Fuel Economy, Causes of poor MPG

5 January 2016 | Bruce Compton

If you have noticed that your vehicle is running less economically than it used to, there could be an underlying mechanical problem. This guide covers the likely issues which could be attributing to you using more fuel.

fuel economy

Driving Style and Circumstances

One of the most common causes of poor fuel economy is the way you drive and the trips that you make. Cars use the most fuel during initial acceleration, so if your car journey is short or involves a lot of stops then you will inevitably use more fuel that longer journeys where your speed and acceleration are constant.  In addition, aggressive driving where you accelerate hard and brake late causes you to flood the engine with extra fuel and therefore use more.  To obtain the best mpg, keep your vehicle moving as much as possible.  Combine short journeys and try to keep the acceleration and speed constant.   For a more in-depth look at eco-driving, please take a look at aff vehicle services eco motoring guide.

Bad Oxygen Sensors

Oxygen sensors guage and help maintain a vehicles fuel/air mix.  As they become older, they can become less responsive.  Having faulty oxygen sensors can reduce mpg by up to 20% because the sensors are telling the car that it needs more fuel when it doesn’t.

Faulty Thermostat

The engine’s thermostat controls the operating temperature of the engine,  helping the engine warm up quickly after a cold start.  The thermostat’s purpose is to close when the engine is cold to block the flow of coolant.  A faulty thermostat does not close properly, allowing the coolant to continue circulating while the engine is warming up.  This could mean that the car never reaches a desired operating temperature.  A cold car uses more fuel.

Faulty Fuel Injectors

Fuel injectors are responsible for putting fuel into the engine.  Fuel varnish deposits can build up inside fuel injectors, preventing them from delivering the correct mix of fuel.  This means that less fuel makes it to the engine and the mix is lean causing backfiring and poor fuel economy.

Engine Misfire

Engine misfire could be the result of many issues, defective fuel injectors, faulty spark plugs, bad plug wires, fuel injectors, low fuel pressure, to name but a few.   Engine misfire will waste a large amount of fuel.

Faulty Spark Plugs

Your spark plugs are responsible for sparking combustion in your engine. Old and worn or faulty spark plugs will cause an engine misfire, waste fuel and in turn give you less mpg.  A car which is using too much oil or making short trips and journeys which involve a lot of stopping and starting all cause the spark plugs to foul up prematurely.

Clogged Air Filter

If your air filter is really dirty, it interferes with normal engine performance causing poor fuel economy.

Wrong Oil Viscosity

Most modern-day vehicles use low viscosity motor oil.  During cold weather, when oils tend to thicken, this can greatly improve fuel economy.  Using a motor oil with a higher viscosity than you need can cause poor fuel economy.  Check your vehicle manual or dealership for the correct oil grade required for your engine.

EGR Valve Leak

The EGR  Valve (Exhaust Gas Recirculation Valve) is a device which has the purpose of controlling emissions.  If the EGR valve does not close at idle or when the engine is cold, it can allow exhaust gases to leak back into the intake manifold causing misfires and in turn less mpg and poor fuel economy.

Check car tyre pressures

Check car tyre pressures

Tyre Pressure and Type

The type and pressure of your tyres have a large and direct impact on your fuel economy.  This is to do with rolling resistance.   If your tyre pressures are low, your vehicle will have to work harder to get itself down the road. The harder your vehicle works, the more fuel it will use. Likewise, better quality tyres often have less rolling resistance and therefore are more economical in the long run.

A Full Boot

If you have a lot of unnecessary weight in the boot of your vehicle, it will take extra power to haul it.  Extra power equals extra fuel.

Air Con Over Use

It takes extra power to use your air con in the car and therefore more fuel.  If you have it on all of the time then this will greatly affect your mpg.

These are some of the main and most common causes of poor fuel economy.  To check what your vehicles fuel economy should be, take a look at your car data statistics on the website.