Savings on Fuel - Part Two

In the first part of this series we looked at choosing the most economical car to suit your needs. (See Savings on Fuel - Part One.)

In this, the second part, we’ll look at maintenance that will keep your vehicle running at its most frugal.

Engine Oil and Gear Lubricants

To wring maximum fuel economy from your car, you should use the thinnest engine oil and gear lubricants recommended by the manufacturer.

As a guide, most modern engines require a multi-grade oil with a viscosity of around SAE 10W/40. This describes an oil with a viscosity rating of 10 when cold and 40 once warmed to normal operating temperature (the higher the number, the thicker the oil). A common alternative for SAE 10W/40 oil is SAE 10W/30 – an oil that gives identical lubrication characteristics when cold but stays relatively thin once up to normal operating temperature. The SAE 10W/30 oil will reduce engine losses and improve fuel economy without compromising engine lubrication.

Eeach car has different engine oil requirements so you should always check your manufacturer’s handbook.

Manual gearboxes and differentials require a dedicated gear lubricant ranging from SAE 60 to 250. This is considerably thicker than typical engine oil. As a guide, the early ‘90s Subaru Liberty can use either SAE 85 or 90 grade fluid for its manual gearbox/front differential assembly. The thinner SAE 85 grade fluid will deliver superior fuel economy without a major effect on lubrication but, again, we recommend consulting the vehicle handbook for recommendations to suit your driving environment.

When it comes to automatic transmissions, there is typically only one type of fluid recommended by the manufacturer – and that’s the one you should use. Automatic transmission fluid is extremely complex and should not be messed around with.

Filters

There are three engine-related filters in a car – the air filter, oil filter and fuel filter.

Of this trio, the air filter can have the biggest effect on fuel consumption - a dirty air filter restricts airflow into the engine, causing a noticeable loss of power and increase in fuel consumption.

One solution is to replace the air filter at - or before - the recommended interval specified in your vehicle handbook. An early ‘90s Subaru Liberty requires an air filter inspection every 25,000km/12 months and replacement every 50,000km/24 months. If you drive in dusty conditions, we recommend more frequent inspection for air filter blockage.

Some people choose to install a generic aftermarket air filter. Many aftermarket air filter manufacturers claim superior airflow to the factory part and, as a result, better power and economy. But a free-flow air filter may not provide engine protection equal to the factory part.

The fuel filter of your vehicle is another item that should be replaced at regular service intervals. A clogged fuel filter can cause poor driveability and, as a result, increased fuel consumption. You can help prevent a clogged fuel filter by filling the car’s fuel tank and driving it to near empty rather than making numerous small stops – this reduces the chance of foreign matter entering the fuel tank through the filler neck.

The engine oil filter should be replaced at each oil change. However, of the three filters, the oil filter generally has the least effect on fuel consumption.

Engine Tune

Your car’s engine should receive more than just routine fluid and filter changes - engine tune is often overlooked.

In most fuel injected vehicles, the engine’s air/fuel ratio does not require regular adjustment - the appropriate air-fuel ratios (ie mixtures) are maintained by the engine management computer. However, if your car is returning worse than expected fuel economy, it is worth checking two aspects of the fuel system – fuel pressure and injectors.

For a small outlay, any mechanical workshop should be able to perform an on-car fuel pressure test. If the fuel pressure is not within the manufacturer’s specified range it’s important that you quickly repair the fuel system to restore fuel economy.

If the fuel injectors are partially blocked (a problem often identified by rough running) you should first try adding a bottle of injector cleaner to your fuel. Injector cleaner is available at all petrol stations. Should a bottle of injector cleaner fail to unblock the injectors, you’ll need to remove the injectors and take them to a specialist workshop to be ultrasonic cleaned.

The correct operation of all engine sensors is also essential.

Most fuel injected vehicles employ at least one oxygen sensor in the exhaust system to monitor air/fuel ratio. The input from this sensor is used to determine how much fuel needs to be injected into the engine for each cycle. If the oxygen sensor is giving a false signal or fails, it will cause the engine to run too rich or lean at light load. It’s also likely that a ‘Check Engine’ warning light will appear on the dashboard when the oxygen sensor - or any other engine management sensor - fails. It’s important to replace faulty engine sensors to maintain efficient engine operation.

In cars fitted with a replacement timing belt or chain, it’s a good idea to check that the camshaft and ignition timing are set to factory specifications. Incorrect cam and/or ignition timing can reduce fuel consumption and potentially cause engine damage. Any workshop should be able to check timing for little cost.

The ignition system is also are important from a fuel consumption point of view.

Spark plugs should be in good, clean condition and provide appropriate spark energy to burn the air-fuel charge. A spark plug that is worn or has a too-wide gap struggles to create a spark to ignite the charge - this means poor combustion efficiency and poor fuel consumption. Use the manufacturer’s recommended spark plugs with the appropriate gap and replace at the specified intervals.

Other engine tune factors that influence fuel economy include idle speed, condition of the ignition system (coils, leads, distributor cap, rotor and other parts specific to your car), engine compression, operation of crankcase ventilation system and exhaust backpressure. Excessive exhaust back pressure is commonly caused by a collapsed cat converter or a dislodged baffle inside a muffler.

Wheel/Brake Drag

One of the easiest steps to improve fuel economy is to ensure tyres pressures are kept high.

For normal driving conditions, we suggest inflating your car’s tyres about 2 – 3 psi over the manufacturer’s recommendation. There should be no concerns with over-inflation so long as you stay below the maximum inflation pressure of your particular tyres (the maximum pressure is shown on the sidewall of the tyre). Increasing tyre pressure reduces rolling resistance which aids fuel consumption. Handling and steering response will also be improved but ride quality will be negatively affected.

The wheel alignment of your vehicle should also be within specifications quoted by the vehicle manufacturer. Poorly adjusted wheel alignment causes increased tyre drag and so increased fuel consumption. A wheel alignment should be part of your vehicle’s regular service schedule.

More important considerations are wheel bearing and brake drag.

A deteriorating wheel bearing (which is often identified by a road speed related rumble) reduces fuel economy by causing extra drag. Similarly, it’s important that the car’s brakes are not dragging. Worn brake calipers and an incorrectly adjusted handbrake are the common problems to watch out for. These items should be checked as part of routine servicing.

Unnecessary Weight and Aerodynamic Drag

Unnecessary weight causes poor fuel consumption.

If your car is packed with items such as golf clubs, a tool set, fold-up chairs and an umbrella, you should be aware that you’re paying extra for these items to travel with you. The heavier your car, the more energy it needs to drive from point A to point B – and more energy means more fuel. Take a good look through the boot, glovebox, centre console and beneath the seats of your car – the more junk you can remove, the better.

Car aerodynamics also play a part in fuel consumption – especially when the vehicle is used for a lot of high-speed driving.

During normal servicing it’s common for car undertrays to go missing – they’re often seen as ‘unnecessary’. Undertrays should be reinstalled to optimise performance of the cooling system and maintain under-car airflow. On the other hand, add-on body accessories such as roof racks and spot lights should be removed after use. These reduce a car’s aerodynamic efficiency and increase fuel consumption.

In Part Three of this series we’ll take a look at driving techniques to reduce fuel consumption...