Is your vehicle delivering poor fuel economy? Have you noticed a gradual or sudden drop in the mileage you were once getting? The following are common causes of poor fuel economy that may or may not turn on your Check Engine Light or cause a loss of fuel economy:
Sluggish Oxygen Sensors
The oxygen sensors on your engine monitor the air/fuel mixture so the powertrain control module can add or subtract fuel as needed to meet changing operating conditions. As Oxygen sensors age, they become less responsive to changes in the air/fuel mixture, and typically produce a lean-bias signal. This tells the engine computer to add more fuel, when in fact the engine really doesn’t need the extra fuel. The end result is a richer than normal fuel mixture that increases fuel consumption.
The fix here is to use a scan tool and/or digital storage oscilloscope to test the response of the oxygen sensors. Or, if your vehicle has a lot of miles on it (over 100,000) to simply replace the O2 sensors if you suspect they are getting sluggish.
You can also use a scan tool to look at Long Term Fuel Trim (LTFT). If the value is negative, it means the engine is running rich. This confirms the engine is wasting fuel, but it does not tell you why the engine is running rich. It could be sluggish oxygen sensors or some of the following causes.
Inaccurate or Defective Coolant Sensor
The coolant sensor monitors the operating temperature of the coolant that is circulating inside the engine. If the sensor is defective and reads lower than normal, or always reads cold, the engine computer will keep operating in “open” loop – which means the fuel mixture remains rich. A richer fuel mixture is required while a cold engine is warming up to prevent it from stalling. But if the mixture remains rich once the engine is warm, it wastes the extra fuel and causes poor fuel economy.
The quickest way to check a coolant sensor is to plug in a scan tool and compare the coolant sensor reading with the inlet air temperature sensor reading when the engine is cold. But should show the same temperature reading. Then start the engine, and look for the coolant sensor to show a gradually increasing reading. If the engine eventually reaches 185 to 195 degrees ( once they warm up), the coolant sensor is probably okay.
If the coolant sensor reading does not change, or never reaches normal operating temperature, the problem could be the sensor or it could be a defective thermostat that is not closing when the engine is cold.
The next step would be to check the sensor’s resistance reading with ah ohmmeter. If the reading does not match specifications for a given temperature, the sensor is bad and need to be replaced. If the sensor reads good, the problem is likely the engine thermostat.
Defective Engine Thermostat
The thermostat controls the operating temperature of the engine, and it helps the engine warm up quickly after a cold start. The thermostat is usually located in a housing where the upper radiator hose connects to the engine. When the engine is cold, the thermostat closes to block the flow of coolant. When a cold engine is started, the thermostat should remain closed until the coolant gets hot (around 185 to 195 degrees). If the thermostat does not close tightly or does not close at all, coolant will be circulating while the engine is trying to warm up. This will prevent the engine from warming up quickly, and it may never reach normal operating temperature. This can delay the powertrain control module from going into closed loop operation, causing a rich fuel mixture and poor fuel economy.
A quick check for this problem is to feel the upper radiator hose while the engine is warming up. If you feel coolant circulating through the hose following a cold start, the thermostat is probably stuck open. The fix is to replace the thermostat.
Engine Misfire
If an engine is misfiring for any reason, it will waste a LOT of fuel and result in poor fuel economy. Misfires can be caused by ignition problems such as worn or fouled spark plugs, bad plug wires, weak ignition coils or arcing between the plug wires or coil and ground. Misfires can also be caused by dirty or defective fuel injectors, vacuum leaks in the intake manifold, or low fuel pressure. Misfires can also be caused by loss of compression in one or more cylinders.
On 1996 and newer vehicles with OBD II, misfires should turn on the Check Engine Light and set a misfire code if the misfires are severe enough to cause an emissions problem. However, if the misfire rate is just below the threshold where a code must be set, you won’t get a code or Check Engine light. If you have access to a factory scan tool or a professional level scan tool that can read something called “Mode $06” data, you can look at the actual misfire rates for each of the cylinders. from this, you can see if one or more cylinders are misfiring (even if they have not yet set a code).
If you do have a Check Engine Light and a cylinder specific misfire code (such as P0301 which would indicate cylinder #1 is misfiring), inspect the spark plug, plug wire (if used) and coil for that cylinder. If the ignition components appear to be working normally (no fouling, no shorting or arcing), the problem is likely a dirty or dead fuel injector.
If you get a P0300 “random misfire” code, the most likely cause is a lean fuel mixture due to an intake manifold vacuum leak, leaky EGR valve, or low fuel pressure.
Intake Manifold or EGR Valve Leak
A vacuum leak at the intake manifold gasket, in the manifold itself or any of its vacuum hose connections can lean out the air/fuel mixture and cause the engine to misfire and deliver poor fuel economy. Likewise, an EGR valve that does not close at idle, when the engine is cold or when it is not under load can allow exhaust to leak back into the intake manifold. This can also have a leaning effect and cause fuel-wasting misfires and poor fuel economy.
