CAUSES OF HIGH FUEL CONSUMPTION (HIGH GAS MILEAGE).

Fuel consumption is peculiar to all internal combustion engine, but when it becomes excessive that's when you know there is fault somewhere causing the excessive consumption.

When you tried to know your car’s current fuel consumption and compared to when you got it new, or you compared it to your friends own, then you discovered that your fuel consumption is on the high side beyond normal. The bordering question that comes to mind is 'why is my car consuming excess fuel?

Read this ;How to determine your fuel consumption. 
 
Here is a list of the possible causes of excessive fuel consumption. ‎some of these  faults causing excessive fuel consumption wouldn't turn the check engine lights on. Examples are from the brake system, when there is a seized cylinder, and the exhaust line, when there's a leakage. And your driving habits, your car wouldn't tell you you're wasting more fuel with the way you drive.

»‎ Delayed engine maintenance service and improper tune up.
 When the routine maintenance service is delayed or the engine is not properly tuned, it will result to an increased fuel consumption. The oil, oil filter has to be changed at regularly intervals when maintenance is required. Other parts such as the air filter, spark plugs also contributes to excessive fuel consumption when they are worn-out. Your routine maintenance service involves the testing of these various parts and replacement of the ones that are worn-out.

» Engine misfire.
 If your engine is misfiring, usually the check engine light comes on, engine misfire wastes a lot of fuel. There are a lot of faults that could be responsible for an engine misfire, the best way to know what is causing the misfire is to run a diagnosis on the car.
 Some of the common causes of engine misfire are; ignition faults such as bad spark plugs, faulty coils, electrical arcing between plug wires and the ground current. Also, faults in the injections such as blocked(due to dirts) or bad fuel injectors, carbon dirts in the throttle body, air leakages around the inlet manifold or fuel leakages along the fuel line. Wrong ignition timing or poor compression due to engine wears. All of these faults will affect your fuel economy negatively, and you waste more fuel.

» Wrong or defective lubricating fluids.
 The use of wrong lubricating fluids, like the engine oil and the transmission oil  will result to a poor fuel economy as these fluids help to reduce friction so as to minimise power loss. A wrong or defective lubricating fluid will not have the right viscosity for the engine or transmission. Some engines mostly recently produced requires lower viscosity fluid which improves fuel economy, but when you're using a higher viscosity fluid you will experience a poorer fuel economy especially during cold weather.  It's important that you follow the manufacturers specifications in selecting the lubricating fluids to use.

» Dirty or bad air filter.
 If the engine air filter is too dirty, it will affect the air-fuel mixture and the engine breathing, also a damaged filter will not be able to filter the intake air properly, dirts or dusts could block some vacuums around the inlet manifold. This also results to a poor fuel economy.

» Faulty coolant sensor or engine thermostat.
 The work of the coolant sensor is to monitor the temperature of the coolant fluid running through the engine. It helps the ECU to know the engine temperature and moderate the operating temperature. When the sensor reads that the engine is cold, the ECU makes the air-fuel mixture reach enough so it can warm up quickly, as soon as the temperature reaches the normal operating temperature, the sensor sends the temperature reading to the ECU so it can reduce the amount of fuel injected. But a faulty sensor will continue to read a temperature that is lower than normal or just reading cold coolant temperature, this will make the ECU to continue injecting rich fuel mixture. This will make the car to consume more fuel as the engine is work on richer fuel mixture.
 The work of the engine thermostat is to help maintain the specific operating temperature by opening and closing the coolant fluid flow when the temp is above and bellow respectively. It helps the engine to warm up quickly at cold start by closing to block the flow of the coolant fluid into the radiator so that the coolant fluid in the engine can get hot quickly. But a faulty thermostat may not close completely after cold start, making the warm up to take longer time and the temperature may not reach the normal operating temperature, this will cause the ECU to continue feeding the engine with rich fuel mixture. More fuel is being consumed by this.

» Faults in the exhaust line.
 When there's a leakage in the exhaust line or a blockage due to bends along the exhaust line or a clogged catalytic converter, it will affect your engine performance which will result to poor fuel economy. A leaking exhaust line is usually indicated from noisy sounds along the exhaust line of smell of exhaust fumes.

» Faulty oxygen(O2) sensor.
 The work of the oxygen sensors which is found along the exhaust line, usually close to the exhaust manifold is to monitor the air-fuel mixture and tell the ECU when to add or reduce the amount of fuel injected into the engine. When the O2 sensor is faulty due to long time use, or inaccurate air-fuel mixture readings. It begins to send wrong signal to the ECU, telling it to add more fuel even when it's not needed, making the air-fuel mixture become richer than normal. This fault will increase the amount of fuel consumed.
 The way to solve this fault is to run a diagnosis to know which of the sensors is faulty and replace it with a good one.

» Faulty brake system.
 A faulty brake system like seized brake cylinders, stiffened callipers  or partially released parking brake will cause drag which  affects your engine to consume more fuel to provide more energy to overcome the brake drag. When you begin to notice that your car does not move freely on an inclined/sloppy road,and you have to accelerate for it to move, it's a sign that one/two of the wheel's brake is dragging.

» Worn-out clutch or defective torque converter.
 When the manual transmission clutch is worn-out it becomes slippery and begin to slide on the clutch disc. Likewise a slipping converter will cause more power loss by the engine and more fuel is consumed. Once you begin to notice a slipping clutch or converter, it is evident from delays in gear change.

» Vehicle overweight.
 Another major problem causing excessive fuel consumption is vehicle overweight. The engine power is primarily to move the weight of the vehicle, but when the weight of the vehicle is too much due to overloading the vehicle. The engine will consume more fuel to provide more power to carry the weight. The solution here is to minimise the loads you carry in your car as much as possible. For every 45Kg of extra weight, the fuel economy reduces by 4%.

» poor tyre conditions.
 Another major fuel consuming agents is an under-inflated tyre. An under-inflated tyre will increase your fuel consumption. Also a worn-out tyre will increase the fuel consumption.

» protruding body parts.
Protruding parts around the vehicle's body will cause more aerodynamic drag, more power will be needed to overcome this drag, especially at high speeds. Examples loose down cover, open sun roof, box on the roof rack.

» poor driving habits.
 Poor driving habits will increase your fuel consumption by several %. Being an aggressive driver and using the gear wrongly, leaving the engine idle unnecessary for too long.

» Air conditioning.
 All time use of the AC will reduce fuel economy.

» loose or missing fuel tank cover.
 A loose or missing fuel tank cover will result to waste of fuel due to vaporisation.

Also read: Fuel saving tips