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Why Am I Experiencing Poor Fuel Economy?

Reduced Fuel efficiency may point to any number of vehicle problems or human behaviors. With gas prices at historic levels, consumers cannot afford to waste fuel. If you’ve noticed a drop in fuel economy recently, consider the following possible causes.

Engine Backfire or Misfire

If an Engine is experiencing misfires, it can lead to a drop in fuel efficiency. Misfires can occur due to several ignition problems such as worn or fouled spark plugs, weak ignition coils, bad plug wires or arcing between the plug wires or coil and ground. Moreover, misfires can also be triggered by dirty or faulty fuel injectors, low fuel pressure, or vacuum leaks in the intake manifold. Additionally, the loss of compression in one or more cylinders can also cause misfires in the engine.

Worn Spark Plugs

Worn or fouled spark plugs can trigger engine misfires, resulting in a loss of power and decreased fuel efficiency. Platinum and iridium plugs are designed to last longer than traditional copper plugs, but short trips and stop-and-go driving can still cause them to foul prematurely. In addition, an engine that is using oil can also contribute to spark plug fouling, as the oil can coat the plugs and prevent them from firing properly.

Reduced Compression

Over time, the piston rings and valves in an engine can wear down, resulting in reduced compression and engine efficiency. This may lead to a gradual loss of fuel economy as the vehicle ages, especially if it has over 100,000 miles on it. Other factors, such as carbon buildup in the engine or worn spark plugs, can also contribute to a decrease in fuel efficiency in high-mileage vehicles.

Tire Pressure

Maintaining proper tire pressure is one of the easiest and most effective ways to improve your vehicle’s fuel economy. By ensuring that your tires are inflated to the recommended pressure, you can reduce rolling resistance, making it easier for your engine to move your vehicle, resulting in significant fuel savings. It’s also important to note that tire pressure should be checked when the tires are cold, as tire pressure increases as the tires heat up during driving. Additionally, it’s a good practice to check your tire pressure regularly, as even small variations in pressure can have an impact on fuel efficiency.

Clutch Slippage or Transmission

When the clutch on a manual transmission or the bands/torque converter on an automatic transmission are slipping, it can result in a loss of power from the engine to the wheels. This means that the engine must work harder to maintain the same speed, which can lead to reduced fuel economy. In addition to the decrease in fuel efficiency, a slipping clutch or transmission may also be a sign of more serious mechanical issues that will require expensive repairs to fix.

Vacuum Leak

A vacuum leak in the intake manifold gasket, manifold, or vacuum hose connections can cause the engine to run lean, meaning that there is too much air and not enough fuel in the air/fuel mixture. This may result in misfires and decreased fuel economy, as the engine has to work harder to maintain the same level of power output. Similarly, a malfunctioning Exhaust Gas Recirculation (EGR) valve can also contribute to poor fuel economy and misfires. The EGR valve is designed to allow a small amount of exhaust gas to enter the intake manifold, which can help reduce emissions and improve fuel efficiency. However, if the valve does not close properly, it can allow too much exhaust gas to enter the intake manifold, causing the engine to run lean and leading to misfires and falling fuel efficiency.

Contaminated Air Filter

A contaminated air filter can restrict the airflow to the engine, which will reduce its efficiency and cause it to work harder to maintain the same speed. This, in turn, can lead to a decrease in fuel economy. It’s important to inspect your air filter regularly and replace it when it becomes dirty.

Timeworn Oxygen Sensors

Your engine’s oxygen sensors are responsible for monitoring the air/fuel mixture and providing input to the powertrain control module, which adjusts the fuel delivery to accommodate for changing operating conditions. As these sensors age, their sensitivity to changes in the air/fuel mixture decreases, causing them to produce a signal that favors a lean mixture. This prompts the engine computer to add more fuel, even when it’s unnecessary. As a result, the fuel mixture becomes richer than normal, leading to increased fuel consumption.

Faulty Coolant Sensor

The engine coolant temperature is closely monitored by the coolant temperature sensor (CTS), which relays this information to the engine control module (ECM) or powertrain control module (PCM) for optimization of engine performance and fuel efficiency through adjustments to fuel injection timing and amount consumed. When the CTS malfunctions and reads a lower-than-normal temperature or always shows the engine as cold, the ECM/PCM may maintain a rich fuel mixture, causing poor fuel economy. To diagnose the CTS, a scan tool should be used to compare its readings with the inlet air temperature sensor readings when the engine is cold, and they should display identical temperature readings. Once the engine is started, observe the CTS for a gradual increase in reading, which should eventually reach the normal operating temperature range (typically 185 to 195 degrees Fahrenheit) if functioning correctly.

Malfunctioning Engine Thermostat

The thermostat is a crucial component within the engine’s cooling system, responsible for regulating the flow of coolant to maintain the engine’s optimal operating temperature. Your observation about the importance of the thermostat remaining closed during the engine’s cold start-up phase is correct, as it allows for efficient and rapid warming up. However, if the thermostat fails to close properly, it may allow coolant circulation through the engine before it reaches the necessary temperature, resulting in decreased fuel efficiency. Moreover, when the engine fails to attain the optimal operating temperature, the powertrain control module (PCM) may not enter closed-loop operation, leading to a rich fuel mixture and poor fuel economy.

Exhaust System Problems

An obstructed or clogged exhaust system can cause backpressure that restricts the flow of exhaust gases and reduces engine efficiency, which can lead to decreased fuel economy. Signs of a clogged exhaust system can include decreased acceleration, difficulty starting the engine, a decrease in power, and increased exhaust noise. If you suspect an issue with your exhaust system, it’s important to have it inspected by a qualified mechanic to ensure proper function and avoid further damage to your vehicle.

Excess Cargo Weight

Carrying unnecessary weight in your vehicle can reduce fuel economy. The extra weight increases the load on the engine, which requires more fuel to maintain the same speed and acceleration. It is recommended to remove any unnecessary items from your car that may be adding weight to improve fuel efficiency.

Negative Driving Behaviors

Poor driving habits will have a significant impact on fuel economy. Some examples of poor driving habits that can decrease fuel economy include:

Aggressive driving:

Frequent acceleration and hard braking wastes fuel, as it requires the engine to work harder and burn more fuel.


Driving at high speeds increases air resistance, which requires more fuel to maintain the same speed. According to the US Department of Energy, fuel economy decreases by about 1-percent for every mile per hour over 50 mph.


Leaving the engine running while parked or waiting in traffic wastes fuel and reduces fuel economy. Turn off the engine if you plan to idle for more than 30 seconds.

Cold starts:

A cold engine requires more fuel to operate efficiently. Try to combine short trips into one. Consider using a block heater in the winter to warm up the engine before starting.


Overloading the vehicle with passengers or cargo can increase weight, which requires more fuel to move.

By avoiding these poor driving habits, you can improve your fuel economy and save money on gas.

Improving Fuel Economy

After addressing the potential causes of poor fuel economy in your car, you should see a noticeable improvement in your fuel efficiency. By maintaining a consistent speed, avoiding quick accelerations and hard braking, keeping up with regular maintenance, and reducing unnecessary weight in the vehicle, you should maximize your fuel economy and save money on gas.

See Also — Making Sense of Dashboard Warning Lights

Image Attribution

Image by Image by Sophie Janotta via Pixabay

Photo by Unknown Photographer via Pixabay.

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Why Am I Experiencing Poor Fuel Economy?


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