If you notice a gradual or sudden drop in the mileage you were once getting from your vehicle. 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:
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 turns to wastage of 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 an 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 most likely from the engine thermostat.
Sluggish Oxygen Sensors
The oxygen sensors on your engine monitor’s the air/fuel mixture so the power-train 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.
To solve this problem you need to get 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 (>100,000) simply replace the oxygen 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.
Defective Engine Thermostat
The thermostat controls the operating temperature of the engine, and it helps the engine warm up quickly after a cold start. 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 best solution to this problem is to replace the thermostat.
If an engine is misfiring for any reason, it will waste a LOT of fuel and result in poor fuel economy. Misfiring can be as a result of 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 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.In the next post we will be concluding the discussion on WAYS TO RESOLVE BAD VEHICLE FUEL ECONOMY, till then don’t forget to get more acquainted with our facebook, twitter and google plus page.