Why Is My Car Using So Much Gas?
Lately, you've noticed that your car is consuming more fuel. Maybe it used to take 2.5 gallons to drive 60 miles, and now it takes four gallons; almost twice as much.
Decreasing fuel efficiency is a cause for concern, whether you are dealing with a small or large increase. Small amounts of money thrown away at the gas pump accumulate over time.
And the list of culprits behind a decrease in mileage can be rather large:
- A bad ECT sensor
- A stuck thermostat
- A fouled spark plug
- Bad ignition timing
- A clogged catalytic converter
- A leaking fuel injector
And the list goes on.
Most of the time, though, you are going to be dealing with systems or components in need of maintenance, parts that have worn out, or, more rarely, parts that have broken.
The next sections deal with those components that typically cause an increase in fuel consumption when they fail, starting with the components that most commonly cause the problem. To make it easier to identify the potential source of trouble, we describe some other symptoms that show up when that particular part or system stops working properly. You'll also find tests you can try for some of them.
Most of the time, bad sensors, especially those in emissions control systems, will trigger the check engine light (CEL). But even if you haven't seen the CEL illuminate, scan the computer memory anyway. You may find information about pending codes that can guide you during your diagnostic.
When you find a potential bad sensor, try to test that part or component to make sure the sensor itself is malfunctioning; the problem may be with its circuit. A little investigation will prevent swapping parts unnecessarily and save you frustration, time, and money.
Also, keep in mind that if more than one system in your vehicle hasn't been properly maintained lately (especially emissions systems), there may well be more than one component in need of attention or replacement.
So let's get started.
This article will cover the following topics:
- How to Calculate Your MPG
- Engine Coolant Temperature (ECT) Sensor
- Oxygen Sensor (O2S)
- Mass Air Flow (MAF) Sensor
- Idle Air Control (IAC) Solenoid
- Ignition Spark
- Air Filter
- Engine Misfires
- Fuel Injectors
- Fuel Pressure Regulator (FPR)
- Vacuum Leaks
- Ignition Timing
- PCV Valve
- Fuel Pump
- Underinflated or Oversized Tires
- Purge Valve
- Engine Oil Viscosity
- Dragging Brakes
- Catalytic Converter
- Transmission Problems
- Fuel System Leaks
- Carburetor Choke
1. How to Calculate Your MPG
If you are not sure whether your vehicle mileage has dropped, it's a good idea to get the actual miles per gallon (mpg) your engine is using.
Instead of relying on your dash read-out, here's a simple method to calculate your mpg:
- Fill up the gas tank until the gas pump shuts off; this will give you reliable information about the level of fuel in your tank. Then, jot down the mileage.
- The next three times you go to get gas, write down the number of gallons you purchase.
- On the third gas station visit, fill up your tank, again allowing the gas pump to shut off by itself. Write down the mileage.
- Subtract this second mileage from the first mileage you recorded.
- Add up all the gallons you purchased in step 2.
- Divide the number of miles driven (the result from step 4) by the number of gallons purchased (result from step 5). The result is your miles per gallon.
Compare the result in step 6 to the mpg rating (factory standard) from your car manufacturer. This information may be found in your car owner's manual, from your dealer, or by searching online.
Just keep in mind that comparing your current mpg with that provided by the manufacturer only gives you a general idea of engine operating condition. A large difference may indicate the engine needs some work, whereas a small difference may just indicate the mechanical wear you expect in an older model. A diagnostic from a trusted shop would be more accurate in this case.
2. Engine Coolant Temperature (ECT) Sensor
The engine coolant temperature (ECT) sensor is one of the first items to check if your mileage has dropped significantly. The ECT sensor monitors engine temperature and sends the information to the car computer as an electrical signal.
The car computer uses this information to adjust the air-fuel mixture. If the engine is cold, the computer will richen the mixture until the engine warms up to operating temperature. Once the engine gradually reaches its ideal temperature, less fuel is sent to the engine.
But the ECT can fail and provide the wrong information to the computer. For example, if the internal circuit breaks, the sensor will report to the computer that the engine is cold no matter how warm it is, So the computer will maintain a rich fuel mixture (open-loop mode) waiting for the engine to reach operating temperature. The result is an increase in fuel consumption. Eventually, performance issues begin to appear.
