WPS500X - petrol intake manifold pressure overview (non-turbo)

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

The purpose of this test is to evaluate the intake manifold pressure under varying engine running conditions from cranking, idling, wide open throttle (WOT) "snap" test, idling, to engine off.

Manifold pressure is directly related to: intake condition and flow, throttle position, valve timing and lift, engine condition, exhaust flow and any boost pressure applied by forced induction.

All numerical readings quoted in this help topic are typical and not applicable to all engine styles.

All values obtained below with the WPS500X are referenced to gauge pressure.

Intake pressure before the throttle (air inlet side, positive pressure) is described here as atmospheric pressure = 0 mbar.

Intake pressure after the throttle (engine side, negative pressure) is described here as vacuum = below 0 mbar.

Ensure that the WPS500X is fully charged before starting this test.

How to perform the test


  1. BNC to BNC test lead
  2. WPS500X Pressure Transducer
  3. TA085 Vacuum Hose
  4. TA129 Vacuum Adapter

PicoScope settings

  1. Channel A Option WPS500X Range 2
  2. Channel A ± 1 bar
  3. Timebase 5 s/div
  1. Connect BNC to BNC lead to Channel A of your scope and the BNC socket of the WPS500X transducer.
  2. Connect the vacuum hose to the inlet port of the WPS500X.
  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).
  4. Fit to any convenient inlet manifold vacuum source, using the vacuum adapter if required.
  5. Press the range button and select Range 2 (–15 to 50 psi) (do not select zoom functions).
  6. Run your scope software by pressing either the space bar on your keyboard or the "start" button in PicoScope.
  7. Crank and start the engine, and allow idle speed to stabilise.
  8. With the engine at the correct operating temperature carry out a momentary WOT snap test and allow engine idle speed to stabilise.
  9. Switch off engine and monitor intake pressure (vacuum) decay.
  10. Repeat steps 8 to 10 if required.

We advise you to recharge your WPS500X after use to ensure it is ready for future measurements.

Channel A shows the intake manifold vacuum during the various stages of the test (Steps 8, 9 and 10).

Green line (at markers) denotes atmospheric pressure 0 mbar
Black ruler (at markers) denotes idle intake vacuum, approximately -710 mbar
Blue line (at marker) denotes maximum manifold vacuum, approximately -970 mbar

Key to Example waveform

ENGINE OFF - Atmospheric pressure
ENGINE CRANK AND START - Note rapid increase in manifold vacuum
IDLE SPEED STABILISING - Varies between idle control systems
STABLE IDLE SPEED - Note the stable manifold vacuum
WOT SNAP TEST - Rapid decrease in manifold vacuum
INCREASED INTAKE "POCKET" DURING OVERRUN - Note the increase in manifold vacuum below the stable and even idle at 4
ENGINE SWITCHED OFF - Monitor the rate of decay of manifold vacuum


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

Typical values (when engine is at correct operating temperature)

1. Engine off

Before engine start the intake manifold vacuum should be identical to atmospheric pressure (zero mbar on our scope scale).

2. Engine crank and start - Approximately 1.5 seconds from minimum to maximum manifold vacuum

During engine crank and start the manifold vacuum will rapidly increase, indicating sufficient cranking speed with reduced pumping loss. Refer to "Intake pressure cranking" preset under the "Automotive" menu for closer analysis during cranking.

3. Idle speed stabilising

Idle stabilisation should settle reasonably quickly depending on engine idle control systems. "Fly-by-Wire" systems will have a larger effect on idle stabilisation vacuum than "Idle air bypass control" systems. Any loads applied here will also have an effect on idle stabilisation vacuum such as air conditioning, cooling fan operation, and alternator output/load after cranking.

4. Stable idle speed

Once idle speed stabilisation has been achieved by the engine management and all loads applied to the engine have settled, the idle speed manifold vacuum should remain reasonably stable on this timebase. For more accurate analysis at idle speed refer to "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-Idle Speed" preset under the Automotive menu.

5. WOT snap test

With the engine at the correct operating temperature, a WOT snap test should result in an instant fall in manifold vacuum, reverting to the atmospheric pressure recorded at step 1 (engine off, 0 mbar). On release of the throttle, the manifold vacuum should rise rapidly at the same rate as the fall. For more accurate analysis of the WOT snap test refer to "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-WOT Snap Test" preset under the Automotive menu.

