What causes high fuel consumption in petrol engines

If your vehicle suddenly uses more fuel than it used to, something has changed: how it is driven, where it is driven, or a part that is wearing out. Start with the free checks below before you pay for bigger repairs.

Check these first

These cost little or nothing.

• Tyre pressure. Soft tyres drag on the road. Check them once a month when the tyres are cold, using the pressure on the sticker inside the driver's door.
• How and where you drive. Stop-start traffic, short trips, long idling and hard acceleration all raise fuel use. An engine that idles in a jam burns fuel while the car goes nowhere.
• Extra weight. A full boot or a loaded roof rack makes the engine work harder on every trip.
• Air conditioning. The AC runs off the engine, so it uses more fuel, most of all in slow traffic.

Common causes in petrol engines

• Worn spark plugs or a weak ignition coil. The fuel does not burn fully. You may also feel the engine shake at idle or hesitate when you press the accelerator.
• A faulty oxygen sensor. This sensor tells the engine computer how much fuel to add. When it fails, the engine often runs rich, which means it burns more fuel than it needs. The check engine light usually comes on.
• A faulty air flow sensor or temperature sensor. Wrong readings make the engine add too much fuel. A coolant sensor that always reads cold makes the engine run as if it has just started.
• A thermostat stuck open. The engine never reaches its normal temperature, and a cold engine uses more fuel. The temperature gauge stays low.
• Leaking or dirty injectors. A leaking injector drips extra fuel into the engine. A smell of petrol or black smoke from the exhaust are signs.
• Dragging brakes. A sticking brake keeps slowing the car while you drive. After a drive, one wheel feels much hotter than the others.
• Wheel alignment. Wheels that point slightly apart scrub the tyres along the road. Uneven tyre wear is the sign.
• A slipping clutch or a worn automatic gearbox. The engine revs up, but the car does not speed up to match.
• A dirty air filter. Older engines with a carburettor lose the most. Modern fuel-injected engines adjust for it, but a blocked filter still costs power.

Can the fuel pump cause high fuel consumption?

Rarely on its own. A weak fuel pump usually makes the car lose power or cut out. A faulty fuel pressure regulator is different: it can send too much fuel to the injectors, which raises consumption and can cause black smoke.

Does a catalytic converter reduce fuel consumption?

No. A working catalytic converter cleans the exhaust gas. It does not save fuel. A blocked one does the opposite: it chokes the exhaust, so the engine loses power and uses more fuel. Removing a working one will not save fuel in a modern car, and the check engine light will usually come on.

What causes high fuel consumption in diesel engines

• Worn or dirty injectors. They spray fuel unevenly, so it does not burn fully. Black smoke when you accelerate is the usual sign.
• A blocked air filter. A diesel engine needs a lot of air, and a blocked filter causes black smoke and higher fuel use.
• Turbo problems. A leaking boost pipe or a worn turbo means less air, so fuel is wasted. You may hear a whistle or feel a loss of power.
• A clogged EGR valve or diesel particulate filter on newer diesels. The engine runs rich, or burns extra fuel to clean the filter. Many short trips make this worse.
• Dirty fuel and an old fuel filter. Over time they damage the injectors, which leads to the problems above. Change the fuel filter as the service book says.

What causes high fuel consumption in a motorcycle

• A carburettor set too rich. The bike burns more petrol than it needs. Signs are black smoke, a sooty spark plug and a strong smell of petrol. A mechanic can set the mixture.
• A dirty air filter. On a carburettor bike, a blocked filter makes the mixture rich. Clean it often, especially on dusty roads.
• A loose or dry chain. It wastes power between the engine and the back wheel. Keep it oiled and at the right tension.
• Soft tyres and heavy loads. Carrying passengers and goods every day raises fuel use, and soft tyres make it worse.
• A slipping clutch. The engine revs, but the bike does not speed up to match.

How do I fix high fuel consumption?

1. Measure it first. Fill the tank to the top, drive as usual, fill it again and divide the kilometres by the litres. How to calculate fuel consumption shows each step.
2. Compare your figure with other owners of the same vehicle on its Kilolita page. If yours is close to theirs, the vehicle is fine.
3. Do the free checks: tyre pressure, extra weight and how long you idle.
4. If it is still high, ask a mechanic to read the engine computer for fault codes and to check the plugs, sensors, injectors and brakes.
5. After the repair, measure again.