Not only do the costs accumulate to a significant amount over time, but it provides no benefits. Actually it increases the risk of contamination of the fuel system as explained above. Is that cheap insurance or false economy? Imagine how the costs accumulate if applying the cheap insurance idea to all the other servicable items in a vehicle.
Some people are upset when the filter light illuminates in their Hilux or similarly designed vehicle. They’re upset because they believe the filter should have been replaced according to a more frequent scheduled replacement before the filter light came on. Actually the filter light coming on is exactly how the system is designed to operate. The filter is still filtering perfectly well when the filter light comes on. The engine is perfectly protected. There is a flow restriction, but not enough of a restriction to effect fuel delivery. The instrument that generates the filter light measures the back-pressure across the filter as a means of early detection of a flow restriction. Flow rate is still ok. The pressure setpoint for the filter light is chosen to give you time to replace the filter. Get it done in the next few weeks, at your earliest convenience. This is the same as how industrial equipment is protected from blocked filters. The blocked filter alarm on an industrial machine is a warning only. The machine can still operate but it tells the operators that they must replace the filter at the next convenient time as the back pressure across the filter is becoming too much. Flow is still ok. Leave it for a few months and it’s likely the flow restriction will become so large that some other protective device will operate and prevent the machine from running.
The fuel filter example is typical of what occurs when deviating from manufacturer’s recommendations. It reinforces the idea that vehicle manufacturers are in the best position to gauge service intervals. Still, it’s personal choice on how often you service your vehicle. The manufacturer’s recommendation is a guide, and everyone has the right to choose as they please based on their own experiences and understanding. The odds are that the manufacturer’s recommendations are on average pretty good, but the manufacturer isn’t perfect and there may be isolated cases where deviating is a valid position to take.