Fiat 124 Spider - The Air Flow Meter and Fuel Pump

FIAT 124 Spider
Engine Maintenance + Modification
40
The FIAT implementation of the Bosch L-Jetronic fuel injection system is relatively
simple and uses the most basic components required to operate the system. Many
additional options including atmospheric pressure sensors, knock sensors, and crank
angle sensors, are all possibilities with L-Jetronic but were never installed. FIAT quite
literally built a system around the existing 2000cc carbureted engine. The simplicity
results in a system that is inexpensive, easy to diagnose, and can be repaired by the
home mechanic. The system consists of a number of components that are located in
the engine and passenger compartments. Figure 33 summarizes the major components
and some of their connections.
5.2 The Air Flow Meter and Fuel Pump
The air flow meter is positioned just above the air filter and is the only point of entry for
air into the fuel injection system. Inside is a spring-tensioned flap connected to a
rheostat. As intake air volume increases the flap moves an arm against the rheostat.
The rheostat converts the movement of the flap into an electrical current that is sent to
the fuel injection computer and is the primary input in the system. Fuel is delivered in
proportion to the volume of air and the fuel injection computer adjusts the fuel delivery
to best find the 14.7:1 air to fuel ratio.
The stream of new air passes through the main air hose (also called the “big air hose”),
connecting to the throttle body. The operation is abbreviated as follows:
1. Air passes the throttle plate into the plenum (the square aluminum box on the
engine intake side) and is drawn through runners (pipes under the plenum) into
the engine.
2. The fuel injection computer opens the injectors and sprays fuel into the intake
runners (the four tubes between the plenum and the cylinder head). All four
injectors fire at the same time.
3. Valves open and draw the mixture into the combustion chamber.
4. The mixture is compressed and explodes via ignition spark.
5. The mixture is expelled.
6. An oxygen sensor measures the amount of unburned oxygen in the exhaust.
7. The F.I. computer uses the reading from the oxygen sensor to adjust the next
mixture.