Fuel pressure regulator vacuum pulsation (ignition triggered) - idling

The purpose of this test is to assess vacuum operated fuel pressure regulators to determine injector activity.

Connection Guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test


Warning: When attaching or removing secondary ignition pickups from damaged HT leads there is a danger of electric shock. To eliminate this possibility, switch off the ignition before attaching or removing the secondary igntion pickup.


  1. Disconnect the vacuum hose from the fuel rail pressure sensor and plug the hose.
  2. Connect the First Look Sensor into PicoScope channel A and use the hose size adaptor to connect onto the fuel rail pressure sensor.
  3. Connect the Secondary Ignition Pick Up lead into PicoScope channel B and around the HT lead for number 1 cylinder. Ensure the Secondary Ignition Pick Up has a good earth.
  4. Start the engine and run at idle.
  5. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  6. Start the scope to see live data.
  7. With your live waveforms on screen stop the scope.
  8. Stop the engine and ensure the ignition is off.
  9. Use the Waveform Buffer and Zoom tools to examine your waveform.

Example waveform

Waveform notes

This waveform was taken using secondary ignition number 1 as trigger on Channel B and the FirstLook sensor signal on Channel A. Let us consider what happens when an injector is opened, as it affects the regulator. The sudden drop in fuel pressure causes a fluctuation in the regulator. The fuel pump then compensates to fill the void left by the exiting fuel, thus returning the regulator to its starting position. In this test we can view the resulting waveform caused by this action. Notice the sharp change as the injector is opened followed by the compensation curve back to the base state.

The waveform displayed will depend on the injector firing order. In this case the injectors are sequential (each injector fires at a present time before the ignition). In the above example it can be seen that the drop in fuel pressure for number 4 cylinder is slightly less than the others, this may suggest a partially blocked injector. In order to identify the cylinder containing the offending injector, shift just over 360 degrees before the ignition spark.

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.


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.

Suitable accessories

  • FirstLook Engine Diagnostic Sensor


  • Secondary ignition pickup (capacitive with BNC)


  • WPS500X Pressure Transducer Kit (with carry case)


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2 comments | Add comment

Bernhard List
June 01 2016

To make things simple I connected my WPS500X to the Schrader Valve on the Fuel Rail and used the ignition paddle on the fuel injector. This saves me from guessing which injector caused which drop by moving the paddle to the next coil until you have the offender. I had a weeker injector but it was with a 0.7psi deviation from the best injector within limits (max. 1.5psi)

Andy Macfadyen
October 11 2015

Looking at the trace it looks like cylinder #4 is the one with blocked injector.
Injection on a 4 stroke with a fully sequential injection system starts close to TDC at the end of exhaust/start of intake stroke; ie. to identify the cylinder shift from just over 360 degrees from ignition spark.

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