Porsche 912 User Manual
TECHNICAL MANUAL
L-JETRONIC
FUEL INJECTION SYSTEM
PORSCHE 912E
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TABLE OF CONTENTS
1.0 Introduction to L-Jetronic Fuel Injection
2.0 L-Jetronic Fuel Injection Theory of Operation
3.0 L-Jetronic Component Identification
4.0 Operating Conditions
5.0 Troubleshooting the Porsche 912E Fuel Injection System
6.0 Component Location
7.0 System Diagrams and Reference Tables
8.0 Bibliography
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1.0 L-JETRONIC INTRODUCTION
The Bosch L-Jetronic fuel injection system used in the Porsche 912E is often
referred to as the Air Flow Controlled or AFC fuel injection system. Originally,
Volkswagen adapted the L-Jetronic for the Type 4 air cooled 411 engines sold in
the United States due to more stringent emissions and fuel consumption
regulations. It was also used in the Porsche 914 with the 1.8L engine and
ultimately used in the Porsche 912E during the 1976 model year. The L-Jetronic
was a further development of the D-Jetronic found in the Porsche 914 1.7L and
2.0L engines. The original L-Jetronic found in the 914 1.8L engine was further
refined which resulted in the system installed in the 912E. The L-Jetronic
principles were later adapted for use in the Porsche 911 series models.
2.0 L-JETRONIC DESIGN APPROACH
The design approach used by Bosch for the L-Jetronic attempted to overcome
some of the limitations found in previous fuel injection systems as well as to meet
the more restrictive emission standards being regulated in the United States.
The design of the L-Jetronic injection system automatically takes into account all
changes in the engine which can occur during the service life of the vehicle
(abrasion, deposits in combustion chamber, changes in valve adjustment, etc.)
Uniform good quality of exhaust gases is therefore assured. In the L-Jetronic,
part of the exhaust gas can be re-circulated to lower the temperature in the
combustion chamber. The air-flow sensor measures only the fresh air drawn into
the engine and the control unit determines the quantity of fuel required only for
the quantity of fresh air. A supplementary mechanism for mixture enrichment
during acceleration is not required because the signal transmitted by the air-flow
sensor precedes charging of the cylinders. In addition, idle stability is improved.
The design of the L-Jetronic allows for fewer compensating adjustments during
the operating cycle of the fuel injection process as compared to the D-Jetronic.
This approach provides a more accurate fuel - air ratio that optimizes
performance under varying requirements. This is due to use of air-flow sensing
which takes direct account of a multitude of factors that influence the fuel
requirements of the engine.
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3.0 PORSCHE 912E L-JETRONIC COMPONENTS
The following table provides a quick reference for the fuel injection components
used in the 912E.
FUEL INJECTION COMPONENTS FOR THE PORSCHE 912E
Component Part Number
Fuel Pump 923 601 111 00 0 580 463 010
Fuel Filter 923 110 176 00 0 450 905 001 0 450 905 062
Fuel Regulator 022 906 035 0 280 160 200
Fuel Injector Valves 923 606 109 00 0 280 150 105
Thermo-time Switch 923 605 101 00 0 280 130 214
Cold Start Valve 923 606 107 00 0 280 170 029
Air Flow Sensor 923 606 111 00 0 280 201 006
Auxiliary Air Regulator 022 906 045A 0 280 140 101
Temperature Sensor II 022 906 041 0 311 906 041A 0 280 130 012
Throttle Valve Switch 022 906 111 D 0 280 120 201
Electronic Control Unit 923 618 101 00 0 280 000 134
Injection Resistor Pack 0 280 159 001
Dual Relay 923 615 101 00 0 332 514 103/104/120
(1)
Part numbers beginning with 923 are Porsche part numbers. Part numbers
beginning with 022 are Volkswagen part numbers. Part numbers beginning with
0, followed by a 3 digit number (0 311 906 041A) are Bosch part numbers.
