Fuel trim calculations are obtained from the oxygen or air fuel sensor signals while the system is in closed loop. Since the driver controls the amount of air flow into the combustion chamber, the PCM controls the amount of fuel. The PCM uses fuel injector pulse width to adjust fuel trim, it controls the amount of time the injector is left on. Fuel trim is a method used by the PCM to add fuel to or subtract fuel from the air fuel mixture. The longer the injector is left open spraying fuel behind the intake valve, the richer the mixture. The shorter the length of time the injector is left open, the leaner the fuel mixture.
If a vacuum line is damaged or left disconnected to the intake manifold, the oxygen sensor will sense this increase of oxygen in the exhaust. The PCM receives this voltage signal and then compensates for the lean mixture by holding the fuel injectors open longer, resulting in an enriched fuel mixture.
Long term fuel trim will increase some either negative or positive as the engine's cylinders wear; this is normal. Typically if LTFT and STFT percentages combined go beyond 10% either negative or positive, the PCM will illuminate the check engine light indicating something is wrong. LTFT is calculated by the PCM over time and over many key cycles. The PCM saves this information for the next time it enters closed loop. Short term fuel trim is between key cycles.