Is a Long-Term Fuel Trim of -14.7 Rich or Lean?
3 Answers
Long-Term Fuel Trim -14.7 indicates a rich condition. Below is additional information: 1. Fuel Standards: Typically, long-term fuel trim parameters range from -23% to 16%. A value of 0 means no fuel compensation is needed to achieve the correct mixture concentration. A positive value indicates the injection system needs to inject additional fuel, suggesting the engine's air-fuel mixture is lean, which usually makes the vehicle hard to start. A negative value means the injection system needs to reduce fuel to correct an overly rich mixture, so a negative long-term fuel trim indicates the vehicle's air-fuel mixture is too rich. 2. Long-Term Fuel Trim Standard: Generally ±5%. Exceeding this standard likely indicates an engine fault. If the vehicle's injectors are clogged, both fuel injection volume and atomization effect will be poor, resulting in a leaner combustible mixture in the engine cylinders.
A long-term fuel trim value of -14.7% is indeed quite interesting. A negative value indicates the ECU is actively reducing fuel, which typically suggests the air-fuel mixture is running rich. Just like the Camry I worked on last month at the shop—it also had a long-term trim of -15%. Upon disassembly, we found a leaking fuel pressure regulator. This negative trim means the oxygen sensor detected excessively rich exhaust, forcing the ECU to reduce injector pulse width as compensation. I recommend focusing on checking whether fuel pressure is too high, if there are any injector leaks, or if the purge solenoid valve is stuck. Sometimes a saturated charcoal canister can also draw in excess fuel vapors. Remember when we repaired that Passat last time? A drifting oxygen sensor also threw similar trouble codes—all these possibilities require systematic diagnosis.
Seeing a value of -14.7% indicates the engine is currently consuming fuel excessively. Negative correction means the ECU is stepping on the brakes to control fuel injection, proving the air-fuel mixture concentration exceeds the standard. This phenomenon is particularly common in German cars I've handled, especially EA888 engines after two years of use. Focus on checking three areas: whether there are cracks in the intake pipe causing vacuum leaks, if the fuel pressure exceeds the standard value of 3.5bar, or if the oxygen sensor is clogged with carbon deposits. Last time when dealing with a BMW, I found that a damaged crankcase ventilation valve could also cause similar issues, with all the oil vapor being sucked into the combustion chamber. Don't underestimate this negative value; long-term conditions like this are especially fuel-consuming and harmful to the catalytic converter.
