Service Repair | ||
Take fuel trim readings at idle, 2,500 rpm, highway cruise and under WOT. If there is a driveability concern, add a recording under the conditions where this concern is evident. Look for variance between banks, indicating a fault common to only that part of the engine. Fuel trims can provide direction when diagnosing misfire codes. Record the fuel trim corrections at the time of misfire. Don't rely on those stored in freeze frame data as this information can be actually recorded after the misfire occurred. Misfires caused by ignition faults are generally less consistent than those caused by fuel or engine related issues, and have less of an impact on total fuel trim corrections. If total fuel trim corrections are less than +18 percent, suspect an ignition system related fault as the culprit. This is one more observation you can add to your diagnostic routine when hunting an elusive miss. Keep in mind that fuel adaptive diagnostic trouble codes (DTCs) are a reaction to another failure or combination of failures. Be sure to repair the causes of any other logged DTCs before chasing down the adaptive codes. Make sure the ECM is receiving accurate information and capable of executing its output correctly. Once you've made sure all is well with the data the ECM is getting, concentrate first on fuel or engine system diagnosis when dealing with a System Rich condition. Think of how these systems all interact at the time of failure. More common are the System Lean codes. Look for factors allowing excess or unmetered air into the engine, or conditions that would allow only partial combustion of the air:fuel charge.Make it a habit of checking fuel trims as part of your regular diagnostic routine. Often, you will catch a fuel adaptive condition before it has a chance to turn the MIL on, only because the thresholds for failure haven't been met yet. Total fuel trims outside of "normal" parameters can be one of your first clues that the engine is not working as efficiently as it could. By addressing those issues now, you will help your customer save money, save fuel and reduce emissions. Remember, fuel trim is how the ECM adjusts air/fuel ratio based on information provided by sensors, and the information it uses to make those decisions is available on the scan tool. Fuel trim diagnostics tells you why the ECM is adjusting fuel trim, and that "why" comes from knowledge and ex-perience stored in your own brain. Your diagnostic skills will become faster and more accurate with practice, even on vehicles that are working correctly. (Author's note: A word of thanks to all those iATN members who shared their knowledge in the preparation of this article.) Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, FL. His experience reaches back more than 30 years, and reflects a wide variety of experience with almost every make and model. You can contact him directly at http://www.autoservicetech.com/. |
