FIAT 124 Spider
Engine Maintenance + Modification
43
airflow is directed through a passageway above the throttle and controlled by a needle
valve. In this way a fixed amount of air is allowed into the engine at idle, and that
volume of air is easy to adjust and control. This operation is depicted in Figure 36.
Figure 36: Idle Speed Control in the Bosch L-Jetronic System
5.6 Cruising, Accelerating, and Deceleration Modes
Normal driving conditions are typified by acceleration, cruising, full load, and
deceleration. During acceleration additional fuel is injected into the engine. This amount
of fuel is governed by the amount of air entering the air flow sensor. Heavy acceleration
(wide open throttle) results in a somewhat rich mixture while deceleration results in the
fuel injectors being shut off. During cruising the fuel injection computer calculates fuel
delivery so that consumption is low. The fuel injection computer keeps the mixture at
the leanest point of efficient operation. During each of these phases air temperature is
measured by a sensor in the air flow meter. Because air temperature has an effect on
the amount of fuel required to maintain good operation, the fuel injection computer will
use this input to alter the amount of fuel.
Fuel mixture management is based on optimum masses (weights) of the air/fuel
combination. It is not a measure of volume. L-Jetronic systems use the air flow meter
to estimate the air mass entering the system by measure air pressure and temperature.
During acceleration the air pressure against the air flow meter flap increases, signaling
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