AUTONEWS

Car suddenly uses more fuel - the culprit is not always the engine

When you drive the same routes every week, you probably have a pretty good idea of how often you need to stop at the gas station. If those stops suddenly seem to be happening more frequently, you may start wondering why your car is using more gas than usual.

Changes in fuel economy aren’t always caused by one major mechanical problem. Something as simple as low tire pressure or a change in driving habits can increase fuel consumption. However, declining gas mileage can also point to problems with your engine, fuel system, sensors, brakes, or other components.

A noticeable drop in fuel economy is worth paying attention to, especially when your driving routine hasn’t changed. Understanding the possible causes can help you determine whether you’re dealing with normal variations or a vehicle problem that deserves professional attention.

Here are some of the most common reasons you may have a car using more gas than usual.

Fuel consumption can also be increased by unsuitable tires or poor wheel geometry.

Most drivers know approximately how much fuel their car uses. When the average consumption suddenly increases by a liter or two, the first suspicion usually falls on the engine. However, the cause of the increased consumption can be much simpler, and therefore cheaper to solve.

First, check whether the car really uses more fuel... The data shown by the trip computer may not always be completely accurate, and short-term average consumption can be significantly affected by traffic jams or several cold starts, writes Autonews.

The most reliable way is to calculate consumption when refueling. Fill the tank to the top, reset the odometer, and divide the amount of fuel poured by the number of kilometers traveled during the next refueling. It is best to compare the results after several tank fills, as a one-time difference can occur due to a different route, weather conditions or other circumstances.

The cause may be the tires...One of the first things to check is the tire pressure. Even a decrease of a few tenths of a bar increases rolling resistance, which is why the engine needs more energy to maintain the same speed. Consumption can also increase due to unsuitable tires or poor wheel geometry.

Don't forget about the roof rack or roof box. They increase air resistance, which is especially noticeable when driving on the highway. Unnecessary cargo in the trunk has a similar effect, although the additional mass is most noticeable in city driving, where the car often starts and stops.

The car may be constantly braking while driving...Increased fuel consumption may not be related to the engine itself, but to the additional resistance that the car has to overcome while driving. A common cause may be a stuck brake caliper or a brake pad that remains pressed against the disc. In this case, the car accelerates less, and when the driver releases the accelerator pedal, the vehicle slows down faster than usual.

The smell of overheated brakes or the car pulling to one side can also indicate a problem. If you suspect this problem, never touch a hot brake disc or other parts of the braking system with your hand. Repair should not be delayed, as this type of failure, in addition to increasing fuel consumption, can endanger safety and cause damage to other parts of the car.

Cold weather and short trips can be the culprit...After a cold start, the engine runs with a richer fuel-air mixture and needs some time to reach the proper operating temperature. If the car is mainly used for trips of only a few kilometers, the engine runs in an uneconomical mode for a large part of the time. Therefore, consumption naturally increases, especially during winter and in dense city driving.

If the engine, however, does not reach operating temperature even after a long drive, the cause may be a stuck thermostat. In this case, the temperature gauge remains low, the cabin heating is weaker, and the engine control unit continues to inject more fuel as if the engine were still cold. Replacing the thermostat is much cheaper than the consequences that can arise from ignoring the problem for a long time.

The problem may also be in the exhaust system or transmission...In cars with diesel engines, increased consumption can also be caused by a clogged particulate filter, or DPF. If the filter regeneration is not completed completely several times, the system uses additional amounts of fuel, while at the same time the resistance in the exhaust system gradually increases.

Attention should also be paid to the automatic transmission. If it unnecessarily holds a lower gear for too long, slips or does not shift into the highest gear, the engine runs at a higher speed, which also leads to higher fuel consumption.

Special attention should be paid if the sudden increase in consumption is accompanied by other symptoms - loss of power, smoke from the exhaust, the smell of fuel, unusual sounds or the lighting of a warning light on the dashboard.

First: Make sure the economy has actually dropped...Before diving into causes, it's worth confirming that the economy actually has changed rather than just seeming like it has. Perception of fuel consumption can be misleading.

The most reliable way to track your real-world consumption is to:

•       Fill the tank completely (to the first click of the nozzle)

•       Reset your trip odometer to zero

•       Drive normally until the next fill-up

•       Fill the tank completely again and note exactly how many litres went in

•       Divide the litres used by the kilometres travelled and multiply by 100 , this gives you L/100km

Do this across two or three fill-ups to get a reliable baseline. Compare it to your manufacturer's stated combined economy figure (in your owner's manual) and to what you were achieving previously if you have that data.

One important caveat: economy varies significantly with how and where you drive. More city driving, more short trips, hotter weather requiring more AC, heavy traffic , all of these increase consumption without anything being wrong with the car. If your driving pattern has genuinely changed alongside the consumption increase, that may be the whole explanation.

Tyre-related causes, often the first place to check...Under-inflated tyres...This is the single most common and most overlooked cause of increased fuel consumption in Australian passenger vehicles. Under-inflated tyres have a larger contact patch with the road and flex more during rotation, creating greater rolling resistance. The engine has to work harder to maintain speed, which means more fuel used for the same distance.

Research consistently shows that tyres running 10% below their recommended pressure increase rolling resistance by approximately 3%, translating directly to 2–3% more fuel per kilometre. At 20% below recommended pressure , which is entirely possible in neglected tyres , the penalty can reach 5–7%.

