10 Common mechanical reasons for poor gas mileage
You probably already know that driving habits, speed and other factors contribute to lower fuel economy, but what about mechanical problems? There are many issues with your car that can cause increased fuel consumption; to know the exact cause, your car must be properly diagnosed. Most of the time, it's not just one faulty part, but rather a combination of worn, old and failing components. Here are the most common mechanical reasons for poor fuel economy:
1. Under-inflated tires:
Under-inflated tires increase the rolling resistance causing your engine to work harder and burn more fuel as a result. According to Fueleconomy.gov, under-inflated tires can lower your gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. In addition, under-inflated tires wear more quickly. Check tire pressure regularly. You can find the proper tire pressure for your car on the tire placard, that is usually attached to the door edge, door post, glove box lid or fuel cap lid. Read how to check the tire pressure »
2. Old or wrong grade engine oil:
Engine oil lubricates all moving parts inside the engine. If oil has not been changed in a long time, it becomes thicker and increases friction. The oil filter gets dirty over time and restricts the oil flow making the engine to work harder. Change your engine oil and filter regularly. This will also help save money on repairs down the road. The proper oil grade matters too: according to Fueleconomy.gov, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1-2 percent.
3. Faulty oxygen sensor:
A front or 'upstream' oxygen sensor is the main component of the fuel injection system that determines how much fuel is injected into the engine. A faulty or "lazy" oxygen sensor is a very common reason for poor gas mileage. Your mechanic can check the oxygen sensor with a scan tool. Replacing the front oxygen may cost from $50 to $200 in labor and $70-$300 for the part. Newer cars have an air/fuel ratio sensor installed instead of an oxygen sensor. The air/fuel ratio sensor works differently, but has the same purpose. A bad air/fuel ratio sensor can also cause increased fuel consumption. Read more about oxygen sensor.
4. Leaking or dirty fuel injectors:
Fuel injectors spray fuel under pressure into the engine. Over time injectors wear out and the spray pattern deteriorates. In addition, fuel injectors may leak. A black smoke from the exhaust and sputtering for a few seconds after the engine is started are the symptoms of a leaking fuel injector. Fuel injectors can be tested in a repair shop. Bad fuel injectors can be cleaned or replaced. Replacing all fuel injectors could be expensive; cleaning is cheaper. Many repair shops offer fuel injection service where a special cleaning solution is run through the fuel injectors. In some cases, the fuel injection service can help improve your gas mileage. Usually it costs from $80 to $150.
5. Lack of tune-ups:
Spark plugs ignite the air/fuel mixture in the engine. Old spark plugs or bad ignition wires can cause incomplete combustion and consequently poor gas mileage. In newer cars spark plugs are designed to last longer, but over time, deposits on the spark plug electrodes increase the resistance causing problems. If your engine is running poorly, a tune up with new spark plugs might be a good start. If your car has a distributor, replacing the distributor cap and rotor is also a good idea. Spark plugs replacement can cost from $80 to $375; a complete tune-up can cost $250-$600.
6. Clogged catalytic converter:
A catalytic converter installed in the vehicle exhaust is a major component of the emission control system. A plugged or restricted catalytic converter can cause many driveability problems including poor gas mileage. One of the symptoms of a clogged catalytic converter is sluggish acceleration, even though at idle the engine may run fine. Sometimes a car may drive normally at low engine rpms but lack power, hesitate and stumble at higher RPMs. A clogged catalytic converter can be diagnosed in a repair shop. Usually the diagnostic includes measuring the exhaust backpressure. A catalytic converter may be covered by the vehicle emission warranty, which is typically 8 years, or 80,000 miles; check your warranty booklet for details. If the catalytic converter is still covered by the warranty, your dealer may replace it at no charge. If it's out of warranty, the repair can be expensive; the part alone can cost up to $900.
7. Stretched timing chain or belt:
A timing chain or belt drives the engine camshaft or camshafts. Over time a timing chain or belt can stretch causing retarded ignition and valve timing, which in turn results in poor gas mileage. Your mechanic can verify a stretched timing belt or chain by checking the timing. A timing belt must be replaced at recommended by the manufacturer intervals, usually from 60,000 to 105,000 miles (96,000-168,000 km). There are no maintenance requirements for a timing chain; it only needs to be replaced when there is a problem with it, including if it's stretched. Replacing a timing belt could cost from $200 to $700 depending on a car. Replacing a timing chain is more expensive: from $370 to $1,500.
8. Misaligned tires:
The rolling resistance increases when tires are not aligned properly. When the car is new, tires aligned "parallel" to the car, but as the suspension and steering components wear, the wheel angles change, altering the alignment. The wheel alignment service is not very expensive. Typically it costs $60-$150. The wheel alignment is recommended to be done at least every year or two. The wheel alignment also needs to be done when replacing tires or any of the suspension or steering components.
9. Dragging brakes:
Over time, due to rust and dirt, brakes can seize up. This cause brakes to "drag," when the brake pads or shoes are not releasing completely. When this happens, you might feel that the car does not roll freely. Sometimes, dragging brakes can cause a car to pull to one side when braking. To avoid this problem, brakes need to be serviced regularly. Brake service typically costs $120-$250 for all four wheels. Servicing brakes usually includes cleaning and lubricating moving parts with specially formulated grease. Brake service is recommended to be done once a year.
10. Stuck open thermostat:
Your engine runs more efficiently when fully warmed up. A thermostat is a part of the engine cooling system. It helps the engine warm up faster and maintain normal operating temperature. A faulty or stuck open thermostat can cause long warm up time and lower engine temperature during highway cruising. This results in increased fuel consumption. The most common symptom of a stuck open thermostat is when the engine temperature drops when cruising on the highway. A thermostat is not very expensive part, but in some cars it might be difficult to get to. Replacing the thermostat can cost from $90 to $350.
