Fuel Injector Return Volume Measurement

Fuel Injector Return Volume Measurement

Due to its operating principle, the return volume of the Common Rail injectors consists not only of the leaked oil volume. The greater part of the injector return consists of the control oil volume of the injectors. The control oil volume is approximately the same as the volume injected into the combustion chamber.
Wear or a defect in the injector can lead to an increase in the return volume. Progressively higher return rates are accompanied by an increase in the amount of fuel that the high-pressure pump must deliver to the rail in order to maintain specified pressure levels. If the return volume exceeds certain limit values, the high-pressure pump can no longer adequately fill the rail and the pressure collapses. This can lead to the following effects:
- Engine jolts, misfires
- Engine shuts down
- Engine fails to start, especially when hot and with low fuel level in tank

This procedure can be employed to measure injector return rates using the return flow tester.

Connection of the measuring equipment
The return volume measuring device must be connected as follows:
- Test probe 1 at MFK1: Measurement of fuel temperature in return line
- Test probe 2 MFK2: Measurement of the fuel return volume
- The hydraulic line "IN" must be connected to the return line of the injector to be measured. The "OUT" line must be connected to the corresponding connection of the return line.

NOTE: For measurements on injectors of the CRI 2.5 (solenoid injectors starting with N47TU and N47C ) generation the special tool 13 0 310 must be used.

- The electrical connector located on the hydraulic line "IN" must be connected to the corresponding socket at the measuring equipment.
- The positive and negative connections must be connected to the vehicle electrical system.

Notes on using the measuring equipment
- The measuring equipment must never be operated without connection to the vehicle network.
- The fan openings on the back must not be closed.
- The measuring equipment must be set up in such a way that as much cool air as possible can be drawn in.

Test preconditions
- Engine at operating temperature (only if engine still starts; measurement at starting engine speed can also be carried out with the engine cold).
- Fuel level sufficiently full (reserve fuel lamp must not light)
- No fault in fuel supply system
- No external leaks in common rail system
- No electrical faults related to fuel supply or the common rail system

Principle of the measurement
A flow sensor is integrated in the return volume measuring device; this works with a ring piston according to the volumetric principle. The sensor can measure volume flows of 10 to 500 cm3/min.
To supply the sensor with fuel with a lower and more even temperature, a cooler and electric fan are integrated.
The fan openings must not be blocked.
The temperature of the fuel in the return line is an important secondary factor when the measurement is conducted (only with engine running). This is why the monitoring point for the temperatures processed by the diagnosis system is located on the tester's input.

Sequence of the measurement

Measurement with the engine running:
The measurement runs at an engine speed of 2000 rpm; the engine speed is raised automatically. Prior to the return volume measurement, the rail pressure is raised to 1600 bar. Here, it is determined whether the system still reaches the maximum rail pressure. If the 1600 bar is not reached, the highest rail pressure reached in this test is stored and used as a second test point for the return volume measurement.
Subsequently, the following measurements are made at 2000 rpm:
- Return volumes at 300 bar
- Return volumes at 1600 bar (or at the maximum rail pressure achieved)

Measurement at starting engine speed:
If the engine no longer starts, the measurement can also be carried out at starting engine speed. No specific engine temperature is necessary for this.

General notes

Sales / distribution of the measuring equipment:
Currently, the measuring equipment is planned for use and has been delivered in the following countries:
- Germany
- France
- Italy
- Spain
- Great Britain
- Belgium

Diagnosis report:
Once the return-flow test has been conducted in a case leading to a warranty claim, any mandatory-return parts must be accompanied by the part's diagnosis protocol, while the protocol for parts without a return requirement should be entered in the vehicle's records file.