WPS500X In-cylinder compression – idle (petrol)

The purpose of this test is to evaluate the in-cylinder efficiency of a petrol engine at idle speed using the WPS500X pressure transducer.

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.

  • Ensure that the WPS500X is fully charged before use.
  • To avoid possible damage to both engine and equipment ensure all connections are clean and dry.


Disable the fuel and ignition systems for the cylinder to be tested.

How to perform the test

  1. Disable the fuel and ignition systems for the cylinder to be tested.
  2. Connect the fully charged WPS500X pressure transducer to PicoScope Channel A.
  3. Switch on the WPS500X and wait for the self-test to complete (LED will scroll from range 1 to 3 and revert to 1). Do not connect the compression hose until the self-test is complete.

  4. Remove the spark plug.
  5. Assemble the compression hose with the correct thread adapter and install into the spark plug hole.
  6. Connect the WPS500X to the compression hose.
  7. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  8. Start the engine and allow it to idle.
  9. Start the scope to see live data.
  10. With your live waveform on screen stop the scope.
  11. Turn off the engine.
  12. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Example waveforms

Waveform notes

During the various stages of crankshaft rotation, cylinder pressures may be either positive or negative.
For the purpose of this test:

Atmospheric pressure = 0 bar

Any value above 0 bar = Positive pressure

Any value below 0 bar = Negative pressure (vacuum)

All values are typical and not specific to all vehicle types.

Channel A indicates the rise and fall in-cylinder pressure with the engine at idle speed.

Refer to vehicle technical data for specific test conditions and results.

  1. Peak positive pressure recorded during idle (3.967 bar) confirmed using the signal ruler. The value is recorded in the ruler legend (marker 4).
  2. Marker indicating 0 bar or atmospheric pressure
  3. Compression tower indicating the symmetrical rise and fall in cylinder pressure during idling.
  4. The ruler legend recording the numerical pressure, time and degree values relative to the position of the signal, time and rotation rulers.
  5. Expansion pocket (negative pressure -698 mbar) formed as the piston descends the cylinder during the expansion stroke. The negative pressure value is indicated by the signal ruler and recorded in the ruler legend (marker 4)
  6. Rotation ruler handles’ rest position. Click on the handle and drag to positions on the waveform that align with two consecutive TDC compression towers (marker 1) This will denote 0–720 degrees of rotation of the crankshaft relative to TDC/peak compression.
  7. Rotation ruler partitions can be added here by clicking on the Rulers button (marker 7) and selecting four rotation partitions from the popup box. The distance/time between the rotation rulers (marker 6) will now be split into four equal partitions to indicate 180-degree rotations of the crankshaft relative to the positions of the rotation rulers (placed at TDC compression stroke).
  8. The time ruler handles are located at the bottom left-hand corner of the waveform. Drag both time rulers to align with the 0 and 360-degree rotation ruler partitions in order to measure the engine speed recorded in the frequency / RPM legend (marker 9).
  9. The frequency/RPM legend displays the engine RPM relative to the position of both time rulers.
  10. Arrows denoting the direction of piston travel labelled with the relevant stroke of the four-stroke cycle.
  11. Zoom tools at your disposal to zoom to two consecutive compression towers
  12. Intake pocket (negative pressure -698 mbar) formed as the piston descends the cylinder during the intake stroke. The negative pressure value is indicated by the signal ruler and recorded in the ruler legend (marker 4).

Waveform Library

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

Further guidance

Peak cylinder pressure is achieved as the piston ascends the cylinder during the compression stroke. (Intake and exhaust valves closed).

Using the Signal Rulers (marker 1) we reveal the cylinder compression peaks at 3.967 bar which appears low but quite normal given the throttle is closed and starving the engine of airflow (low air intake = low compression). However, we can now see repeated, even and symmetrical compression towers as the crankshaft rotates and more importantly, events taking place between compressions that could never be visible with a compression tester. Marker 2 denotes 0 bar (atmospheric pressure) where the cylinder pressure should remain from approximately 180 to 360 degrees of crankshaft rotation during the exhaust stroke

NB! Peak cylinder pressure of the compression stroke can be considered as Top Dead Centre (TDC)

At the base of each compression tower during the expansion stroke, you can see the expansion pocket (marker 5) dropping below 0 bar, indicating the cylinder pressure to momentarily drop to negative (vacuum). This indicates adequate sealing of both intake and exhaust valves that should remain closed as the piston descends down the cylinder towards the end of the power stroke, referred to here as the expansion stroke as there is no combustion. Valve timing, the integrity of the piston compression rings and cylinder face, can also be confirmed by looking at the expansion pocket. You can measure the depth of the expansion pocket (and so the vacuum level) with the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) at -698.6 mbar.

At approximately 360 degrees of crankshaft rotation, you can see the commencement of the intake pocket dropping below 0 bar indicating the cylinder pressure to now fall into a vacuum. This indicates adequate sealing of the exhaust valve that will now close as the piston descends the cylinder during the intake stroke. Valve timing, the integrity of the air intake, piston compression rings and cylinder face, can also be confirmed by looking at intake pocket. The depth of the intake pocket (the vacuum level) can be measured with the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) at -698.6 mbar.

In most petrol engines at idle speed (throttle closed) the depth (vacuum level) of the expansion pocket is equal to the depth of the intake pocket. Identified here by the signal ruler (marker 5)

Engine speed (= Frequency x 60)

You can use the time rulers (marker 8) placed at the 0-degree rotation ruler and 360-degree rotation partition, to measure and display the engine speed. The time it took (frequency in Hz) for the crankshaft to rotate 360 degrees (measured by the time rulers) is multiplied by 60 to reveal the engine speed where the value (719 RPM) will be displayed in the Frequency / RPM legend (marker 9).

The rotation rulers are used to denote 0 to 720 degrees of rotation about the captured cylinder pressure waveform. Given that peak cylinder pressure occurs at TDC of the compression stroke, positioning the rotation rulers at two consecutive compression peaks/towers will denote 0–720 degrees of crankshaft rotation relative to TDC / peak compression.

Based on our knowledge of the four stroke cycle, we know the events that should take place between the rotation rulers at 0 and 720 degrees (TDC Compression to TDC Compression). To make the diagnosis easier, you can divide the distance between 0 and 720 degrees into four equal divisions. This will reveal the position of the crankshaft at key stages throughout the four-stroke cycle (TDC and BDC). When you know the position of the crankshaft, you can identify each of the four-stroke cycles between compression events and attributing anomalies found to specific four-stroke events/activities.

You can read more about Compression testing and find our troubleshooter for compression testing on our training page.


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

  • WPS500X Pressure Transducer Kit (with carry case)


Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

2 comments | Add comment

January 10 2019

Thank you for your feedback. Now fixed.

Carsten Gotfredsen
December 14 2018


Figure: Zoomed to display two consecutive compression towers

Zoom does not work, can not read what it says.

Your email address will not be published. Required fields are marked *

Guided test: In-cylinder compression – idle (petrol)