Accent GT Coupe L4-1495cc 1.5L DOHC MFI (1997)
Technical Service Bulletin # 98-36-001
Date: 980401
Air/Fuel Mixture - Trim Code Diagnosis
Group: ENGINE ELECTRICAL
Number: 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.
Short/Long Term Fuel Trim (ST/LT Trim)
^
"Short Term" Fuel Trim is the value that represents the quick small changes the PCM makes to the A/F ratio.
^
On a Sonata, the PCM continuously responds to the signals from the oxygen sensors to maintain the 14.7 to 1 A/F ratio by increasing or decreasing
fuel delivery.
^
On Accent, Elantra and Tiburon, the "Short Term" Fuel Trim indicates the amount of correction at idle and low rpm/load.
^
"Short Term" Fuel Trim values range from -25% to +25%.
^
"Short Term" Fuel Trim values are positive when the PCM is increasing fuel delivery, negative when the PCM is decreasing fuel delivery. "Short
Term" Fuel Trim may read "0.0" for brief periods during transition from positive to negative (or negative to positive) operation. If the "Short Term"
Fuel Trim reads "0.0" for longer periods, it may indicate that the vehicle is in open loop.
^
On a Sonata, the "Short Term" Fuel Trim values are not retained in the PCM memory when the ignition is turned off.
^
On a Sonata, "Long Term" Fuel Trim values change to compensate when the "Short Term" Fuel Trim has reached its limit in one direction and is
unable to maintain the 14.7 to 1 A/F ratio.
^
On Accent, Elantra and Tiburon vehicles, the "Long Term" Fuel Trim indicates the amount of correction at high rpm/load.
^
The "Long Term" Fuel Trim correction range is -25% to +25%. Exceeding the -20% to +20% range will set a code on an Accent, Elantra and
Tiburon. Sonata vehicles will set a code when the -12.5% to +12.5% range is exceeded.
