GM Cadillac 1967-1989 Repair Guide

General Information



See Figures 1, 2 and 3

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Fig. Fig. 1: Cadillac DFI system - all cylinders are fed by 2 fuel injectors located in the throttle body

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Fig. Fig. 2: Cadillac EFI system - each cylinder has its own fuel injector

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Fig. Fig. 3: Cadillac DFI pressure regulator and fuel injector system - both are contained in the throttle body

Cadillac has used two different types of fuel injection systems: Electronic Fuel Injection (EFI) and Digital Fuel Injection (DFI).

The EFI system is an indirect, or ported fuel injection system, on which each cylinder has its own fuel injector mounted behind the intake valve. The EFI system was used on 1977-79 Sevilles as standard equipment and on 1977-79 full size Cadillacs as an option.

The DFI system is a throttle body fuel injection system, on which two solenoid actuated fuel injectors are mounted in the throttle body and inject fuel down into the intake manifold. The DFI system, introduced on 1980 Sevilles, is used on 1980 6.0 liter (368 cu. in.) engines and all 1981 and later Cadillacs except the 252 cu. in. V6 engine and the 368 4 bbl. engine used in the limousine and commercial vehicles.

Both systems control the air/fuel mixture for combustion by monitoring selected engine operating conditions and electronically metering fuel requirements to meet those conditions.

The EFI system consists of four basic sub-systems: the fuel delivery system, the air induction system, the network of sensors, and the Electronic Control Unit (ECU). The DFI system includes the same sub-systems as the EFI, but uses a more detailed Electronic Control Module (ECM) and adds several more sub-systems. These include the electronic spark timing system (EST), idles speed control system (ISC), EGR (all) and charcoal canister (1981 and later) control system, modulated cylinder displacement (on cars so equipped), closed loop/open loop oxygen sensor system (1981 and later), and the failure operation circuit and diagnostics readout system.

Fuel Delivery System

The EFI fuel delivery subsystem is made up of an in-tank fuel pump, a chassis mounted fuel pump, fuel filter, fuel pressure regulator, fuel rails, and an injector for each cylinder.

The DFI fuel delivery system consists of an in-tank fuel pump, fuel filter, fuel feed and return lines, and a TBI unit which includes fuel injectors and a pressure regulator.


The electric fuel pump(s) are connected in parallel to the ECU/ECM and are activated by the ECU/ECM when the ignition is turned ON and the engine is cranking or operating. If the engine stalls or if the starter is not engaged, the fuel pumps will stop in about one second. The fuel is pumped from the fuel tank, through the supply line and the filter, through the pressure regulator, fuel rails (EFI) and to the injectors, with excess fuel being returned to the fuel tank. The fuel tank pump supplies fuel to the chassis mounted fuel pump or to the throttle body pressure regulator.

Vehicles with EFI have two electric fuel pumps; one is mounted in the fuel tank and is integral with the fuel level sending unit and the other, a chassis-mounted pump, is located in front of the rear axle either on the right or left side. The chassis mounted fuel pump is a constant-displacement, roller-vane pump with a check valve to prevent fuel from flowing back into the tank. This pump has a flow rate of 33 gallons per hour and maintains a minimum pressure of 39 psi. An internal relief valve opens at 55-95 psi to protect the system from excessive pressure. The pump is mounted under the vehicles except on the Eldorado, where it is mounted in front of the right rear wheel. A fuel filter is mounted on a bracket at the lower left front of the engine. On Seville models the fuel filter is mounted on the left side of the frame near the fuel pump.

Vehicles with DFI have one, in-tank, fuel pump. The fuel filter is located on the left side of the chassis just ahead of the rear axle.


