We do not always have insights into why our customers buy the cars they do, but as technicians, we are called upon to diagnose and repair all manner of problems and issues on the vehicles they do buy, including on hybrid vehicles as they occur- even if we are not always as up to speed on all the various hybrid technologies as we perhaps should be. One such problem involves excessive fuel consumption on older hybrid vehicles, and since many of these vehicles are now out of their warranty periods, or are approaching the end of their warranty periods, more and more of these vehicles will start to appear in independent workshops, which begs the question of how ready we are to diagnose and resolve issues on technologies we often don't know very well.
Therefore, in this article we will discuss some of the primary causes of excessive fuel consumption on older hybrid vehicles, as well as how to resolve at least some of these issues, starting with this question-
This is a very good question, and to find an answer, the University of Western Australia ran a study between 2010 and 2012 that produced some surprising insights, such as the following key findings-
1) “Buyers of conventionally fuelled vehicles reported that they considered quality and performance to be the most important determinants of choice. They rated the image they get from driving a particular car and social influence as being the least important determinants of purchase.” (Italics added)
2) “Hybrid car buyers reported opposite results, that social influence and projecting a “green” image were most important, and quality and appeal was least important.” (Italics added)
3) “That these consumers [buyers of hybrids] are willing to pay premium prices of up to 16,000 Australian dollars more for a Prius over a comparable Toyota Corolla appears to support this conjecture." (Italics added)
Source: https://www.researchgate.net/publication/228073181_Why_Do_People_Buy_Hybrid_Cars
While the above explains some of the reasons why some people buy hybrid vehicles, this writer understands a “green” image to mean a desire to use less fossil-based fuel as a means to inject less pollution into the atmosphere, which is/was the primary driving force behind the development of hybrid vehicles, in the first place.
Having said the above, the question now becomes one of how much fuel older hybrid vehicles should be using, or perhaps more precisely, what constitutes excessive fuel consumption on an older hybrid. This is an important point because this writer had recently encountered a Gen 1 Toyota Prius whose fuel consumption had increased by more than 30% in the space of only four months, so let us look at some-
NOTE: From this point forward, this article will focus on early iterations of Toyota Prius and Honda Civic hybrids, since these models were among the first hybrids to be marketed in large numbers. Therefore, these vehicles now account for the largest combined number of hybrids that are out of their warranty periods.
We all know that fuel consumption figures released by car manufacturers should be treated as guidelines and not as actual values, but in an attempt to address this problem, the EPA (Environmental Protection Agency) in the USA tests all new vehicle models before they can be sold to the public and based on their observations and tests, the EPA then releases an estimated fuel consumption rate for each model. Note that a) these tests and standards have become a de facto standard all over the world, including in the Australian market, and b), that the way the EPA estimated fuel consumption values changed in 2008, meaning that the figures listed here might differ slightly between sources.
So what did the EPA say about fuel consumption for first-generation Prius and Civic hybrids? Here are some details to serve as context or background against which to judge excessive fuel consumption issues-
Toyota Prius
The EPA estimated that Gen 1 Prius hybrids should not use more than 5.7 litres of fuel per 100 km in combined city/highway driving. Note though that when the Gen III Prius was released in 2009/10, this value had been revised downward to 5.1 litres / 100 km, which was further revised downward to only 4.7 litres / 100 km after Toyota had introduced some weight-saving measures in late Gen III models.
Honda Civic
NOTE: Honda Civic hybrids use a VTEC-based cylinder deactivation system that requires a very specific oil formulation to work reliably. The primary function of the system is to prevent the valves from opening on two of the cylinders on 3-cylinder engines, and all four cylinders on later, 4-cylinder engines to shut off the engine under certain conditions to save on fuel. In practice, this is a Stop-and-Go system, and if it stops working because the engine is filled with anything other than the recommended fully synthetic oil, the result is a dramatic rise in fuel consumption because the engine either does not shut off as often as it should, or it does not shut off at all during normal driving.
When the Honda Civic hybrid was introduced into the US market in 2002/3, the EPA estimated that the 5-speed manual version would use 5.0 litres of fuel /100 km in city driving, and 4.6 litres /per 100 km during highway driving. The version with the CVT transmission was estimated to use 4.9 litres / 100 km in both city and highway driving.
So now that we know how much (or how little) fuel these early versions of Toyota and Honda hybrids should be using, we are in a better position to ask more pertinent questions and draw more informed conclusions from the answers about these vehicle’s fuel consumption rates when the practice departs drastically from the ideal.
For instance, did the decline in fuel economy start suddenly, and is it getting worse? Or, did the decline start slowly, and is getting progressively worse? Did the way the vehicle is used change suddenly? Is it being maintained /serviced according to the original service schedule? These are just some example questions, and there are many more you could ask a customer that is complaining about excessive fuel consumption on his/her hybrid but to keep things moving along, let us look at some-
Like conventional vehicles, hybrids also require regular maintenance and servicing, but it has been this writer's experience that many owners of hybrid vehicles think that their vehicles are somehow immune to the effects of regular use. Some of the typical causes of decreased fuel economy that this writer sees regularly include-
We don’t want to spend too many words on this subject, except to say that the added weight of three or four passengers could cause a marginally degraded HV battery to discharge quicker, which translates into increased charging times, and therefore, into more fuel used to run the ICE (Internal Combustion Engine) for longer to keep the HV battery charged.
