Products suited to this guided test*
  • FirstLook Engine Diagnostic Sensor

  • Secondary ignition pickup (capacitive with BNC)

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Fuel pressure regulator vacuum pulsation during idling

The purpose of this test is to assess fuel injector activity using the First Look sensor by measuring fuel pressure regulator diaphragm fluctuations with the engine at idle.



Uninsulated HT pickups are designed to clip around double-insulated HT leads only – they are not designed for direct connection to a hazardous live voltage.

To prevent injury or death, when connecting or disconnecting an HT pickup:

  1. switch off the ignition
  2. clean the HT leads
  3. inspect them for damage
  4. clip the HT pickup over the desired undamaged HT lead
  5. ensure all test leads are kept clear of hot or rotating parts before starting the engine


How to perform the test

View connection guidance notes.

  1. Disconnect and seal off the vacuum hose from the fuel rail pressure regulator.
  2. Connect the First Look Sensor to PicoScope Channel A. Use the hose size adaptor to connect onto the fuel rail pressure regulator vacuum port.
  3. Connect an HT pick up around the HT lead for cylinder number 1 and to PicoScope Channel B. Ensure the HT pick up earth lead has a good earth connection.
  4. Start the engine and run at idle.
  5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  6. Start the scope to see live data.
  7. With your waveforms on screen stop the scope.
  8. Stop the engine and ensure the ignition is off.
  9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

These waveforms have the following characteristics:

  • A series of pulses (Channel A) with each having a decaying oscillation.
  • Their peak to peak amplitudes are around 400 to 450 mV.
  • The pulse oscillations decay to around 0 mV over a period of 35 ms.
  • The secondary voltage (Channel B) shows cylinder number 1’s ignition event, as a reference to identify the injector pulses.
  • The first pulse (Channel A) begins just before cylinder number 1’s ignition event (Channel B).
  • The peak to peak amplitude of cylinder number 1 pulse is lower than that of the other pulses (indicating a possible fault – see further guidance).

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Fuel pressure regulator pressure waveform.

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.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

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Guided test: Fuel pressure regulator vacuum pulsation (ignition triggered) - idling