Injection nozzles provide each cylinder with the required fuel and simultaneously atomise the fuel so that it can burn most effectively in the combustion chamber.
Here, HELLA explains how injection valves work and how to identify potential fault symptoms. You will also find useful tips for performing troubleshooting work on injection valves with the engine running and switched off.
How do injection nozzles work?
The purpose of injection valves is to precisely inject the fuel quantity calculated by the control unit in all engine operating states. To ensure the fuel is atomised effectively while minimising condensation losses, a certain engine-specific distance and injection angle need to be observed.
Injection valves are actuated electromagnetically. The control unit calculates and controls the electrical pulses for opening and closing the injection valves on the basis of the current sensor data regarding the engine operating state.
Injection valves comprise a valve body, which houses a magnet winding and a guide for the nozzle needle, and a nozzle needle with magnet armature. When the control unit applies voltage to the magnet winding, the nozzle needle rises up from its valve seat and releases a precision bore hole. As soon as the voltage drops, a spring presses the nozzle needle back onto the valve seat and the bore hole is closed again.
The flow rate when the injection valve is open is precisely defined by the precision bore hole. In order to inject the fuel quantity calculated for the operating state, the control unit calculates the opening time for the injection valve in a comparison with the flow rate. This ensures that the precise fuel quantity is always injected. The design of the valve seat and the precision bore hole ensure that the fuel is optimally atomised.
Symptoms of a faulty injection nozzle
If an injection valve is faulty or not working properly, it can cause the following fault symptoms:
- Problems starting
- Increased fuel consumption
- Loss of power
- Fluctuating idle speed
- Poor exhaust emission performance (e.g. emissions test readings)
Potential consequential damage:
- Shorter engine service life
- Damage to the catalytic converter and diesel particulate filter
Causes of failure
If the injection nozzle is faulty or has reduced functionality, the case may be:
- A blocked filter screen in the injection valve as a result of contaminated fuel
- A poorly closing needle valve caused by tiny dirt particles from the inside, combustion residue from the outside, additive deposits
- A clogged, closed outflow bore hole
- A short circuit in the coil
- A cable break to the control unit
Troubleshooting can be performed both when the engine is running and when it has been switched off.
Troubleshooting when the engine is running
Compare the injected fuel quantity
Using a cylinder comparison measurement and simultaneous exhaust measurement, the injected fuel quantity can be compared based on the drop in speed and the HC and CO values for the individual cylinders.
In the best case, the values will be identical for all cylinders. If there are major deviations between the values, it may be that not enough fuel is being injected (a large quantity of unburnt fuel = high HC and CO values, whereas little unburnt fuel = low HC and CO values). The cause may be a faulty injection valve.
Read off and compare the voltage and the pulse duration
The injection signal can be depicted using an oscilloscope. For this purpose, connect the measurement line to the signal line, and the other line to a suitable ground pin. With the engine running, it is possible to read off the voltage and the pulse duration (opening time) from the signal pattern.
When opening the throttle valve, the pulse duration must increase during the acceleration phase, and with a constant engine speed (around 3,000rpm) it must fall back to, or just below, the idle value. The results of individual cylinders can be compared with each other, and this may provide an indication of potential faults, e.g. a poor voltage supply.
Measure the fuel pressure and check the intake and exhaust systems for leaks
Other important tests are measuring the fuel pressure, in order to identify other components that may be faulty (fuel pump, fuel filter, pressure regulator), and checking the intake and exhaust systems for leaks, to prevent false measurement results. If the sensor has a two-pin connector, it is most likely to be an inductive sensor. In this case, the internal resistance, a potential short circuit to frame, and the signal can be determined.
To do this, remove the plug connection and check the internal resistance of the sensor. If the internal resistance value is 200 to 1,000 ohms (depending on the reference value), the sensor is OK. If the value is 0 ohms, there is a short circuit, and in the case of M Ohm there is a break.
The test for a short circuit to frame is carried out using the ohmmeter from a connection pin to vehicle ground. The resistance value must tend toward infinity. The test using an oscilloscope must result in a sinus signal of sufficient strength.
In the case of a Hall generator, only the signal voltage in the form of a square wave signal and the supply voltage are to be tested. This must result in a square wave signal depending on the engine speed. It’s worth noting that at this point the use of an ohmmeter can destroy the Hall generator.
Troubleshooting when the engine/ignition is switched off
Check the continuity of the cable connection
Check the continuity of the cable connection between the injection valves and the control unit (the circuit diagram for the pin assignment is required). In order to conduct this measurement, pull out the control unit connector and check the individual cables of the injection valve connectors to the control unit. Reference value: approximately 0 ohms.
Check the cable connection for a short circuit to frame
Check for a short circuit to frame in the cable connection between the injection valves and the control unit. With the control unit connector pulled out, measure the cables from the injection valve connectors to the control unit against vehicle ground.
Check the continuity of the injection valve coils
Check the continuity of the injection valve coils. To do so, connect the ohmmeter between the two connection pins. Reference value: approximately 15 ohms (note manufacturer specifications).
Check the injection valve coils for a short circuit to frame
Check the injection valve coils for a short circuit to frame. For this purpose, check the continuity of each individual connection pin against the valve housing. Reference value: >30 MOhm.
On a stationary test stand, it is possible to test the spray pattern of the injection nozzles when they are disassembled. Depending on the device manufacturer, it may also be possible to clean the injection valves.
Click here for more information.