A simple method to test the ECT sensor is to check its resistance values when the engine is cold and when hot:
- Pop the hood open and locate the ECT sensor. It is usually sitting next to the thermostat on the cylinder head. The thermostat connects to the upper radiator hose.
- Unplug the ECT electrical connector.
- Measure the electrical resistance across the sensor terminals using a digital multimeter (DMM). You may get around 40,000 ohms, depending on your particular model.
- Then start and idle the engine for about 10 minutes to allow the engine temperature to increase.
- Stop the engine and repeat step 3. This time, you may get around 2,000 ohms.
- Compare your readings in steps 3 and 5.
There should be a difference in the sensor resistance, reflecting the difference in coolant temperature. If the difference is not significant, probably your thermostat isn't working or is stuck open. If both readings remain the same, most likely the sensor isn't working.
A bad ECT sensor may disrupt other things besides the rich/lean fuel mixture:
- ignition timing
- temperature gauge readings
- cooling fan
- transmission operation
3. Oxygen Sensor (O2 Sensor)
An oxygen sensor (O2 sensor) measures the amount of oxygen in exhaust gases. The sensor sends this information as an electrical signal to the computer.
The higher the oxygen content, the lower the voltage signal (as low as 0.1 volts), and the lower the oxygen content, the higher the voltage signal (up to 0.9 volts).
The car computer uses this information to adjust air-fuel mixture and spark timing.
Depending on your vehicle model, there may be one or more O2 sensors in the exhaust system. You may found one in the exhaust manifold, another located right before the catalytic converter, and another one located after the catalytic converter (catalytic monitor).
A common oxygen sensor problem is carbon buildup around the tip that prevents it from reacting fast enough, or prevents it from working at all. If the O2 sensor can't properly read oxygen content, it will send faulty information to the computer.
Usually, a bad O2 sensor will trigger the check engine light. Whether you find a diagnostic trouble code indicating a possible bad oxygen sensor, always test the oxygen sensor using a digital multimeter to make sure the sensor itself is bad.
Consult your vehicle repair manual for the correct procedure to test the oxygen sensor in your particular vehicle model. If you don't have one yet, you can get an inexpensive aftermarket copy from Amazon. provide troubleshooting steps for many components and systems, maintenance procedures, and many photos to guide you. Haynes manuals
Symptoms of a lazy or bad oxygen sensor:
- lean fuel-mixture condition
- rich fuel-mixture condition
- bad engine performance
- increase in emissions
4. Mass Air Flow (MAF) Sensor
The mass airflow (MAF) sensor measures the amount of air flowing through the intake manifold and into the engine. It produces a voltage signal that is sent to the engine control module (car computer), which regulates the air-fuel mixture accordingly. You can find the sensor in the air intake assembly.
If the air flow meter is sending no signal to the computer, or an incorrect signal, the injector pulse width can be four times longer than usual, increasing fuel consumption.
You can test the MAF output signal using a digital multimeter (DMM). Consult your vehicle repair manual for the exact method for your vehicle model.
It's not uncommon for a MAF sensor to fail after miles of operation or because the sensing element becomes dirty. Make sure there are no air leaks between the MAF sensor and the throttle body. The air cleaner assembly should be properly mounted and clamps should be tight. Any air leaks between the sensor and the intake manifold will cause it to send wrong data to the computer.
Other possible symptoms of a bad MAF sensor:
- poor idling
- stalling at idle
- lack of engine power
- driveability problems
5. Idle Air Control (IAC) Solenoid
The idle air control (IAC) valve allows more air into the intake manifold. It is used by the car computer to bypass the throttle valve to increase engine speed during a cold start.
A common issue with the IAC valve is carbon buildup that can interfere with valve operation. Buildup can make the valve stick open, causing an increase of fuel feed to the engine as well.
Commonly, the IAC valve is readily accessible, and you can remove it from the throttle body by unscrewing a couple of mounting bolts and unplugging the electrical connector. Once it is removed from the vehicle, you can use carburetor cleaner to remove carbon deposits.
The internal electrical circuit of the valve can also fail.
Troubleshoot the valve using a digital multimeter (DMM):
- Set your DMM to the ohms scale to measure resistance.
