 Most 1980-on Fiats and 1982-88 Alfa Romeos use a version of the popular
Bosch L-Jetronic fuel injection system. Though bewildering at first, the
system is actually not conceptually difficult to understand. We won't
try to explain how to repair your fuel injection system, but we can give
you a better idea of how it works.
An electric fuel pump provides pressurized fuel, controlled by a
pressure regulator, to the four or six fuel injectors located in the
intake manifold (an additional injector on most L-Jet cars functions
only during cold starts). Each injector contains a tiny electromagnetic
valve which can be opened and closed with a simple electronic signal.
A series of sensors feeds data to a “brain” (typically called ECU for
Electronic Control Unit). The ECU “studies” the data and determines the
correct duration of electrical pulses to the injectors. The sensors are
spread around the engine compartment, and locations vary from car to
car. Still, all L-Jetronic cars use the following sensors:
Airflow Meter (A)
The airflow meter’s function is just what you’d think: it measures the
flow of air through the intake snout. A spring-loaded flap, positioned
in the path of the air, swings open in proportion to the amount of air
entering the engine. The ECU reads voltage through a potentiometer
connected to the flap. Basically, the more the airflow, the more fuel
the ECU orders. The potentiometer can become worn, giving false readings
and leading to power loss, backfiring or stalling. A more common woe is
simply an air leak in the intake air system, allowing for “un-measured”
air to enter the engine.
Ambient Temperature Sensor
Usually located in the airflow meter, this sensor measures intake air
temperature - which roughly reflects air density. Cooler air is denser,
and requires a richer mixture than warm air. (Density and air flow can
be combined to calculate air mass, which is actually what the ECU needs
to know.) The ECU measures resistance through the sensor, which is
merely a temperature-sensitive resistor (thermistor), and adjusts
mixture accordingly. A bad sensor will not stop you dead, but mixture
will be slightly off. Poor mileage is the most common effect of a
defective unit.
Coolant Temperature Sensor (B)
As the engine warms up, its mixture requirements are reduced (think of
the choke on a carburetor. Another thermistor, this temperature sensor
dips into the cooling system, usually near the thermostat housing. The
ECU evaluates the resistance and adjusts the air/fuel ratio. On some
later cars, this sensor even handles enrichment duties during cold
cranking (starting), a function left to the thermo-time switch on most
L-Jet cars. Depending upon the type of failure, a faulty sensor could
cause very rich running or difficulty in cold-starting.
Thermo-Time Switch (similar to B)
If your car incorporates a cold-start injector, it will be controlled by
a thermo-time switch. While you crank the engine-and if it is cold- the
thermo-time switch will allow the cold-start injector to open. It
operates much like the thermostat in your home, using two metals
sandwiched together, each with a different rate of expansion from heat.
A second, heating circuit in the switch begins to warm the bi-metallic
switch as soon as you start cranking, so the switch will shut off before
you flood the engine. The thermo-time switch works by grounding out the
current to the cold start injector. If it fails to energize the
cold-start injector, a bad switch can cause difficult starting (but not
poor running). At the other extreme, if it doesn't shut off after
startup and allows the injector to continue spraying fuel, mixture will
be extremely rich.
Engine Revolution Sensor (C)
The ECU needs to know the speed of the engine. In the Fiat Spider, for
example, the signal comes from the distributor. The Alfa Spider uses a
magnetic flywheel sensor located in the bell housing. In either case,
the sensor lets the ECU count engine revolutions. From this, it can
calculate how fast the engine is spinning. Failure of this sensor can
leave you stranded.
Throttle Position Switch (D)
The airflow meter tells the ECU most of what it needs to know about
throttle position. For more accuracy, a switch on the throttle butterfly
signals the ECU when your accelerator pedal is either floored (wide
open) or at idle. A defective switch can cause poor take-off from idle,
or higher fuel consumption.
Oxygen Sensor (a.k.a. Lambda Sensor, O2 Sensor) (E)
The ECU requires a final test to fine-tune the mixture: the oxygen
sensor. Located in the exhaust downpipe, close to the engine, this
sensor doesn't work until it reaches about 600ºF. At that point, it
generates a voltage that is proportional to the amount of oxygen left in
the exhaust gas. Too much oxygen indicates lean running; too little
means the mixture is rich. The ECU uses the feedback to constantly
adjust mixture. The O2 sensor is quite sophisticated, and can be killed
in a number of ways. Inappropriate gasket sealants can poison the
sensor. A sudden shock (such as dropping the sensor), or excessive oil
or fuel in the exhaust can poison it. If you blow a head gasket, you
should replace the sensor, because coolant can ruin it. A bad O2 sensor
may not be immediately noticeable. Just remember that a rich mixture can
damage your expensive catalytic converter.- Mark Lee |
