mustangchris09

What does this mean, and where abouts should the numbers be sure my car is running right. I only ask because i was playing with my livewire today and saw those parameters

cliffyk

The short term fuel trims represent the amount of fuel, expressed as a percentage, the PCM has to add or subtract--in closed loop mode--to keep the AFR at 14.7:1. A positive number means fuel is being added, negative numbers indicate fuel being pulled.

In closed loop mode the PCM uses what it knows of air flow (from the MAF), IAT, ECT, load and other factors to calculate the fuel injector pulse width needed to make a stoichiometric (14.7:1) fuel mixture. It then uses input from the front O2 sensors to monitor air/fuel ratio, and attempt to keep the average mixture 1t 14.7:1. If however the calculated injector pulse is creating a a lean or rich mix it applies a correction factor "next time round"--this is the short term fuel trim.

In the ideal world they would be 0.0%, on a normally operating engine a constantly varying range of -5.0% to +5.0% is to be expected. Anything over +/- 25% is bad.

Many things affect the amount of trim necessary. In the tune the major factors used to "dial-in" fuel trim and keep it close to 0.0% are the MAF transfer function, the low and high injector slopes, and the injector breakpoint.