- With the engine off, unplug the IAC valve electrical connector.
- Measure the resistance across the valve terminals. You may get around 14 ohms of resistance, depending on your particular vehicle model.
- Then test the resistance between each terminal and the valve's body. You should get around 10,000 ohms of resistance, depending on your particular model.
Other symptoms of a bad IAC solenoid:
- Engine speed too low at cold start
- Engine speed high after warm up
6. Ignition Spark
Ignition system problems can come from a bad ignition module, ignition coil, or spark plug wires, or from a bad spark plug itself. These system components wear out over time, interfering with the spark. Ignition coils or modules circuits can malfunction, wires may short out or break, and spark plugs electrodes wear out, widening the gap in between.
But it's also common for the plugs to foul with carbon deposits or oil, if leaks are present, failing to produce a strong spark or no spark at all.
This type of fault leads to misfires.
- You can remove the spark plugs and analyze them with the help of your vehicle repair manual, if necessary.
- Often, the spark plug electrodes require a simple adjustment Check the gap specification for your particular model and adjust the gap using a wire gauge.
- Consult your vehicle repair manual to test spark plug wires, the ignition coil, and modules, if necessary.
Other symptoms of a bad ignition system:
- rough idle
- poor engine performance
- lose of engine power
7. Air Filter
The air filter cleans incoming air flowing into the intake manifold. Eventually, trapped particles clog the filter element. It's easy to forget to replace the air filter. Still, many drivers think it unnecessary to replace the filter because it looks clean. But this may be a false impression.
The fact is that air filters do get clogged. Over time, the engine finds it hard to breathe, which gradually increases fuel consumption, until you begin to notice. Many drivers fail to connect an old, clogged air filter with higher fuel consumption.
A simple air filter test is to look at the filter against the sun or a strong light. If light doesn't come through the filter, or you can barely notice it, replace it.
Also, look up the service schedule for your air filter in your car owner's manual or vehicle repair manual.
- low engine power
- erratic idle
- engine surges
The thermostat is part of the engine cooling system. It is an automatic valve that controls coolant flow at a predetermined engine temperature. It allows hot coolant to flow out of the engine and into the radiator to maintain the engine at operating temperature.
A pellet inside the thermostat reacts to hot temperature, depending on the thermostat rating, expanding and opening the thermostat's valve to allow the water pump to push coolant to the radiator.
But thermostats can fail as well. And when they do, they may get stuck open or closed.
- If the thermostat fails to open, the engine will overheat and the warning light or temperature gauge will try to alert you of the situation; otherwise, you'll see coolant steam blowing out through the overflow.
- If the thermostat fails to close, combustion efficiency will decline, the engine will perform poorly, and fuel consumption will increase.
Other overcooling symptoms include:
- insufficient heat from the heater
- lower temperature reading on the temperature gauge
- engine running poorly, especially in cold weather
- a drop in engine power
A quick way to check for an open thermostat:
- Remove the radiator cap.
- Start the engine and let it die.
- While the engine idles, watch the coolant through the radiator filler neck.
- If you see coolant flowing through the radiator, the thermostat is stuck open.
However, not all vehicle models use a radiator with a cap. In this case:
- Start and let the engine idle.
- Feel for the flowing of coolant by placing your hand on the upper radiator hose.
- Wait for a few minutes with the engine at idle and try to feel the difference in temperature between the engine surface and the upper radiator hose.
- If you notice the engine surface much hotter than the upper radiator hose after 5 to 10 minutes of engine idling, the thermostat is working fine. If there's not much difference in temperature, probably the thermostat is stuck open.
- To confirm your findings, remove the thermostat for testing.
- If the thermostat is stuck open when you remove it from the engine, replace it.
- Grab the thermostat with a pair of pliers and submerge the thermostat in boiling water.
- The thermostats should open.
- Remove the thermostat from the boiling water. The thermostat should begin to close.
- If the thermostat fails to operate correctly, replace it.
Other symptoms of a stuck-open thermostat:
- Gauge temperature remains below its normal reading
- insufficient heat from the heater
- engine runs poorly, especially in cold weather
- engine power drops
- driveability problems
9. Engine Misfires
A misfire happens when one or more cylinders fail to properly combust the air-fuel mixture or the mixture doesn't burn at all, causing raw fuel to flow into the exhaust system.
