Intake manifold pulsation (ignition triggered) - cranking

The purpose of this test is to assess the engine condition during cranking by using the FirstLook™ sensor to observe intake manifold pulsation.

 

WARNING

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. Disable the fuel injection system before carrying out this test.
  2. Connect the FirstLook™ sensor to PicoScope Channel A and to an appropriate intake manifold vacuum source.
  3. Connect a secondary ignition pick up to PicoScope Channel B and around the HT lead for cylinder number 1. Ensure the secondary ignition pick up earth fly lead has a good earth.
  4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  5. Start the scope to see live data.
  6. Crank the engine until the waveform stabilises (3 to 5 seconds).
  7. With your waveforms on screen stop the scope.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

Some Engine Control Modules (ECM's) will operate the throttle independent to pedal operation whilst attempting to start. This will alter your waveform. If this is the case you will need to disconnect the throttle body connector and re-test.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

  • A repeating series of alternate peaks and troughs, representing the pulsations (Channel A).
  • All pulsation peak to peak amplitudes are approximately the same.
  • There are no missing or anomalous peaks, troughs or inflexions in the pulsation waveform.
  • The secondary voltage (Channel B) shows cylinder number 1's ignition event, as a reference to identify the individual cylinder pulses.
  • The number of pulsations per secondary ignition event equals the number of engine cylinders.

Waveform Library

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

Further guidance

The FirstLook™ sensor contains a piezo crystal device which converts pressure pulsations to a voltage signal output. The output can be taken as an indication of the underlying physical actions causing the pressure pulsations.

Waveform features

We check for potential engine issues by examining the intake manifold pressure pulsations. Specifically, we can compare intra-cylinder variations by looking at the effects of each of their respective inlet valve opening events.

The intake pulsations should have the following features:

  • Similar pressure pulses for every induction stroke event.
  • A well-defined trough due to the initial “suck” after the inlet valve opens and as the piston accelerates downwards on its induction stroke.
  • An upward trend after the trough as the intake piston slows down towards Bottom Dead Centre (BDC) and fresh air moves in to refill the intake manifold.
  • A peak as the next inlet valve (in the cylinder firing order) opens just before the current cylinder's induction stroke has reached BDC.

Any waveform showing periodically occurring anomalies, for example a missing trough every 4 x intake events (with a 4 cylinder engine), will warrant further investigation.

Cylinder identification

The process for cylinder identification with an intake manifold pressure pulsation waveform is:

  1. Retard the engine cycle reference points from the secondary ignition events: they should be between 20 and 35 degrees afterward. As a rule of thumb, place Phase rulers at “30/720 x the time between two consecutive ignition events” after each.
  2. Tag the cylinder number one engine phases (given that the secondary ignition event occurs during the compression stroke).
  3. Find the corresponding cylinder number one intake pulse.
  4. Mark the remaining pulsations in engine firing order, relative to the cylinder number one pulse.

Causes of waveform anomalies

Intake-related faults which may cause waveform anomalies are:

  • General intake leaks, manifold cracks, gaskets etc.
  • Burnt valves/seats.
  • Valve not closing.
  • Valve not opening.
  • Incorrect timing/valve clearance issues.
  • Badly worn cam profiles.

Other engine faults also may affect the waveform:

  • Lack of cylinder compression.
  • Exhaust issues (e.g. catalytic converter).

Exhaust valvetrain issues (e.g. those listed for the intake valvetrain above).

GT131-EN

Disclaimer

Suitable accessories

  • FirstLook Engine Diagnostic Sensor

    £449.00

  • Secondary ignition pickup (capacitive with BNC)

    £58.00

Help us improve our tests

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2 comments | Add comment

MikeW
November 20 2018

Hi Anthony, thank you for your observation. Test amended.

Anthony
May 01 2017

Disconnect electronic throttle body, newer vehicles will play with the throttle during cranking to try and start the vehicle. This will mess up a pattern as the throttle is opening and closing during test drastically changing manifold vacuum.

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Guided test: Intake manifold pulsation (ignition triggered) - cranking