How to Interpret Data for Over-Rich Air-Fuel Mixture?
3 Answers
An over-rich air-fuel mixture can be identified by checking fault codes and long-term fuel trim values. Here are the details: 1. Fault codes: P0172 and P0175 both indicate an over-rich mixture, with the former referring to Bank 1 and the latter to Bank 2. During engine operation, the ECU adjusts fuel injection quantity via injector opening/closing times, primarily relying on voltage signals from various sensors. Short-term and long-term fuel trim values are key data for assessing air-fuel mixture combustion quality. If the sum of short-term and long-term trim values falls within ±5%, it indicates good engine combustion. 2. Long-term fuel trim: Also called the average learning value, if this value exceeds +35%, it means the ECU is continuously increasing fuel injection, indicating a lean mixture condition. Conversely, if the learning value exceeds -35%, it confirms an over-rich mixture.
I've been repairing cars for 20 years and often use an OBD scanner to diagnose rich mixture issues. After connecting the device, you mainly look at a few key data points: First, the oxygen sensor voltage, which should normally fluctuate between 0.1 and 0.9 volts. However, if it consistently stays above 0.8 volts, especially at idle, it indicates too much fuel and too little air. Second, the short-term fuel trim, ideally fluctuating around ±5%. If it's consistently negative, say -15%, it means the ECU is desperately reducing fuel injection to compensate for the rich condition. Third, if the long-term fuel trim is also significantly negative, it further confirms the overall issue. Possible causes include leaking fuel injectors or a dirty air filter restricting airflow. I advise car owners not to just look at the data but also check the spark plugs; if they're blackened, that's evidence. Addressing the issue early can improve fuel efficiency and reduce foul exhaust emissions.
It's quite simple to check this at home using an onboard diagnostic tool. I often use a Bluetooth OBD device connected to a mobile app. Monitor the fuel trim data stream—if the short-term trim frequently stays below -10%, the air-fuel mixture is likely too rich. Don't overlook the oxygen sensor signal; it should fluctuate during driving, but a consistently high reading above 0.7V is problematic. Abnormal MAF sensor readings can also mislead fuel injection—for instance, the system may inject more fuel when the value is unusually low. Related troubleshooting: Clean the throttle body or check for vacuum leaks. This issue can affect power output, making the engine noticeably sluggish when climbing hills.
