Is a Negative Short-Term Fuel Trim Lean or Rich?
2 Answers
Indicates that the air-fuel mixture is too rich, and the fuel injection quantity is being reduced. The short-term fuel trim value generally ranges between -5.2% to -8.6%. Below are detailed explanations: 1. How short-term fuel trim works: Short-term fuel trim is a real-time correction control of fuel injection quantity based on the oxygen sensor's feedback regarding the richness or leanness of the air-fuel mixture in previous working cycles. Through continuous short-term fuel trim, the mixture concentration is maintained as close as possible to the theoretical air-fuel ratio. 2. Factors affecting the fuel trim coefficient: Factors that influence the fuel trim coefficient include engine misfires, poor exhaust flow, low fuel pump pressure, or clogged fuel injectors. These aspects should be checked. If the combined value of long-term and short-term fuel trim exceeds 5%, it may indicate potential issues such as clogged injectors, eroded spark plugs, poor fuel quality, faulty air flow meters, or overdue fuel filter replacement.
Short-term fuel trim is a system in the car that controls fuel injection to balance the air-to-fuel ratio. If the trim is negative, such as showing -10%, it means the system is reducing the fuel supply. The reason is simple: the car detects that the air-fuel mixture is too rich, meaning there's too much fuel and not enough air, leading to incomplete combustion. It quickly adjusts to a leaner state to better match the fuel and air. This issue could be due to an air intake leak, a faulty mass airflow sensor, or a bad oxygen sensor. If left unchecked for a long time, the engine will work harder, consume more fuel, and be prone to carbon buildup. As an ordinary car owner, I've found that regular checks on these components can prevent trouble—like cleaning the sensors or replacing a part—to restore normal operation. In short, a negative value indicates a rich mixture, and the system is correcting it to a more reasonable lean state. Remembering this is crucial.
