To Use the VF Terminal to Confirm Air Fuel Ratio


Approximate D-Type L-Type

VI Voltage |

V 1-25V Decreased Volume ov

Under this condition, the VF voltage represents the learned value correction factor to fuel injection duration. As you learned in Chapter 5, final injection duration is the sum of basic injection plus injection corrections. Learned value is simply another correction factor which is used to bring the corrected air/fuel ratio as close to the stoichiometric air/fuel ratio as possible.

The ECU fuel injection duration program is the same for every engine; however, each engine is a little bit different from the next. The purpose of the learned value correction is to tailor the standard fuel injection duration program to each individual engine. The injection duration calculation, before oxygen sensor correction, is the ECU's best guess at a stoichiometric air/fuel ratio. The oxygen sensor correction fine-tunes injection duration precisely to 14.7 to 1. The learned value correction factor ensures that oxygen sensor corrections do not become too large to manage.

In this mode, the VF voltage signal will be at one of five different steps (three steps on D type EFI) depending on how close the calculated air/fuel ratio (before oxygen sensor correction) is to stoichiometry. With the engine operating in closed loop, learned value VF should be somewhere in the 1.25 to 3.75 volt range with a nominal value of 2.5 volts.

Generally speaking, a lower voltage indicates the ECU is decreasing fuel to correct for some long term rich condition. Examples of conditions which could cause low learned value VF:

• Crankcase diluted with fuel

• Loaded evaporative canister

• High fuel pressure

A higher voltage indicates that the ECU is increasing fuel to correct for some long term lean condition. Examples of conditions which could cause high learned value VF:

• Atmospheric leaks into intake system

• Worn throttle shaft

• Low fuel pressure

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