Further guidance
The fuel rail acts as a fuel reservoir to supply the injectors in a multi-point injection system. The fuel rail pressure regulator acts to adjust the fuel rail pressure according to the intake manifold vacuum at its vacuum connection.
With this system, changes in pressure on the vacuum side of the regulator’s diaphragm induce changes in pressure on the fuel rail side. Conversely, changes in fuel rail pressure, for example from injector opening and closing, can affect the air pressure on the vacuum side of the regulator diaphragm.
Therefore, by removing the vacuum connection to the regulator, we can use the First Look pressure sensor to measure air pressure pulsations caused by rail pressure pulsations as the injectors open and close. Note that the First Look sensor output is not an actual measure of pressure but is a representation of component/pressure activity and used, in this case, for comparative purposes.
Waveform features
When an injector opens, fuel and pressure are lost from the fuel rail. This produces the first negative peak at the start of each pulsation event.
The action of the fuel pump causes a volume of fuel to flow into the rail and replace the injected fuel. Once the fuel volume has been replaced, the fuel’s inertia causes a rail pressure overshoot seen as the first positive pulse within each pulsation.
The remaining, decaying, oscillations result from complex fluid dynamic reactions as the fuel flow stabilises, and as the injector closes.
With constant engine operating conditions, the pulsations should start at precisely regular intervals and have equal peak to peak amplitudes.
The pulsations will occur in cylinder firing order.
Waveform diagnosis
A comparison of the initial peak to peak amplitudes across all pulsations will indicate whether the injectors are functioning equally and in a timely manner.
As the pulsations occur in cylinder firing order (in a sequential multi-point injection system), the cylinder number 1 reference (Channel B) allows us to identify which pulsation belongs to which cylinder’s injector.
In the example above, the pulsations are timely but the peak to peak amplitude of the first pulsation is lower than those in the next three pulsations.
As the test engine was a conventional 4-cylinder four stroke, with a 180° crank throw, a firing order 1-3-4-2, and sequential injection on its intake stroke, it is possible to identify the problem injector:
Ignition sparks are normally timed to occur at a crank angle Before Top Dead Centre (BTDC) on the respective cylinder’s compression stroke. The cylinder that is on its intake stroke whilst cylinder 1 is on its compression stroke is the cylinder that follows cylinder number 1 in the firing order. Therefore, the example waveforms are indicating that there is a problem with fuel injection on cylinder number 3.
Typical symptoms that might arise from injector faults are:
- Engine misfire.
- Reduced performance.
- Increased fuel consumption.
- Increased emissions.
- Difficulty starting.
Injector, or related, faults can be:
- Electrical, such as:
- Open or short circuits and high resistances.
- Control unit signal failure.
- Mechanical, such as:
- Partially blocked injector (fuel contaminant deposits).
- Weak or broken return spring.
- Internal seal failure.
- Excessive in-cylinder pressure (causing less fuel to be injected by the injector).
- Pressure regulator issues.
- Excessive or reduced fuel supply pressures.
