Engine Maintenance and Modification Bradley Artigue

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FIAT 124 Spider Engine Maintenance + Modification 5.4 Warm-Up Components The warm up phase starts when the thermo-time switch heats up and closes the cold start injector. The engine temperature sensor, located in the water coolant “T” in front of the cylinder head, sends coolant temperature readings to the fuel injection computer. The engine is still cool enough to have increased frictional resistance and, without additional fuel and an increased idle speed, will stop running. To compensate the fuel injection computer turns on a device called an auxiliary air regulator. This device allows air to be drawn directly into the plenum, bypassing the throttle plate. The effect is an increase in idle speed – the computer will give the engine more fuel to create an appropriate ratio to the inbound air. Once engine temperatures reach the normal operating range the auxiliary air valve closes. The engine now enters a “normal” operating phase, with no cold or warm up devices operating. Figure 35: Warm-up Operation in the Bosch L-Jetronic System 5.5 Idle Speed Control Components Your FIAT fuel injection system maintains idle by allowing a small amount of air to bypass the throttle plate and enter the throttle body. This amount of air is controlled in three ways. First, the mixture is statically controlled by turning a bypass screw on the air flow meter body. Next, the air strikes the throttle plate, which at rest is open approximately 1 degree and allows a small amount of air into the plenum. Additional 42
FIAT 124 Spider Engine Maintenance + Modification 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 43
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Engine Maintenance and Modification Bradley Artigue

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