FUEL TRIM(MING) DIAGNOSTIC TIME

FUEL TRIM(MING) DIAGNOSTIC TIME

No matter what the driveability issue happens to be, beginning by checking the PCM's fuel trim decisions can get you pointed in the right direction, and may end up cutting your diagnostic time in half.
The year is 2006, and for those who may have overlooked it, this is the 10th anniversary of On-Board Diagnosties II (OBD II). I believe its cause for celebration. Here's an example of how it used to be: I was recently called to help a shop with a 1992 Subaru. In order to retrieve the diagnostic trouble codes (DTCs), I had to remove the driver's kick panel, visit a vehicle repair information source for the instructions on how to jumper the diagnostic connector, then count the flashes of the malfunction indicator light (MIL). The final step was looking up the DTC description. Total time from start to finish was approximately 15 minutes. If this had been an OBD II vehicle, I would have had the information in under 30 seconds. The standardization associated with OBD II, which gives us easy access to fuel trim data, has really simplified the diagnostic process.

What is fuel trim? Fuel trim is a window that allows you to see what the computer is doing to control fuel delivery and determine how the PCM's adaptive strategy is operating.
Why was fuel trim created? In order for vehicle manufacturers to comply with EPA emissions regulations, catalytic converters were added to reduce tailpipe emissions. Catalytic converters need a stoichiometric air/fuel ratio of approximately 14.7:1 to obtain the greatest emissions reductions. Vehicle engineers designed closed-loop engine control systems to maintain that ratio, adjusting injector pulse width based on information from die oxygen sensor and other inputs. Short-term fuel trim (STFT) and long-term fuel trim (LTFT) are expressed as a percentage, and the ideal range should be within ±5%.
Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to richen the fuel mixture, to compensate for a perceived lean condition. Negative fuel trim percentages indicate the PCM is attempting to lean out the fuel mixture, to compensate for a perceived rich condition. STFT and LTFT percentages are the adjustments made by the PCM to maintain the 14.7:1 ratio.

No matter what the driveability issue happens to be, the fuel trim window should be used first to check the STFT and LTFT parameters.
There are two basic fuel control systems used on most vehicles: speed density systems, which use rpm, manifold absolute pressure (MAP) and barometric pressure (BARO) to calculate engine load, and mass airflow systems, which use the mass airflow sensor (MAF) and rpm. to calculate engine load. In both cases, the PCM begins with a standard injector pulse width calculation, based on various inputs and internal fuel cell tables.
The equation used by early Chrysler speed density OBD II vehicles to establish initial pulse width is: Injector Pulse Width = (RPM × MAP/BARO) × TPS × ECT × IAT × Battery Volts × O2 (Short Term x Long Term). Once the vehicle is running and the engine control system enters closed-loop, the PCM relies primarily on feedback from, the oxygen sensor to determine if the stoichiometric air/fuel ratio is being maintained.
Think of closed-loop operation as a Sense-Decide-React sequence. The Closed Loop System Operation sequence in Fig. 1 on page 66 provides an explanation of the Sense-DecideReact process. The PCM determines the base injector pulse width as described above. Once the system enters closed-loop, the Sense phase begins, and is handled by the oxygen sensor. In the Decide phase, the PCM uses the oxygen sensor data to determine if the proper 14.7:1 air/fuel ratio is being maintained. If the ratio is correct, the PCM decides that no change should be made to the injector pulse width. In this scenario, the React phase maintains the same injector pulse width. However, if the air/fuel ratio is 16.1:1 (lean) during the Sense phase, the PCM makes the decision to increase the injector pulse width to correct the lean air/fuel ratio condition. In the React phase, the PCM commands the fuel injector to stay open longer. The Sense-Decide-React sequence continues throughout closed-loop operation, maintaining the proper air/fuel ratio.
During closed-loop operation, the PCM reports changes in fuel trim calculations via the OBD II generic data parameters short-term and long-term fuel trim. STFT for most vehicles will normally sweep rapidly in response to the oxygen sensor. In many cases, if you graph Bank 1 STFT and BlSl O2 sensor, you'll see the oxygen sensor go rich and STFT go lean to adjust the air/fuel ratio. The oxygen sensor will then go lean and STFT will go rich.
LTFT for most vehicles will remain more stable, adjusting over a longer period of time. On some vehicles, if STFT has reached the specified limit, LTFT will change in a few seconds. On other vehicles it may take 15 to 20 seconds before a change occurs. The LTFT calculation is normally kept in memory, so the PCM is ready to use the last known injector pulse width following a restart. STFT will normally begin at 0% and adjust to the current conditions. Both STFT and LTFT will normally reset when all trouble codes are cleared.