Vacuum - Triggered cranking test

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

To perform this test disconnect the vacuum hose from the fuel pressure regulator and plug the vacuum hose. Then place the reducer hose on the FirstLook sensor and connect it to Channel A of the scope using the BNC to BNC lead as shown in Figure 1. Now connect the reducer hose and FirstLook sensor to the regulator port as shown in Figure 2.

Now connect the Secondary Ignition pick-up Lead to Channel B of the scope and attach the clamp to the cylinder 1 plug lead. Once you have disabled the fuel system (may not be possible on a carburated engine), crank the engine until the display pattern stabilizes. You can adjust the timebase and voltage scales to achieve best signal display.

This test is useful in the assessment of the intake air and valve system for cylinder specific defects on the intake side of the engine.

Example waveform

Waveform notes

This waveform was taken using injector 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.

AT132-2

Disclaimer
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

    £439.00

  • Secondary ignition pickup (capacitive with BNC)

    £58.00

  • WPS500X Pressure Transducer Kit (with carry case)

    £1,155.00

Share your experience

If you have any suggestions to improve this guided test please do so using the 'add comment' button.

4 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)

Bryan Brandau
January 17 2016

I don’t deny that there may be an issue with one of the injectors in the waveform, but the set up instructions say to connect to cyl #1 ign for the trigger while the waveform notes say fuel injector trace. which I believe is the difference in time between the changes in the trace of the two traces.

Steve Smith
October 19 2015

Well spotted and thank you as you are indeed correct. I will advise publishing to amend the explanation in the “Waveform notes” section

Take care…....Steve

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 vs Ignition