You need to check for vacuum leaks, and/or remove and clean the bottom of the EGR valve. Vacuum leaks can be found by spraying throttle cleaner along the edges of the intake manifold while the engine is idling. If the idle suddenly changes, it means some of the cleaner is being pulled into the engine through a leak. The fix usually requires replacing the intake manifold gasket, or the manifold itself if it is cracked. A cheaper fix is to apply a high temperature epoxy sealer to the crack and hope it seals the leak.
NOTE: It does not take much of a leak to upset the air/fuel ratio. Even a very small leak can cause problems. Professional technicians often use a device called a “smoke machine” to find small leaks. The machine generates a mineral vapor smoke, which is fed into the manifold (engine off). If there are any leaks, you will see the smoke seeping through the crack.
If no vacuum leaks are found, remove the EGR valve and check the underside of the valve and the port in the intake manifold for carbon deposits that may be preventing the valve from closing. Also, check the EGR valve’s vacuum connections and solenoid to see if they are operating properly. There should be NO vacuum reaching the valve at idle or when the engine is cold.
Worn or Fouled Spark Plugs
Worn or fouled spark plugs will obviously cause fuel-wasting engine misfires. Platinum and Iridium plugs should last 100,000 miles, but short trip stop-and-go driving may cause the plugs to foul prematurely. An engine that is using oil can also foul out its spark plugs.
Remove and inspect the spark plugs. Clean the plugs if they are dirty, and regap to specifications, or better yet, just install a new set of spark plugs.
Dirty Fuel Injectors
Fuel varnish deposits can build up inside fuel injectors, preventing them from delivering their normal dose of fuel. This can cause a lean air/fuel mixture that results in lean misfires and wasted fuel.
Try adding a bottle of good quality (not the cheapest stuff) fuel injection cleaner to your fuel tank. It may take several tankfulls before any improvement is noticed. If that does not work, having the injectors professionally cleaned will often restore normal performance. If an injector is too badly clogged to be cleaned, or it is defective, you’re looking at replacing one or more fuel injectors (which aren’t cheap to replace!).
Low Compression
If you are driving a high mileage vehicle (over 100,000 miles), you may be getting poor fuel economy because your engine does not have the compression it once had. As the miles add up, so does the wear on the piston rings and valves. This can result in a gradual loss of compression that reduces engine efficiency and fuel economy.
If you suspect low compression, do a compression test on the engine. If low, there is no easy fix other than an overhaul. There’s no miracle cure in a can that will restore lost compression.
Wrong Oil Viscosity
Most late model passenger car engines today require a low viscosity 5W-20 or 5W-30 motor oil. Some even specify 0W-20. Such oils improve fuel economy, especially during cold weather when the oil tends to thicken. If you are using a heavier viscosity motor oil, it can reduce your fuel economy (maybe 5 to 10 percent depending on what you are using).
Dirty Air Filter
If your air filter is really dirty, it will interfere with normal engine breathing and hurt fuel economy. Remove and inspect the filter, and if it is dirty replace it with a new one.
Clogged Converter or Exhaust Restriction
Any obstructions in the exhaust system will create power-robbing backpressure that also hurts fuel economy. You can inspect the outside of the system for any obvious signs of damage such as a crushed or crimped pipe. But internal problems such as a clogged converter or collapsed muffler or double walled pipe can’t be seen from the outside.
You can check for an exhaust restriction by connecting a vacuum gauge to the intake manifold. At idle, the engine should show a high and steady vacuum reading (say 18 inches or higher). If the reading is less than this or it gradually drops, you have an exhaust restriction.
Slipping Clutch or Transmission
If the clutch on a manual transmission is slipping, or the bands or torque converter lockup on an automatic transmission are slipping, some of the engine’s power will be lost before it can reach the wheels. This can cause a noticeable drop in fuel economy — and be VERY expensive to fix because it will require replacing the clutch or transmission.
Low Tires
To achieve maximum fuel economy, your tires must be inflated to the recommended pressure for your vehicle and load. For most passenger car tires, that means 32 to 34 PSI. A low tire increases rolling resistance (and tire wear), and can result in a loss of 5 to 10 percent fuel economy. Check all four tires (when cold) with an accurate gauge, and inflate as needed to the recommended pressure.
Dragging Brakes
A parking brake that is not fully releasing, or a brake caliper that is sticking can cause the brakes to drag and your engine to waste fuel. A quick check for this kind of problem is to park your vehicle on a slight incline, put the transmission in neutral, then release the brake pedal. If your car does not start to roll immediately, the brakes may be dragging.
Too Much Junk in Your Boot
More weight equals less fuel economy. It takes power to move mass, so if you are hauling a lot of unnecessary weight in the trunk or cargo area of your vehicle, you are not going to achieve maximum fuel economy.
Poor Driving Habits
This is probably the most common problem and biggest fuel waster of all. Aggressive driving and jack rabbit starts flood the engine with extra fuel. Take it easy, as if you were driving with a raw egg under the gas pedal and you’ll get the most miles per gallon from the fuel in your tank.