Air/Fuel Mixture - Trim Code Diagnosis: Overview
Group: ENGINE ELECTRICALNumber: 98-36-001
Date: APRIL, 1998
Model: ALL 1996-98 MODELS
Subject
FUEL TRIM CODE DIAGNOSIS
DESCRIPTION
The Powertrain Control Module (PCM) on Hyundai OBD II vehicles (all 1996 and later models) can adjust the engine Air to Fuel (A/F) ratio to compensate for variations in operating conditions. This adjustment only takes place during "Closed Loop" operation.
The PCM uses input from the following components to determine the amount of fuel to deliver:
^ Oxygen sensors
^ Air flow sensor
^ Coolant temperature sensor
^ Intake air temperature sensor
^ Throttle position sensor
^ Engine speed sensor
The ability to adjust the A/F ratio enables the PCM to compensate for engine, fuel and sensor variations as well as temperature, altitude and humidity. This enables the vehicle to maintain a stoichiometric (14.7 to 1) A/F ratio that will ensure optimal engine performance and minimal emissions.
Since the PCM can change the A/F ratio to compensate for a wide range of conditions, an indicator is needed to identify that a compensation has been made. Fuel Trim values shown on the HDS scan tool are the primary indicator that an adjustment in fuel delivery has taken place.
The scan tool displays the PCM's A/F ratio compensation as "Short Term" Fuel Trim (ST TRIM), Long Term Fuel Trim (LT TRIM), and "Oxygen Sensor" Trim (TRIM B1 S1 and B1 S2). The most beneficial diagnostic information comes from Short Term" Fuel Trim and "Long Term" Fuel Trim.