•       Check all four tyres including the spare when cold (before driving or after less than 3 km)

•       Find the correct pressure on the tyre placard inside the driver's door frame , not on the tyre sidewall, which shows maximum pressure

•       Check monthly , tyres naturally lose 1–2 PSI per month through normal permeation

•       Also check for uneven pressure between tyres , one noticeably lower than the others suggests a slow puncture or valve issue

Wheel alignment out of specification...If your wheels are not properly aligned , pointing in slightly the wrong direction , the tyres scrub against the road rather than rolling freely. This creates significant rolling resistance that saps both fuel economy and tyre life simultaneously.

Signs that alignment may be off: the car pulls to one side, the steering wheel isn't centred when driving straight, or you notice uneven wear across the tyre (more worn on the inner or outer edge than the centre). A wheel alignment costs $60–$90 at most tyre shops and is worth doing if you notice any of these signs.

Wrong tyre type or size...If your tyres have been replaced recently and a different size or specification was fitted, this can affect fuel economy. Wider tyres and high-performance 'sticky' compound tyres have higher rolling resistance than standard touring tyres and will increase consumption. Check that the replacement tyres match the size and load/speed rating specified in the owner's manual.

Maintenance issues that drain economy...Dirty or clogged air filter...The air filter prevents dirt and debris from entering the engine. Over time, it accumulates particles and becomes progressively more restrictive. A clogged air filter reduces the volume and flow of air entering the engine, forcing the fuel management system to compensate with richer fuel mixtures , more fuel per combustion event.

A significantly dirty air filter can reduce fuel economy by 6–10%, which on a car that normally does 8L/100km means it's doing 8.5–8.8L/100km instead. Over 15,000 km per year, that's 75–120 extra litres consumed , several hundred dollars' worth at current prices.

•       Air filters should be inspected at every service and replaced when dirty , typically every 20,000–40,000 km depending on conditions

•       In dusty conditions , outback driving, dirt roads, dusty worksites , filters clog much faster and should be checked every 10,000 km or sooner

•       Replacement cost is typically $20–$40 in parts, often done as part of a standard service

Worn or fouled spark plugs...Spark plugs ignite the air-fuel mixture in each cylinder. When spark plugs are worn, their electrode gap has increased and the spark becomes weaker. When they're fouled with deposits, the spark may be inconsistent or may misfire entirely.

A weak or missing spark means incomplete combustion , the fuel doesn't fully burn, energy is wasted, and the engine compensates by using more fuel to maintain power output. Misfiring cylinders also cause rough running, increased emissions, and in severe cases, damage to catalytic converters.

•       Most modern vehicles have service intervals of 60,000–100,000 km for iridium or platinum spark plugs

•       Standard copper plugs in older vehicles should be replaced every 25,000–40,000 km

•       If your car is rough at idle, hesitates on acceleration, or has a noticeable misfire, spark plugs are a priority check

Degraded engine oil...Engine oil's viscosity changes as it ages and accumulates contaminants. Older oil becomes thicker, more viscous, and harder to pump through the engine's internal galleries. The additional internal friction this creates means the engine uses more energy , and therefore more fuel , to overcome it.

This effect is gradual and hard to notice day-to-day, but an engine running on oil that's 5,000 km overdue for a change may have measurably worse economy than the same engine on fresh oil. The improvement from a full oil service with the correct grade is typically 1–3% in fuel economy.

Blocked or partially clogged fuel injectors...Modern fuel injection systems deliver precisely metered amounts of fuel directly into each cylinder. Over time, carbon deposits from combustion accumulate on the injector nozzles, affecting the spray pattern and the volume of fuel delivered per injection event.

Clogged injectors either deliver too much fuel (rich running) or spray it in a poor pattern that doesn't atomise correctly , both waste fuel. The engine's management system may compensate by adjusting fuelling upward, increasing consumption further.

•       Symptoms: rough idle, hesitation on acceleration, poor cold start performance, increased consumption

•       Diagnosis: a mechanic can perform an injector flow test or use fuel system cleaner through the injectors

•       Treatment: professional injector cleaning ($100–$250) or in severe cases, injector replacement

Failing oxygen sensor...The oxygen sensor (also called a lambda sensor) monitors the oxygen content of the exhaust and feeds this data back to the engine management system, which uses it to maintain the correct air-fuel mixture. When this sensor fails or provides inaccurate readings, the engine may run with an excessively rich mixture , too much fuel relative to air , wasting fuel and increasing consumption.

A faulty oxygen sensor is one of the most common causes of a check engine light illumination, and it's also one of the most common causes of sudden, unexplained fuel economy drops of 10–20%.

•       If your check engine light is on alongside worsening economy, an oxygen sensor fault is high on the diagnosis list

•       Replacement cost: typically $150–$400 fitted depending on vehicle and sensor location

Faulty thermostat...The engine thermostat regulates coolant temperature, keeping the engine in its optimal operating temperature range. A thermostat stuck in the open position allows coolant to circulate continuously, preventing the engine from reaching full operating temperature.

A cold-running engine has higher internal friction, runs with a richer fuel mixture (cold engines need more fuel), and doesn't operate at peak efficiency. If you notice your temperature gauge never reaching its normal position, or the heater not getting properly warm, a stuck-open thermostat may be the culprit , and it's a relatively inexpensive fix.

Dragging brakes...If a brake calliper has seized or is partially dragging, the brake pad is in constant light contact with the rotor. This creates continuous mechanical resistance , the engine constantly has to work against braking force to maintain speed. The effect is similar to driving with the handbrake slightly on.

Signs of a dragging brake include a burning smell, unusual heat from one wheel after driving, the car pulling to one side, or uneven wear between front and rear brakes. A sticking calliper requires immediate attention , it's a safety issue as well as an economy one.