Service Repair | |||
LTFT is a percentage modifier applied by the ECM after all other base factors are considered and before actually firing the injector. It is a learned value based on the amount of additional correction needed to return the STFT to a point that it is once again moving relatively equally across "0." It's the goal of the ECM to keep STFT switching across "0," and it will adjust the LTFT until it does so. These corrections are kept in memory to be used under similar rpm/load conditions. All of this action is going on in what is called "closed-loop" operation. Closed loop occurs when the oxygen sensor has warmed up to operating temperature and starts producing its voltage signal. Ideally, the engine would remain in closed loop at all times, but certain driving conditions prevent this. Cold start, acceleration and deceleration are examples of times when closed-loop operation is suspended, because of the mixture changes and needs that occur at those times. Modern sensors have significantly reduced "open-loop" operation times, and AF sensors are able to maintain closed-loop operation under nearly all conditions. The LTFT corrections learned in closed loop are also applied in open loop. When LTFT exceeds its preset thresholds, the ECM knows that it's having a hard time maintaining lambda. If this correction is positive (indicating the ECM is constantly adding fuel), a "System Lean" DTC will be set (P0171 and/or P0174). If the correction is negative (indicating the ECM is reducing fuel quantity), a "System Rich" code will be recorded (P0172 and/or P0175). There may also be related, manufacturer-specific codes set. These are typically "one trip" codes that will illuminate the MIL at the first noted failure.Diagnosing Fuel Trim-related Codes-gathering Information When dealing with any powertrain diagnostic code, the first step is to understand what conditions were seen by the ECM that caused it to illuminate the MIL and record the failure. In the case of fuel adaptive codes (System Lean or Rich), the typical ECM strategy determines that the LTFT correction needed to keep STFT crossing "0" (and thus maintaining lambda) is at its limits, either positive (adding fuel) or negative (reducing fuel). It does not mean that the engine is getting too much or too little fuel alone.
So before we clear the codes, let's look at Freeze Frame data. It is important to write this information down and understand that this is a snapshot of the rpm/load conditions where the ECM lost control over lambda. On generic scan tools, you will have PIDs for STFT and LTFT only, so now is the time to make note of the one you are really interested in: Total Trim. This is done by adding the average STFT (remember, it is constantly changing) to the LTFT. Is that all? Not quite. We can't forget the influence the post-cat sensor has on trim. This should also be added to get Total Trim. Unfortunately, not all generic scan tools have this PID listed. Keep in mind that Rear Trim is a factor, regardless of whether you can read the information. (Some enhanced scan data will list a Total Trim PID; Toyota, for example.) |
