This basic description is part of a set of pages which deal with basic fuel injection concepts. Some variation occurs from manufacturer to manufacturer.

Note: There are two fuel pumps on most marine EFI gas engines. A low pressure primary fuel pump draws fuel from the fuel tank and a high pressure fuel injector pump pressurizes the fuel lines to the injectors. Both have to be working properly

These should be tested  to verify operation, as a starting point. Many "No start" situations are traced to this issue.

They can be unplugged from the engine and activated with a set of jumpers hooked to 12 volts. Fuels pump are polarity sensitive. Obvious amperage draw but the pump doesn't run means either it's just stuck or it's time for a new one. There are several other places on this web site that I cover fuel pumps so I won't cover that here. See this and there are other pages also.


The ECM is the control center for the fuel injection system. It constantly monitors information from various sensors (engine temperature, throttle opening, engine speed, air temperature and pressure) and controls the systems that affect engine performance (engine timing and injector pulse width).

There are three types of memory storage within the ECM: ROM, RAM, and EEPROM.

Read Only Memory (ROM) is the permanent memory inside the ECM. The ROM contains the overall control programs and once programmed cannot be changed. The control program is the list of instructions the ECM will follow in performing its routines. The ROM memory is non-erasable and does not need power to be retained.

Random Access Memory (RAM) is the microprocessor "scratch pad". The processor can write into, or read from, this memory as needed. The ECM uses RAM to store temporary values and data like coolant temperature or manifold pressure signals. This memory is erasable and needs a constant supply of voltage to be retained.

Electronic Erasable Programmable Read Only Memory (EEPROM) is the portion of the ECM that contains the different engine calibration information that is specific to each marine application. Information like fuel curve, spark advance and default values are stores in EEPROM. This type of memory will retain information until erased for insertion of new information. Changing EEPROM memory requires special equipment and is usually not preformed at the dealership without factory assistance.

The ECM supplies 5 or 12 volts to power various sensors or switches. This is done through resistance in the ECM which is so high in value that a test light will not light when connected to the circuit. Care should be taken to use a 10 meg-ohm input impedance digital meter for accurate readings.

The ECM can also perform a diagnostic function check of the system. It can recognize operational problems and store a code or codes which identify the problem areas to aid the technician in making repairs.

The following sensors interact with the ECM

See Diagram


The Manifold Air Temperature Sensor (MAT) is a two wire sensor mounted on the under side of the intake air plenum. It measures the temperature of the incoming air. Low temperature produces high resistance, while high temperature causes a low resistance. A failure with the map sensor system will store in the ECM a failure code of 23.


The Manifold Absolute pressure Sensor (MAP) is a three wire sensor and is mounted at the rear of the intake plenum. It is a pressure transducer that measures the changes in intake manifold pressure caused by engine load and speed. The MAP sensor also is used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes. Signal voltage will vary from 1-1.5 volts at idle to 4-4.8 volts at wide open throttle (WOT)). A map sensor failure will log in a #33 failure code.


The Coolant Temperature Sensor (CTS) is located on the port side of the thermostat housing. It sends a signal to the ECM letting it know if the engine is warm or cold. The voltage is high when the engine is cold and low when the engine is warm. The CTS is a three wire sensor using a 5 volt signal from the ECM. A typical reading is 1 60 deg. and if the sensor fails a 14 failure code is stored inside the ECM.


The Knock Sensor is mounted in the lower right side (starboard) of the engine block. When abnormal engine vibrations occur because of spark knock, the sensor produces a signal that is sent to the Electronic Spark Control Module (ESC).


The Electronic Spark Control Module (ESC) is located under the cover of the electrical component mounting bracket which is on the top of the engine, along side of the intake plenum;

The module receives signals from the Spark Knock Sensor if engine knock is present and it grounds a 8-10 volt signal from the ECM to retard timing. It is important to use the correct Electronic Spark Control Module (ESC) for the engine application because they are "tuned" to each engine. An incorrect ESC module will not recognize spark knock occurring and engine damage could result.


The Position Throttle Sensor (TPS) is mounted on the underside of the throttle body assembly. As the throttle is opened the TPS sends out a changing voltage signal to the ECM so that it can adjust the fuel delivery. The TPS also signals the ECM when the throttle is opened rapidly so it can add extra fuel for acceleration. The TPS is a 3 wire sensor using a 5 volt signal from the ECM. Signal voltage will vary from .7 volts at idle to 5 volts at wide open throttle (WOT). If the TPS fails the ECM will hold a 21 failure code.