Usually, this will trigger the check engine light (CEL), but not always, depending on the frequency of the misfire.
If you suspect a misfire, there are several systems or components you need to check:
- ignition system
- fuel injectors
- vacuum leaks
- failed EGR valve
If necessary, consult this other article on diagnosing engine misfires.
10. Fuel Injectors
Bad electronic fuel injectors can also cause poor fuel management. The most common problems are dirty injectors that get stuck open and leaking injectors that richen the air fuel mixture.
On throttle body injection systems, you can visually check the injector(s) by removing the air cleaner from the throttle body:
- Start and idle the engine.
- Watch the fuel pattern as the injector sprays.
- You should see a fine spray as an inverted V pattern.
- Thick droplets of fuel or an irregular pattern may indicate problems with the injector(s).
On some multiport fuel injection systems it is possible to detach individual injectors to make a visual check for leakage and operation. On other systems it is almost impossible to remove the injectors for this purpose. Instead, it is necessary to do an injector balance test. The test can tell you if one or more injectors have an abnormal pressure drop (more fuel being injected) during operation.
If dirt or buildup is the cause of a bad fuel injector operation, the system may be cleaned using fuel injector additives. Injectors with internal mechanical or electronic problems need replacement.
Keep in mind that a bad engine control module (car computer) or sensors can also cause a fuel injector to deliver too much fuel. If the injectors seem in good condition, check the module or sensors that may be causing excessive pulse width.
Other symptoms of bad fuel injectors:
- rough idle
- engine hard to start
- driveability problems
11. Fuel Pressure Regulator (FPR)
A bad fuel pressure regulator can also empty your fuel tank sooner than usual. If the regulator pushes too much fuel through the system, the mixture will become richer.
You can test the FPR using a fuel pressure gauge. Multiport fuel injection systems usually provide a Schrader valve (a test fitting) to connect a pressure gauge. On other systems, consult your vehicle repair manual for the suggested method to connect the gauge.
- Connect the fuel pressure gauge
- Start the engine and let it idle
- Read fuel pressure on the gauge and compare it to the specification listed in your repair manual
- If the gauge indicates pressure higher than normal, more likely the pressure regulator is bad
- engine hard to start
- rough idle
- driveability problems
... bad sensors, especially those in emissions control systems, may trigger the check engine light (CEL).
12. Vacuum Leaks
Vacuum leaks usually happen around deteriorated vacuum hoses and loose fittings. Over time, hoses may crack, collapse, swell and tear open small holes and disconnect from fittings. Leaks can also happen around the intake manifold gasket or throttle body gasket.
A leak can affect the operation of the computer and many sensors or actuators that need vacuum to operate like the EGR valve, MAP sensor or purge solenoid.
This type of leak can be hard to detect. While the engine runs, it's hard to hear the characteristic hissing sound of a vacuum leak. That's why car technicians use a mechanic's stethoscope, but a length of hose can also work for this purpose.
- With the engine at idle, put one end of the hose at one ear and use the other end of the hose to trace vacuum hoses.
- To check gaskets, you can spray carburetor cleaner or even soapy water around the gasket. The cleaner will cause a change in engine speed when found; soapy water will cause bubbles to show up on the leaky spot.
When checking sensors that might be pointed to by trouble codes retrieved from the computer memory, don't forget to check vacuum hoses that might be connected to the sensor or actuator.
If you are checking the EGR valve for example, check the vacuum hose and under the valve for leaks, and remove the valve to check for carbon buildup that might be interfering with valve operation.
- poor acceleration
- hard starting
- rough idle
- high idle
13. Ignition Timing
Increase in fuel consumption may also come from ignition timing problems. This is a symptom of abnormally retarded timing. Timing is retarded at low engine speeds so the spark plugs fire later in the combustion process to prevent fuel from burning ahead of time, causing spark knock or ping.
However, if timing doesn't advance as engine speed increases, fuel will not burn properly. Abnormal timing retardation will cause engine power to drop. You'll notice this during acceleration.