As a matter of interest, many owners of hybrid vehicles seem to be unaware of the fact that much of their vehicles’ motive power derives from a battery that needs to be in a good to excellent condition to perform as intended. Thus, the combination of poor maintenance and HV batteries that may be beyond their best years means that previously-normal loads (not to mention additional loads) on the HV battery place greater demands on the battery than similar loads would have placed on a new HV battery.
While the issues listed above also have the potential to increase fuel consumption on conventional vehicles, the effects of these kinds of issues on fuel consumption are bigger on hybrid vehicles than they are on conventional vehicles. This is simply because the motive power of hybrid vehicles does not derive directly from their internal combustion engines, but from a battery that can only be kept in an acceptable state of charge by burning more fuel more often when the battery is in a less than perfect condition.
Nonetheless, this writer has “fixed” many excessive fuel consumption issues on a variety of hybrid vehicles simply by doing some overdue maintenance, and educating his customers on how their vehicles work, why they require regular maintenance, and particularly on how poor maintenance/servicing practices negatively affect HV battery life and by extension, the fuel consumption of hybrid vehicles, which brings us to-
If you have never seen an HV battery pack from a hybrid vehicle before, this image shows some details of the 200 or so sealed lead-acid cells that comprise a typical non-nickel metal hydride battery pack. The strip of electronics running across the battery pack is part of the battery monitoring system, whose function it is to monitor the resistance, state of charge/health, and charge/discharge rate of each cell in the battery pack.
We need not delve into the complexities of battery chemistries and the differences between them here, but suffice it to say that while connecting lots of cells to create a battery is a cost-effective way of designing a battery for a specific application, the fact is that designing batteries in this way has major implications for both battery life and performance.
The problems with connecting lots of cells are a) that it takes a lot of connections to do it, and b), that there is no guarantee that every cell in the pack is identical to every other cell in terms of both internal resistance, and the integrity of internal connections. Nonetheless, HV battery packs in many hybrid vehicles have been known to perform faultlessly for anything up to 15 years, but since there are many exceptions to this, most manufacturers guarantee their batteries for only eight years, which is an important point to bear in mind.
Why is this important? It is important because regardless of their chemistry, almost all HV hybrid battery packs start to degrade noticeably after about three to five years of continuous use. The first thing that happens is that some cells begin to not only discharge faster than others but they also take longer to recharge again. If this condition is allowed to continue, some cells may degrade to the point where their polarity might reverse, which if it happens, renders the entire battery pack useless.
Before this happens, though, the battery control module will set "battery imbalance" codes, but at this point, the internal combustion engine has been running for longer than it should have over extended periods to keep the degraded battery pack in an acceptable state of charge. Depending on the age/condition of the battery, and the degree of imbalance(s) in the battery fuel consumption can increase dramatically, and increases of 25 to 30 per cent are not uncommon under these conditions.
Battery imbalances are arguably the leading cause of excessive fuel consumption issues on most hybrids, but the really bad news is that while a battery pack can often be reconditioned to resolve imbalances, this is a highly technical procedure that requires specialised equipment and expert-level knowledge of the underlying chemical properties and characteristics of batteries in general, and of hybrid battery systems, in particular.*
*As a practical matter, attempts to resolve battery imbalance codes cannot, and should not be undertaken by workshops that do not have the required equipment, skills/ knowledge/experience, and access to the required service information. The risk of electrocuting yourself is very high, as is the risk of causing fatal damage to the battery. Rather play it safe and refer the vehicle to a specialist repairer for proper diagnosis and repair/reconditioning of the battery, which brings us to-
Having said the above, it must also be said that poor HV battery performance and the concomitant excessive fuel consumption is often the result of things that we as independent technicians don’t need exert level knowledge for to correct. For instance, high battery temperatures are often the result of nothing more than blocked or restricted air vents, or in some cases, under-performing battery cooling fans. This can be confirmed /verified or eliminated as the cause of battery overheating with the use of a suitable scan tool (with bi-directional control capabilities) to test the operation of battery cooling fan(s).
Other things we can do include checking for, and clearing away obstructions in, over, or from air vents in the battery cooling system, or ensuring that pet hair, sweet wrappers, long-lost socks, and/ or gym/sport kit bags are not impeding the operation of battery cooling fans. These simple inspections and corrective actions have resolved many battery-operation and excessive fuel consumption issues on many hybrid vehicles in this writer's workshop, and they will do the same thing for you, as well.
Based on lengthy conversations with many owners of older hybrid vehicles, it has become clear to this writer that in many cases the reality of excessive or steadily increasing fuel consumption of older hybrid vehicles is a strong motivating factor for many owners of hybrid vehicles not to replace their current hybrid vehicle with another hybrid, especially since the fuel consumption rates of many new conventional vehicles are improving rapidly and consistently.
We cannot control our customers’ buying decisions, but from our perspective as technicians, the fact is that hybrid technology is not only here to stay; it will also become more prevalent among more manufacturers and in more market segments. What this means for the independent repair industry is that hybrid vehicles will soon begin to account for progressively larger percentages of our daily/weekly car counts and unless we learn to diagnose and repair these vehicles reliably, accurately, and consistently, we could begin to lose a lot of business to competitors.
