Diesel Engine Fuel Pressure Control


Determining Common-rail Fuel Pressure

A fuel pressure that matches the operating conditions of the engine is calculated according to the actual injection volume that has been determined based on the signals from the sensors, and the engine speed. The ECU sends signals to the SCV to adjust the fuel pressure that is generated by the supply pump.


Determining Common-rail Fuel Pressure

Diagnostic Functions

As with the EFI system of the gasoline engine, the EFIdiesel also features a diagnostic MOBD (OBD) function. The MIL (Malfunction Indicator Lamp) illuminates if a malfunction is detected in the ECU itself or in the electrical system. The area of the malfunction will be indicated by a DTC(Diagnostic Trouble Code) number. After the malfunction has been resolved, the MIL will disappear. However, the DTC will remain stored in the ECU memory.

Check (Test) mode

The diagnosis function consisted of a normal mode and a check (or test) mode. While the normal mode performs normal diagnosis, the check (or test) mode features an increased level of sensitivity to detect even more detailed malfunction conditions.

Freeze frame data

The ECU stores in its memory the conditions of the engine at the time the malfunction occurred. The conditions that existed at that time can be later retrieved and reviewed through the use of a hand-held tester.


The ECU assumes the fail-safe mode if a malfunction occurs in some of the diagnostic items. This mode initializes the signals to their prescribed values in order to enable the vehicle to be driven.

DTC (Diagnostic Trouble Code) Display

Depending on the model, the check connector might come as a DLC or DLC3. The DTC (Diagnostic Trouble Code) can be monitored by shorting the terminals of the connector and counting the number of blinks. If no malfunctions have occurred, the number of blinks will correspond to normal conditions. One of the methods for accessing the DTCs (Diagnostic Trouble Codes) is to use a hand-held tester. The DTC numbers can be displayed on the hand-held tester screen. A hand-held tester can be used also for displaying the conditions of the engine or of the sensor signals (reference values) in addition to displaying the DTC numbers.

Active Test

During an active test, a diagnostic tester is used for outputting commands to the ECU in order to drive the actuators. This test determines the integrity of the system or parts by monitoring the operation of the actuators or by reading the engine ECU data.

Service Hint

Reading the DTC (Diagnostic Trouble Codes)

In the Repair Manual, the detection item, detecting condition and trouble area are included for each DTC, so refer to the Repair Manual when troubleshooting.

Inspection Using a Circuit Tester

Perform an inspection in accordance with the Inspection Chart for each diagnostic trouble code. The inspection method is identical to the electronic fuel injection (EFI) system of a gasoline engine.

ECU inspection

Measuring the voltage and resistance at the ECU/EDU terminals.

Relay inspection

Measuring the voltage and resistance of the relay terminals.

Sensor inspection

Measuring the voltage and resistance between the sensor terminals.

Inspection Using a Hand-held Tester

Through the use of a hand-held tester, the conditions of the ECU, EDU and sensor can be monitored on the tester. In the negative test mode, the hand-held tester can activate the actuators in order to simulate the vehicle’s operating conditions. Follow the instructions given bellow to clear the DTCs(Diagnostic Trouble Codes) that are stored in ECU memory. Execute “Clear” on the hand-held tester. Disconnect the specified fuse and the positive terminal for the battery (this procedure differs from model to model).

Related Post