On modern vehicle models the computer controls the timing advance using input from several sensors like:
- throttle position sensor (TPS)
- manifold absolute pressure (MAP) sensor
- engine coolant temperature (ECT) sensor
- camshaft and crankshaft position (CMP, CKP) sensors
To correct timing on a modern vehicle engine you'll need to locate the sensor or module causing the fault.
On older vehicle models, timing is controlled by a distributor centrifugal force or vacuum advance and can be adjusted using a timing light and rotating the distributor.
For more information on ignition timing checks and adjustments, consult your vehicle repair manual.
14. PCV Valve
A positive crankcase ventilation system helps remove blowby gases out of the crankcase and routes them to the engine for reburn.
Blowby gases pollute the environment if released into the atmosphere; if left inside the engine, they'll corrode engine parts, dilute oil and accumulate sludge.
Basically, a PCV system uses one or more vacuum hoses, a valve, and grommets. Some modern systems may be electrically controlled or use a single vacuum hose.
Most problems with the PCV system are caused by vacuum leaks or restrictions. A restricted PCV system can enrich the fuel mixture, make the engine surge, and force black smoke out the tailpipe.
The PCV system should be checked regularly.
To check for a restricted PCV system:
- Disconnect the PCV valve from the engine or vacuum hose and shake it.
- On most models the valve should rattle. Otherwise the valve is blocked or inoperative.
- Now, start and idle the engine.
- With the hose disconnected from the valve, block the end of the valve with your finger.
- You should feel suction from the vacuum and engine speed should drop about 40 to 60 rpm. If not, the system is restricted.
To clean a restricted PCV system:
- Trace the hoses from the valve cover to the other end.
- Mark each hose so that you know how to reinstall them.
- Disconnect each hose and check its condition and make sure it isn't clogged.
- Replace damaged hoses and clean clogged hoses with a long wire and solvent.
Other symptoms of a restricted PCV system:
- a rich fuel mixture
- engine surges
- black smoke out the tailpipe
15. Fuel Pump
Another component in the fuel system that may cause high fuel pressure is a bad fuel pump.
An electric fuel pump may have a bad pressure relief valve. If the valve can't open, fuel pressure will increase, enriching the air-fuel mixture and, possibly, flooding the engine as well.
On modern multiport fuel injection systems, it is easy to test fuel system pressure. Most come equipped with a test fitting (Schrader valve) that you can use to connect a fuel pressure gauge. You may need to relieve fuel system pressure and disconnect the battery before connecting the gauge. Consult the repair manual for your particular vehicle model.
- After connecting the pressure gauge, start the engine and let it idle.
- Take note of the system pressure.
- Then, depending on your model, disconnect the vacuum line from the fuel pressure regulator
- take note of the pressure indicated on the gauge.
- Compare your readings to the specifications.
For the best method for the fuel system pressure test for your vehicle, consult your repair manual.
16. Underinflated or Oversize Tires
Underinflated tires are one of the most common sources of increase in fuel consumption. One of the reasons is that a radial tire looks properly inflated, even if it's not.
Since 2007, vehicles in the Unites States come equipped with tire inflation monitoring systems. However, most systems won't alert you that a tire is underinflated until pressure goes down by 25%. Even at 24% you can expect fuel economy to go down by 4%. So your mileage can drop significantly if you take into account the four tires.
You can drive for weeks with underinflated tires without noticing unless you check tire pressure yourself. And driving with underinflated tires for a few months will cause your outer threads to wear more than the middle thread, so your maintenance expenses go up.
Buy an inexpensive tire pressure gauge and carry it with you in the glove compartment so you can check tire pressure at least once a week.
Most tires in passenger vehicles have a maximum inflation pressure of 32 psi (220 kPa) or 44 psi (303.16 kPa). You can find this pressure indication on the tire sidewall markings. However, it's recommended to inflate tires one to three pounds below the maximum recommended pressure to compensate for tire heating and air expansion.
Also, using oversize tires can accelerate fuel consumption.
Look for the manufacturer's recommended tire inflation pressure and tire size on the label located on the driver's side door pillar, frame, or the door itself.
17. Purge Valve
Problems with the evaporative emissions control (EVAP) system can also cause an increase in fuel consumption, specially when the canister purge valve fails.
The EVAP system collects toxic fuel vapors from the fuel tank inside a charcoal canister, where they are drawn into the engine and mixed with fuel for burning.
