Incorrect Injection Duration

In addition to the problems mentioned above, false sensor input from any of the six major input sensors can also cause the air/fuel ratio to shift sufficiently to cause driveability and/or emissions concerns. If engine load is incorrectly calculated, fuel requirements are also miscalculated, resulting in a driveability or emissions concern. This type of a condition can be identified by reading sensor signals and comparing them to standard values. With this type of condition, the ECM adaptive fuel program will probably be making major corrections to bring the air/fuel ratio back into a neutral range (stoichiometry).

Fuel Trim Data Interpretation

As shown, fuel trim parameters and Interpretation methods differ depending on the vehicle's diagnostic system.

Diagnostic System

Long Fuel Trim Data Parameter

Normal Operating Range

'possible problem indicated outside of this range

Diagnostic Interpretation

OBD (w/o serial data)

Learned Voltage Feedback (LVF)*

'Access at VF1 terminal of check connector, TE1 off

2.50 ±1.25 volts

LVF represents command to injection system 5.0 volts = lean condition 0 volts = rich condition

OBD (with serial data)

Target A/F

2.50 ± 1.25 volts

Target A/F represents command to injection system 5.0 volts = lean condition 0 volts = rich condition

OBD-II

Long FT

0%±10%

Long FT represents command to injection system > 10% = lean condition < 10% = rich condition

The best way to confirm that a neutral air/fuel ratio is being delivered to the engine, is to monitor the adaptive fuel correction to injection duration. This can be accomplished several different ways, depending on the engine being tested:

1. OBD vehicles without serial data: Use a voltmeter on terminal VF1 at DLC 1 (check connector)

2. OBD vehicles with serial data: Use a scan tool to monitor Target A/F data

3. OBD-II vehicles: Use a scan tool to monitor Fuel Trim data

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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