The components listed in the table can be functionally divided into three major
systems that are integrated to provide the correct injection pulse based on
specific operating conditions. The major systems are the fuel system, sensors,
and control unit. The fuel system provides the gasoline from the tank to the
injection valves, creates the pressure necessary for injection and maintains the
pressure at a constant value. The fuel system includes the pump, filter, pressure
regulator, cold start valve, and injectors. The sensors detect the various
parameters necessary to assure the correct injection pulse duration. The most
important parameter is the quantity of air that is used by the engine. The intake
air flow sensor provides this value as well as the temperature of the air entering
the engine. The throttle valve switch determines the position of the throttle (either
idle or wide open), and the temperature sensor determines the engine operating
temperature. Two additional devices, the thermo-time switch and the auxiliary air
regulator, are used during the starting phase to control fuel enrichment and
amount of air drawn into the engine. All of the parameters generated by the
sensors are then provided to the electronic control unit. The electronic control
unit processes these inputs and the input from the ignition distributor, which
represents engine speed, and provides the correct injector fuel pulse duration.
(1)
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The following diagram provides an overview of how the fuel injection components
are integrated.
AIR FLOW
METER
Amount of Air Draw In
Temperature of Air Drawn In
THROTTLE
SWITCH
IGNITION
DISTRIBUTOR
TEMPERATURE
SENSOR II
THERMO-TIME
SWITCH
Engine Load (WOT)
Engine Speed
Engine Temperature
Engine Temperature/Time
ELECTRONIC CONTROL UNIT
PUMP and
POWER RELAYS
STARTER
Battery
Injection
Pulse
IGNITION
SWITCH
INJECTORS
CYL 1
CYL 2
CYL 3
CYL 4
AUX AIR REGULATOR
Excess Air for Starting
Excess Fuel for Starting
COLD START
VALVE
4.0 OPERATING CONDITIONS
There are six major operating conditions that the components of the L-Jetronics
fuel injection system must detect and provide inputs to the control unit in order to
provide the correct fuel-air mixture. These operating conditions are:
1. Cold Start
2. Warm Up
3. Idle During Warm Up
4. Load Adaptation
5. Temperature Adaptation
6. Coasting Adaptation
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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
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at temperatures below 35
C or 950F. 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 550F.
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
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air during the idle period. This additional air is obtained via the auxiliary air
regulator that provides a bypass around the throttle valve and allows air to enter
directly into the manifold. This bypass route avoids any deflection by the throttle
valve. However, the additional intake air that is provided through the auxiliary air
device, does get detected by the air-flow sensor, which results in additional fuel
being supplied at the fuel injectors. The compensation for the additional air
provides more air-fuel mixture during warm up. During a cold start, the auxiliary
air device will be wide open. As the engine temperature increases, the size of
the opening decreases and eventually closes altogether. Additionally, the
auxiliary air device has an internal heater that is designed to provide a limited
opening time. The auxiliary air device should remain closed after normal engine
operating temperature is reached due to ambient heat from the engine.
LOAD ADAPTATION: The 912E engine continuously encounters varied engine
loading conditions. These can be broken down into four categories; idle, partial
load, full load, and acceleration. Air flow sensing takes the varied conditions into
account and provides inputs to the electronic control unit that provides the correct
injection duration for the load encountered.
The sensors and the electronic control unit normally control the idle load
condition. The air-flow sensor is designed to allow a small amount of air to
bypass the air flow sensor. The size of this opening can be adjusted to
compensate for lean conditions. This can be accomplished by adjusting the idlemixture screw found on the front of the air flow sensor. This adjustment does not
provide additional fuel to the engine, only air. Specific steps for this adjustment
are provided in Section 5.0. The 912E throttle valve switch does not provide an
input to the electronic control unit during the idle load condition as previously
found in the 914 1.8L L-Jetronic Fuel Injection System.
The majority of the time the engine encounters partial load conditions. The
electronic control unit has an internal program for this condition and provides the
correct injection pulse duration when normal driving conditions are detected by
the sensors. If a full load condition is detected, the mixture is enriched to provide
maximum output from the engine. The full load condition is detected by the
throttle valve switch that is connected to the throttle valve shaft. When the
accelerator pedal is fully depressed, a contact in the throttle valve switch is made
and this condition is detected and processed by the electronic control unit.