The purge valve controls the flow of these fuel vapors from the canister to the intake manifold. The valve itself can be vacuum or electrically controlled, depending on your particular vehicle model. Usually, the valve is located on top of the canister or in-line mounted.
Depending on your particular vehicle model, you probably can test the purge valve using a DMM and a hand-held vacuum pump.
Check your vehicle repair manual for the best method to test this valve in your vehicle.
18. Engine Oil Viscosity
Incorrect oil viscosity can not only cause an increase in fuel consumption but lead to engine mechanical damage as well.
Engine oils come under different viscosities (or weights) that tells you how thick (or fluid) a particular oil is. Engine oils usually range from 5 to 50. The higher the viscosity number, the thicker (less fluid) the oil.
Car manufacturers always recommend a particular oil weight for their engines. Using a thicker oil can burden the engine with an extra load during operation. Moving components will require more energy to move, with a consequent increase in fuel consumption.
Check your car owner's manual or vehicle repair manual, to make sure you are using the correct oil viscosity for your engine.
However, a high mileage engine may require a thicker oil to properly fill the wider clearance between moving components caused by wear. If necessary, consult a trusted shop for the best oil for your vehicle.
Also, don't forge to check your oil filler cap. A worn out, damaged cap can also cause fuel consumption to rise. Watch the next video for a quick tip on checking the filler cap.
19. Dragging Brakes
Dragging brakes can also cause your mileage to drop. This is usually caused by stuck wheel cylinder pistons, sticking or overadjusted push rod in the master cylinder, problems with the master cylinder, or an overadjusted or malfunctioning parking brake.
You can diagnose a sticking brake problem by feeling the brake calipers soon after driving your car. The sticking caliper will be unusually hot.
20. Catalytic Converter
Exhaust backpressure can also cause your fuel consumption to increase. This usually happens when the catalytic converter or muffler become restricted.
Other symptoms you may notice include loss of engine power and stalling at idle.
If you suspect a blockage at the exhaust system, check for a clogged converter.
21. Transmission Problems
Particular transmission problems, whether automatic or manual, can also cause an increase in fuel consumption. A clutch in a manual transmission, for example, can wear out (the friction material) and cause slippage. You'll notice the engine racing while accelerating slowly after a stop, when shifting, or going uphill.
In an automatic transmission, worn bands or torque converter can also cause similar problems. If the transmission slips in gear, there may be a problem with a stuck valve, internal leaks, low fluid level or a clogged filter. If necessary, take in your vehicle for a diagnostic.
22. Fuel System Leaks
Fuel leaks can happen practically anywhere in the fuel system, especially on line connections, in-line components like a fuel filter, and the fuel tank. Rust can puncture a steel fuel tank and road debris can damage a plastic fuel tank.
If you can smell raw fuel while inside or standing near the vehicle, inspect for potential leaks around the fuel tank: trace the fuel lines from the tank to the engine. Look closely inside the engine compartment. Look for wet spots around system components and wet spots on the floor from dripping.
23. Carburetor Choke
If your engine uses a carburetor, you might want to check the choke system.
In a carburetor, the choke supplies a rich air-fuel mixture during cold starts. When you start a cold engine, the choke plate (valve), located near the top of the air horn, remains closed. This creates a high intake manifold vacuum that pulls extra fuel to prevent a lean condition that might cause the engine to idle poorly or stall.
Once the engine warms up, the choke opens to lean out the mixture. But, if the choke sticks and can't operate properly, a rich condition can be created, increasing fuel consumption. The engine will run rough. Probably you'll see black smoke coming out the tailpipe. And engine power will fall and possibly the engine will stall as it warms.
Either the choke is binding, the mechanism has failed, or it may need adjustment. Inspect the system with the help of your vehicle repair manual.
For a Faster Diagnostic
When your car consumes more fuel than usual, you don't have to guess where the problem is or start throwing money away on parts hoping that it'll fix the problem. Start by scanning for DTCs and pay attention to any symptoms you may have noticed; and check those systems and parts in need of maintenance, using this guide as a reference. Soon, you'll find the "culprit" and your fuel consumption will drop back to its normal level.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
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© 2018 Dan Ferrell