The fuel filter is located on a bracket on the lower left front of the engine or on the frame near the right rear wheel. The filter consists of a casing with an internal throwaway type paper filter element. FUEL PRESSURE REGULATOR The EFI fuel pressure regulator, located on the fuel rail at the front of the engine, maintains a constant 39 psi pressure across the fuel injectors. The DFI fuel pressure regulator is integral with the throttle body and cannot be serviced separately. The regulator contains an air chamber and fuel chamber separated by a spring-loaded diaphragm. The air chamber, on EFI systems, is connected by a hose to the throttle body assembly. The pressure in the air chamber of the regulator is identical to the pressure in the intake manifold. The changing manifold pressure and the spring control the action of the diaphragm valve, opening or closing an orifice in the fuel chamber of the regulator. At this point excess fuel is routed out of the regulator and back to the fuel tank. FUEL INJECTOR The fuel injector, on EFI systems, is a solenoid operated pintle valve that meters fuel to each cylinder. The injectors are controlled by an electronic pulse signal from the ECU. When energized, the valve opens for precisely the proper amount of time to spray the exact amount of fuel droplets required by the engine. When the injector is de-energized, it prevents any further fuel flow to the engine.

On the EFI engine, the injectors are located on the intake manifold above the intake valve of each cylinder. The eight injectors are divided into two groups of four each. Cylinders 1, 2, 7, and 8 form group 1 and the remaining injectors form group 2. All four injectors in each group are opened and closed simultaneously; the two groups operate alternately.

The DFI system uses two fuel injectors located in the throttle body. They are controlled by the electronic control module (ECM) and meter the atomized fuel into the throttle bore. Each injector contains a spring loaded ball valve controlled by a solenoid. When the ball valve is lifted from its seat by the solenoid plunger, fuel is fed through the atomizer/spray injector nozzle.

Air Induction System

The air induction system is made up of the throttle body assembly, idle speed control, and the intake manifold.


Air for combustion enters the throttle body and is controlled by the throttle valves which are connected to the accelerator pedal linkage, much like a conventional carburetor. The throttle body consists of a housing with two bores and two shaft mounted throttle valves. The throttle valves are pre-set slightly open when the throttle lever is resting against the idle stop position. The adjustment is not to be tampered with. An adjustable set screw on the front of the throttle body adjusts an idle bypass air passage incorporated within the throttle body and allows a regulated amount of air to bypass the throttle valves, adjusting warm engine idle speed.

A large port on top of the EFI throttle body contains the fast idle valve. Starting 1978 on EFI equipped Sevilles, a solenoid operated idle air compensator is added to provide more air to the engine when the air conditioner clutch is engaged at idle.


The EFI system fast idle valve, installed on the top of the throttle body, consists of a plastic body that houses an electric heater, a spring and plunger, and a temperature sensitive unit.

The fast idle valve is connected electrically to the fuel pump circuit through the ECU. When the engine is started cold, the open valve allows extra air to bypass the throttle valves.

The heater warms the thermal element which expands and forces the spring and plunger toward the air orifice, restricting the flow of extra air and gradually reducing the engine speed to the normal idle rpm. The fast idle valve has no effect after the thermal element reaches about 140°F. The rate at which the valve closes is a function of time and temperature. The warmer the air, the faster the valve closes. At 68°F (20°C) the valve will close in about 90 seconds and at -20°F (-29°C) the valve will require about 5 minutes to close.


The DFI idle speed control subsystem is controlled by the ECM. The system acts to control the engine idle speed using a small electric motor which, when used in conjunction with the throttle switch, adjusts idle speed by opening or closing the throttle valves. When the engine is cold, the idle speed motor opens the throttle valve to provide faster warm-up time, and as such acts as a fast idle device. INTAKE MANIFOLD The intake manifold is basically the same as those installed on carbureted engines. There are, however, a few minor differences: On EFI systems only air travels through the intake manifold. There is a hole above each cylinder for injector installation. A port is made available for the installation of the air temperature sensor. There is no exhaust heat cross-over passage. The exhaust passage from the right cylinder head is for EGR only.

On the DFI system, both air and fuel travel through the intake manifold, much in the same manner as on a carbureted engine.

Engine Sensors

All the engine sensors are electrically connected to the Electronic Control Unit (ECU) or the Electronic Control Module (ECM). Each of the sensors operates independently, monitors a specific engine operating condition, and transmits this information via electronic signal to the ECU/ECM. The sensors continuously send information signals to the ECU/ECM while the ignition switch is in the On or Start position.