Sudden acceleration could result in variations in the fuel-air mixture that could
impact performance. Acceleration enrichment is required to avoid fuel-air
mixture problems. The additional fuel above the requirements detected by the air
flow sensor is provided to the engine as a result of the design of the sensor flap
in the air flow sensor. Sudden acceleration causes the sensor flap to swing
beyond its full open position for a short period of time before it returns to the
normal position. This over-swing of the sensor flap is detected by the electronic
control unit and results in an increase the quantity of fuel to the engine and
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provides good acceleration response. If sudden acceleration is required during
the warm up phase of the engine, the acceleration enrichment provided by the
over-swing of the sensor flap may not be sufficient. If this condition is
encountered, then the electronic control unit also detects the speed of the
deflection of the sensor flap and adjusts the fuel accordingly.
ADAPTATION TO TEMPERATURE: Compensation for variations in air
temperature must be made. This is due to the fact that the density of the air will
affect the efficiency of combustion. Colder air is denser and provides a more
efficient combustion process than warmer air when the same throttle valve
position is used. The air flow sensor has a temperature sensor (Temperature
Sensor I) in the intake which measures the temperature of the air drawn into the
engine. The temperature measurement is provided to the electronic control unit
that compensates for the various temperatures by adjusting the amount of fuel
provided for the combustion process.
COASTING ADAPTATION: During normal driving conditions, the operator will
often lift his foot completely off of the accelerator pedal and coast. The electronic
control unit detects this operating condition and the result is that the injector
valves will be closed above a certain speed and temperature. If a load condition
is sensed again, or if the speed sinks below a preset value, then the injection
process returns to normal operation.
5.0 TROUBLESHOOTING THE PORSCHE 912E FUEL INJECTION
This section is designed to assist the Porsche 912E owner in locating faults in
the L-Jetronic system, isolating the component involved and testing the
component for correct function. The tools required will include a fuel pressure
gauge and fittings, a tachometer, a CO meter and volt-ohmmeter. The following
safety and maintenance tips are provided:
SAFETY/MAINTENANCE TIPS
Never jump the battery to start the car.
Never start the engine without battery cables firmly connected.
Always remove cables from battery before charging.
Never remove cables from battery with engine running.
Never remove or attach wiring harness plug to Control Unit with the ignition on or
with battery connected.
When turning the engine over to check compression, unplug the red cable from
the battery to the relays. Remember, there is not a fuse on this line
Before testing the L-Jetronic system, make sure the timing, dwell and spark plug
gaps are within specification.
Make sure all vacuum hoses and fuel hoses are connected and in good working
order.
Check for air leaks in associated with the air filter and connecting hose.
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Failures in the L-Jetronics fuel injection system can be categorized into five major
categories. The components associated with the potential failures are identified
and measurements are provided which will allow the verification of each
component’s operating condition. Please refer to the system diagrams provided
in section 7.0 to assist in isolating the problem area.
FAILURE CATEGORIES
• Engine does not start • Acceleration not smooth
• Engine misses • Rough idle
• Engine starts and dies
ENGINE DOES NOT START
Possible Problem Procedure
Fuel Pressure or
Fuel Pump
1. Check the fuel pressure by connecting a pressure
gauge with an adapter to the tee found on the driver’s
side of the fuel rail.
2. Disconnect the vacuum hose between the air
distributor and pressure regulator.
3. Turn ignition switch to the on position and open the
air flow sensor valve slightly to start the fuel pump.
4. Fuel pressure should read 35 +/- 1.4 psi with vacuum
hose disconnected. If reading is not correct, proceed
with step 5.
5. Turn the ignition switch to the start position and listen
for the fuel pump. If pump can be heard, then check for
blockage in the fuel filter and fuel line. If pump does not
operate or cannot hear the pump, then verify fuel filter is
good and proceed with step 6.
6. Unscrew the dual relay set and measure for 12 volts
at pin 88d while cranking the engine. If voltage is not
present, proceed with step 7. If voltage is present, then
verify the continuity between pin 88d and the fuel pump,
or verify voltage is present at the fuel pump.
7. Verify that 12 volts is present on the double relay at
relay terminals 88y at all times and at 86a while
cranking the engine. If voltage is not present at 86a
while cranking the engine, then go to stop 9.
8. If voltage is not present at 88y, then check for
continuity between regulator, one side of the fuses (S23,
S24) in the rear relay panel and then to pin 88y on the
dual relay. If voltage is present at 88y, go to step 9.
9. Verify continuity between pin 50 of the ignition switch
and pin 88a of the dual relay. If continuity is good, then
the ignition switch may be bad.
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