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COLD START: When the ignition switch is initially turned on during the Cold
Start phase, it energizes the dual relay set which provides battery voltage to the
control unit and injection valves. When the starter is engaged, the relay set
provides battery voltage to the electric fuel pump, the cold start valve, the
thermo-time switch, and the auxiliary air regulator. During cold start and all other
load conditions, the fuel pressure regulator found in the 912E is able to maintain
an equal difference in pressure on all the injector valves regardless of load
conditions. This is due to a pressure diaphragm in the fuel regulator that is
controlled by the pressure levels in the intake manifold.
If the engine starts successfully, then battery voltage is maintained to the fuel
pump and auxiliary air regulator through contacts in the air flow sensor. If the
starting effort is not successful, then the battery voltage is removed from the fuel
pump to prevent cylinder flooding.
Additional fuel is provided to the engine during the initial period of the cold start
phase. This is necessary due to condensation of the fuel-air mixture as a result of
the cold engine. This cold start enrichment is based upon temperature
conditions and is accomplished by extending the duration period of the actual
injector valve opening time based on calculations by the electronic control unit.
Under certain temperature conditions, cold start enrichment is supplemented by
directly injecting atomized fuel from the cold start valve into the intake manifold
behind the throttle body. The length of time that cold start valve enrichment
occurs is controlled by the thermo-time switch. Several factors determine how
long the duration is. The length of time the cold start valve provides additional
fuel-air enrichment is dependent on temperature of the engine, outside air
temperature, and the heating element inside the thermo-time switch. Normally,
the cold start valve does not provide additional enrichment when the engine is
warm. The 912E thermo-time switch is designed to stay energized for 8 seconds
at temperatures below 35
0
C or 95
0
F. This is considerably different from
previous L-Jetronic fuel injection systems. In the 914 1.8L L-Jetronic system, the
cold start valve was not utilized at temperatures above 13
0
C or 55
0
F.
WARM UP
: Considerable fuel enrichment is required for a period of time after a
cold start. For the first 30 seconds, up to 60% additional fuel will be provided to
facilitate the initial warm up period. The actual percentage varies based upon
the temperature. After the first 30 seconds, only a small amount of enrichment
is being provided. The amount of fuel enrichment is controlled by the
temperature sensor (Temperature Sensor II) installed in the cylinder head. The
electronic control unit calculates the amount of enrichment based upon the
Temperature Sensor II input.
IDLE DURING WARM-UP: When an engine is cold it has more frictional
resistance and requires compensation to overcome this condition. To adjust for
the frictional resistance, the L-Jetronic system is designed to provide additional