The Idle Air Control Valve (IAC) is mounted into the back side of the throttle body assembly and controlled by the ECM. This valve controls engine idle speed by controlling the amount of air that bypasses the throttle valves through the idle air passage in the throttle body. It also prevents stalls due to a change in engine RPM. The valve moves in and out of the idle air passage to decrease or increase air flow as required.


A Distributor Reference Signal (Dist. Ref) is sent to the ECM from the Electronic Ignition Module in the EST distributor housing. This reference signal is the timing signal for pulsing the fuel injectors as well as the RPM counter for the ECM.


Two discrete switch inputs are used to identify abnormal conditions that may effect engine operation. a low oil pressure switch and low drive unit fluid level switch are wired to the ECM. They signal the module of low oil pressure or low drive unit fluid.The ECM will put the engine into the "Power Reduction Mode" to protect it from possible damage. Both discrete switches are in a normal open mode when the engine is running.


The provision for communicating with the ECM is the Assembly Line Diagnostic Link (ALDL) Connector. It is part of the EFI engine wiring harness and is a 1 0-pin connector which is electrically connected to the ECM. The coded stored in the ECM can be read through the ALDL connector.

There are three fuses #16, #1 7, #18 located under the cover on the Electrical Component Mounting Bracket. One 1-15 Amp. fuse is for the fuel pump and relay. A 10 amp fuse for the ECM/injectors and a 10 amp fuse for the ECM/ Battery. There are spare fuses located inside the electrical component mounting brackets.

The Lanyard Stop Circuit #20 is a safety feature incorporated in boats to stop the engine in the event that the operator is removed from a safe control position during normal operation. The Lanyard Stop Switch is a normally open switch that when closed will cause the ECM to cease engine operation.

The harness Connector for Dual Engine Data Link Cable #21 connects both ECM together on dual engine rigs. If one engine goes into a power reduction mode, the second one will go into a power reduction mode. There are six different modes of engine operation.


With the ignition switch in the start position the ECM will turn on the fuel pump relay. The electronic fuel pump (located in the vapor separator tank) runs and pressurizes the fuel in the fuel rail (34-38 PSI). The ECM then checks the coolant Temperature Sensor (CTS) and Throttle Position Sensor (TPS) to determine the proper air/fuel ratio for starting. The ECM controls the fuel delivered to the engine by changing the pulse width of the injectors.


When the engine is running the ESC checks the inputs from the following sensors to calculate the required air/fuel ratio:

1. Distributor Reference Signal for engine RPM 2. Manifold Absolute Pressure Sensor (MAP) 3. Manifold Air Temperature Sensor (MAT)
4. Coolant Temperature Sensor (CTS)

Higher RPM or higher Manifold Absolute Pressure (equals lower vacuum in the manifold) or Lower Manifold Air Temperature or Lower Coolant Temperature signals the ECM to provide a richer fuel/air ratio for the engine.

Lower RPM or lower Manifold Absolute Pressure (equals higher vacuum in the manifold) of higher Manifold Air Temperature or higher Coolant Temperature signals from these sensor would cause the ECM to provide a leaner fuel/air ratio to the engine.


Rapid changes in Throttle Position Sensor (TPS) and Manifold Absolute Pressure Sensor (MAP) signals to the ECM will cause the ECM to provide extra fuel to the engine. The ECM achieves this by holding the fuel injectors OPEN for a longer period of time.


If a engine floods it can be cleared by opening the throttle half way. Open the throttle handle until resistance from the secondary throttle is felt. The ECM will then open or "pulse" the fuel injectors at an air/fuel ratio of 20:1. The ECM will hold the air/fuel ratio at 20:1 as long as the throttle stays half open and the engine RPM is below 300. If the throttle is more than 75% or less than 50%, the ECM returns the air/fuel ratio to the richer "starting" ratio.


The ECM cuts off fuel delivery to the engine; when the key is off (to prevent dieseling), when no distributor reference pulses are sent (means the engine is not running) and at high engine RPM (overspend protection- the ECM will pulse the fuel injectors until RPM falls to a safe level)


The two discrete switches and Coolant Temperature Sensor are used by the EFI system to identify abnormal conditions that affect engine operations. If engine oil pressure drops too low, or if drive unit fluid leaks out and the fluid reservoir level drops too low, or if the engine coolant temperature raises too high, these switches will send a signal to the ECM. When the ECM receives this signal it causes the engine to go into the "power reduction mode". The "power reduction mode" allows normal fuel injection and full power up to 2,800 RPM. Above 2,800 RPM fuel delivery is limited to four injectors (four injectors are disabled) until engine RPM falls below 1,200. The engine then will resume normal operation with all eight injectors. This will allow the operator good maneuverability at low speed.

Go to diagram?

Go to part 2?

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