The manifold absolute pressure (MAP) sensor monitors pressure changes within the intake manifold which are the direct result of engine load, speed, and barometric pressure. As pressure in the intake manifold increases, additional fuel is required. The MAP sensor sends this information to the ECU/ECM so that the length of time the injectors are energized is increased or decreased accordingly.

The sensor is mounted within the electronic control unit. A manifold pressure line is routed with the engine harness and is connected to the front of the throttle body at one end to the MAP sensor at the other end.


The throttle position switch (TPS) is mounted to the throttle body, connected to the throttle valve shaft, and monitors the opening or closing of the throttle valves. The switch senses the shaft movement and position and transmits electrical signals to the ECU/ECM. The ECU/ECM processes these signals to determine the fuel requirement for the engine. MANIFOLD AIR TEMPERATURE SENSOR (MAT) The MAT sensor is used on the DFI system and is installed in the intake manifold in front of the throttle body. This sensor measures the temperature of the air/fuel mixture in the intake manifold and provides this information to the ECM. COOLANT TEMPERATURE SENSOR This coolant temperature sensor is used on the DFI system and is installed in the right front corner of the engine directly below the thermostat. The sensor provides data to the ECM for fuel enrichment during cold operation, for idle speed control, ignition timing and EGR operation. TEMPERATURE SENSORS These sensors are used on the EFI system. The two air and coolant temperature sensors vary electrical current resistance as a function of temperature. Low temperatures provide low resistance and vice versa. Voltage changes across each sensor are monitored by the ECU.

The air temperature sensor is located on the rear of the intake manifold and is connected to the engine harness. The coolant temperature sensor is located on the heater hose fitting at the rear of the right cylinder head or on the right side of the block below the thermostat.

The sensors are identical and completely interchangeable.


On EFI systems the speed sensor is incorporated within the ignition distributor, and consists of two components. The first is a plastic housing containing two reed switches. The second is a rotor with two magnets attached to it and rotating with the distributor shaft.

The rotation of the magnets past the reed switches causes them to open and close, providing two signals: one for synchronization of the ECU and the proper injector group with the intake valve timing; and the engine rpm for fuel scheduling.

On DFI systems the engine speed signal pulses are picked up by an electronic module in the distributor. The pulses are sent to the ECM where they are used to calculate engine speed and spark advance.


The oxygen sensor system used on 1981 and later cars controls fuel injection quantity by monitoring the amount of oxygen present in the exhaust gases and sending this information to the ECM, which adjusts the amount of fuel injected to provide the ideal air/fuel mixture ratio (14.7:1). The oxygen sensor is attached to the exhaust system ahead of the catalytic converter. When the oxygen sensor system is controlling the air/fuel mixture, the DFI system is said to be in closed loop operation. When the oxygen sensor is not controlling the air/fuel mixture (engine cold, etc.) the DFI system is said to be in open loop operation. BAROMETRIC PRESSURE (BARO) SENSOR This unit senses ambient or barometric pressure and provides information to the ECM or ambient pressure changes due to altitude and/or weather. This sensor is used only on the DFI system and is mounted under the instrument panel near the right-hand A/C outlet. The sensor's atmospheric opening is covered by a foam filter. ELECTRONIC CONTROL UNIT/MODULE (ECU/ECM) The electronic control unit/module (ECU/ECM) located under the instrument panel or in the glove box is a pre-programmed computer. The ECU/ECM is electrically connected to the vehicle's power supply, all of the EFI/DFI system electrical components, plus the EGR activation solenoid and other emission controls by a harness routed through the firewall.

When the ECU/ECM is energized by the turning ignition switch ON , it receives information from all of the engine sensors and activates the fuel pump(s), fast idle valve, fuel injectors, and emission control components.

The commands for proper air/fuel ratios for various driving and atmospheric conditions are designed into the ECU/ECM. As the electronic signals are received from the sensors, the ECU/ECM analyzes the signals and computes the exact fuel requirement for the engine. The ECU/ECM then causes the fuel injectors to open for a specific amount of time. The duration of time the injectors are open varies as the engine operating conditions change.

The electronic control units are calibrated differently depending on where the car is sold (California or 49 states) and in which vehicle the unit is installed. Each ECU/ECM is labeled for its intended use. The proper unit must be used for each application.