Service Repair | ||||
Aging sensors may not set a code, but can cause driveability problems, increase emissions and lower fuel economy. Look for Short Term Fuel Trim (STFT) corrections going ± 10 percent or more, while still crossing "0" that could indicate the sensor is taking too long to react to the change in lambda. Also look for sensors that cannot reach peak voltages or switch less frequently than described above. If one of the sensors is failed, what caused it to fail? Contamination from coolant leaking into the exhaust is not an uncommon cause of premature sensor failure. Age is also a factor, with some manufacturers of these sensors recommending replacement at intervals as low as 30,000 miles for Zirconium Dioxide (ZrO2) and Titanium Dioxide (TiO2) sensors, and 60,000 for Wide Band and Air/Fuel (AF) sensors. Be careful when replacing an aged post-cat sensor only. The new, fresh sensor — when combined with an older, slower pre-cat sensor — could result in a false catalytic converter efficiency code. This will certainly result in a comeback and a potentially unhappy customer. The Feedback is Correct...What's the Next Step?
MAP Sensors and the Speed/Density System If the actual airflow into the engine is not reported accurately, fuel trim codes are on the way. In a speed/density system that has only a Manifold Absolute Pressure (MAP) sensor or Vane Airflow (VAF) sensor, the actual mass of air is not directly measured. Instead, it's calculated using that sensor's input, the rpm sensor input and a programmed volumetric efficiency table.
MAF Sensors The Mass Airflow (MAF) sensor can actually measure the mass of air being drawn into the engine. However, the ECM can be fooled by a problem in the MAF. Contamination of this sensor, especially a "hot wire" type sensor, is not uncommon. Dirty air from the intake, as well as leaking combustion byproducts from a poorly operating Positive Crankcase Ventilation (PCV) system, can contaminate the sensor, skew the readings and result in fuel trim codes. There are several ways to tell whether the sensor is indeed reading correctly. First, check the MAF voltage at idle and compare it to specification. Look at fuel trims at idle and cruise. Generally, a contaminated MAF sensor will over-report air mass at idle (resulting in a negative LTFT correction) and under-report air mass at higher loads (resulting in a positive LTFT correction). |