6. Increased intake "pocket"

Given the engine speed is high but falling after the WOT test (throttle now closed) the intake manifold vacuum continues to rise above the "stable idle speed" level to form the "intake pocket". Here we are further able to confirm the mechanical efficiency of the engine and the integrity of the intake system. A small intake pocket could indicate a mechanical defect or intake leak. For more accurate analysis of the intake "pocket" refer to "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-WOT Snap Test" preset under the Automotive menu.

7. Engine switched off - Approximately 1.8 second decay time maximum manifold vacuum

During the engine shutdown period, the rate of decay is all–important and should be progressive as opposed to a rapid fall in vacuum to atmospheric pressure measured at "Engine off" (0 mbar). Once again a rapid fall here would indicate a potential engine efficiency issue or intake leakage. Refer to "Pressure Sensors > WPS500X > Pressure Transducer > Intake pressure additional tests", cylinder pressure and crankcase test to identify the area of decay.
Note: Leaks in vacuum auxiliaries such as the brake servo and vacuum switching valves will contribute to a rapid decay in vacuum.

Technical information

The internal combustion engine can be likened to a mechanical air pump, where air is drawn in through the intake and forced out through the exhaust. Engine efficiency relies heavily on this process, which is often referred to as "Engine breathing". During the intake stroke on our petrol engine below, air is drawn into the relevant cylinder, but the flow of air is met with a restriction in the form of our throttle butterfly valve. The butterfly valve will be held near to the closed position leaving a very small area for air to be drawn in and reach the cylinder on the intake stroke. A comparison can be made here with a bicycle pump, where placing your finger over the inlet to the pump while drawing back on the grip will restrict the air flow into the pump and generate a vacuum under your finger.

This test will provide you with an overview only of the sequence of events and vacuum values present within the intake manifold during the conditions 8, 9, and 10 mentioned in "How to perform the test". Should you discover an area of concern, the zoom function of your scope will enable you to analyze the waveform further. You will then need to select the relevant preset test under "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure" within the Automotive menu.

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


  • PicoScope WPS500X Vacuum tap


  • Vacuum hose


  • Universal vacuum adaptor


  • WPS500X Pressure Transducer Kit (with carry case)


Share your experience

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

2 comments | Add comment

Steve Smith
July 05 2016

Hello Bernhard, thank you for your continued feedback. 
There is an error in the title here as it should read WPS500X - petrol intake manifold pressure overview (non-turbo) and not Intake Manifold – Intake Pulses using Zoom 3 (Running)
I will ask the team to amend the title for the above Intake Manifold pre-set as you are correct NO ZOOM is required.

Thank you also for the information surrounding atmospheric pressure as you are correct at 1013 mB.
All values obtained with the WPS500X are referenced to gauge pressure, and so the scope will read zero bar at atmospheric pressure.
With regards to “zooming” using the pressure transducer, I have copied a section below from the following Guided Test that will help clarify the expected PicoScope reading

Engine idling - Zoom level

Whilst the engine is idling, select Zoom level 3 by pressing the ZOOM button 3 times on the front panel of your WPS500X. This has the effect of magnifying only the pulsations/ripple present in the waveform due to the open and close events of the cylinder head valves. Do not refer to the pressure scale on your scope when using the zoom function as only the ripple is displayed on screen, not the manifold vacuum value. With the ripple now magnified you are able to analyse the formation of the waveform, as any irregularity in the peak-to-peak formation or saw-tooth across the peaks/troughs could indicate poor sealing of the intake/exhaust valves.

This principle Bernhard is the same as AC coupling whereby we reject the DC and look purely at the AC (ripple) centred about 0 Bar.

I hope this helps, and thank you for the feedback, take care……..Steve

Bernhard List
June 01 2016

Btw. You mention not to use the zoom function in this setup but the title says Zoom 3. if no zoom is used the atmospheric pressure should be around 1013mB, not 0mB. With zoom selected I believe the static pressure is removed from the display to get a reading showing only deviation from a Zero axis.

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Guided test: Intake Manifold – Intake Pulses using Zoom 3 (Running)