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Fuel Economy is Down

Today’s engine management systems rely on many different sensor inputs to regulate fuel economy, performance and emissions.
Symptom:
  • Poor Fuel Economy
Probable Causes:
  • Fuel Rich Codes (P0172 and/or P0175)
  • Coolant Sensor (P0115 to P0119)
  • Misfire Codes (P030x)
  • Mass Air Flow (P0100 to P0105)
  • Oxygen Sensor Codes (P0130 to P0167) or (P0036 to P0064)
  • MAP Sensor (P0105 to P0109)

With the price of gasoline, you don’t want to ignore this kind of problem for long. When fuel economy is down, you may find any number of codes:

P0172 and/or P0175 are rich codes, and indicate a general rich air/fuel condition. The underlying cause may be anything that increases fuel delivery, such as excessive fuel pressure, a defective fuel pressure regulator, plugged return line, or leaky injectors. Decreased airflow from a dirty air filter or restricted air intake misleads the mass airflow sensor or engine management system into believing the engine is using more air or is under more load than actual.

Misfire codes P030X, where X indicates the cylinder number that is misfiring. Misfire codes are bad because they mean a whole cylinder full of air/fuel mixture is wasted every time a cylinder fails to fire. The cause may be a worn or dirty spark plug, a bad plug wire, a weak ignition coil in a distributorless ignition system (DIS) or coil-on-plug (COP) ignition system, or a dirty or dead fuel injector. Misfires can make an engine run rough and reduce horsepower every time it happens. Worse yet, the unburned fuel that passes right through the engine goes into the exhaust. When that fuel reaches the catalytic converter, it will ignite and may cause the converter to overheat and suffer damage. Misfires are one of the two leading causes of catalytic converter failures (the other is leaky exhaust valves).

Oxygen sensor codes (P0130 to P0167), or oxygen sensor heater code (P0036 to P0064) indicate a fault with that sensor. If the O2 sensor fails, it usually causes the engine to run rich leading to poor economy, pollution, and catalytic converter failure. Never replace an O2 sensor without testing it first. Figure 3 shows failed Bank 1 Oxygen Sensors.

Figure 3

Figure 3

P0115 to P0119 point to a problem with the coolant sensor. The coolant sensor tells the engine computer the temperature of the coolant. The engine computer needs this information to adjust the fuel mixture and ignition timing as the engine warms up. If the coolant sensor is faulty and tells the computer the engine is cold when it really is hot, the fuel mixture will be too rich. Fuel economy will drop like a rock and the engine will pollute.

P0100 to P0104 are Mass Airflow (MAF) sensor codes. The mass airflow sensor measures airflow into the engine. If it is not reading correctly, the air/fuel mixture won’t be right. This is an expensive sensor to replace, so in many instances cleaning the delicate sensor wire with an aerosol electronics cleaner can return it to normal operation.

P0105 to P0109 Manifold Absolute Pressure (MAP) sensor codes. The MAP sensor monitors engine load by reacting to changes in intake vacuum. If the sensor reads incorrectly, the computer may think the engine is under more load than it actually is and give it more fuel than it needs.

P0070 to P0074 Inlet Air Temperature sensor. Some fuel injected engines do not use a mass airflow sensor. Instead, these engines calculate air flow using inputs from this sensor, throttle position, and the MAP sensor. If the air temp sensor is reading colder than it should, the computer will give the engine too much fuel. The key to solving a fuel economy problem is figuring out which of these inputs is feeding the PCM bad information. If the computer receives bad sensor data, it will make the wrong adjustments and waste gas. Using AutoTap Express DIY you can graph and read important parameters and monitor fuel economy (as shown in Figure 4).

Figure 4

Figure 4