WPS500X - Petrol intake manifold pressure (running)

The purpose of this test is to observe the intake manifold pressure at engine idle speed in order to analyse the dynamic changes in pressure due to valve open/close events.

Connection guidance

Note: Manifold pressure is directly related to intake condition and flow, throttle position, valve timing/lift, engine condition, exhaust flow and any boost pressure applied via 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. Connect the fully charged WPS500X to PicoScope channel A.
  2. Switch on the WPS500X and wait for the self-test to complete (LED will scroll from range 1 to 3 and revert to 1).
  3. Select range 2 and connect to the intake manifold vacuum.
  4. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Start the scope.
  6. Start the engine and run at idle.
  7. With your live waveform on screen Stop the scope.
  8. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Waveform notes

Channel A

① indicates the intake manifold vacuum value (-690 mbar) during idling with the WPS500X pressure transducer set to Range 2 with no zoom. Note the ripple evident, see Figure 3 below.

Green ② denotes atmospheric pressure 0 mbar.

Channel A

indicates the intake manifold vacuum pulsations (ripple) in relation to valve open/close events at idle speed, magnified 1000 times. The waveform no longer represents the value of the vacuum: see Diagnosis section below.

Further guidance

Channel A : When selecting zoom level 3 of your WPS500X, the manifold vacuum value obtained in Figure 2 (-690 mbar) is brought up to the zero line (black marker) so removing the static vacuum value. This reveals the dynamic pulsations hidden within the vacuum signal by magnifying the ripple 1000 times, allowing you to analyze the waveform for irregularities attributed to cylinder head valve operation.

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

Typical values (when engine is at correct operating temperature and idling with no loads applied.)

  1. Engine idling - No zoom

    Once the engine idle speed has stabilised and all loads applied to the engine have settled, the idle manifold vacuum should remain reasonably stable with a typical value of −690 mbar.

    Note:A reduction in manifold vacuum value (with reduced ripple) may indicate a mechanical defect due to pumping losses (inefficient piston sealing) or a leak in the intake system downstream of the throttle (between throttle butterfly valve and engine).

    Irregular pulsations/ripple in manifold vacuum, possibly accompanied by unstable idle speed and a low intake manifold vacuum value, may indicate cylinder head valve concerns. To identify the source of the pulsations select zoom level 3 below.
  2. 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.

Here we can see how the vacuum pulsation/ripple is formed. As the piston moves down the bore, air is drawn into the cylinder so creating a negative pulse. Now imagine 4 cylinders drawing in air at different times at high speeds. The result is the pulsations /ripple you can see in Figure 3.

Assessing engine conditions using the WPS500X and PicoScope will reveal more information about the condition of your engine than was ever thought possible given the resolution and speed of both the transducer and scope. For this reason we have to be aware that the variety of engine designs, intake, exhaust systems, and elaborate variable valve timings will all have an effect on the waveform that will differ from vehicle to vehicle.

Be very careful when analysing intake manifold pulsations. Remember what we are looking for is anomalies in waveform patterns, something irregular that stands out in a repetitive fashion.

Knowing how the pulsations are formed is key to a non-intrusive evaluation and diagnosis of an engine's condition.

  • The Trough is formed during the intake stroke as a vacuum develops above the descending piston whilst air is drawn into the cylinder.
  • The Peak is formed during the pistons transition from BDC on the intake stroke to the compression stroke. Note that dependant on engine design the intake valve closer may be delayed by up to 40 degrees after BDC of the intake stroke.
  • The Saw-Tooth is formed during periods of valve overlap where exhaust gases and intake air will momentarily blend and so the effects are felt inside the intake manifold. However, the saw tooth can also indicate potential areas of air flow disturbance due to poorly seated or sticking valves. The saw-tooth tends only to be formed when the engine is running and NOT cranking.

Once again be aware that we are looking for irregularities in the waveform and so a saw- tooth pattern across all pulsations is most likely to be normal for the style of engine under test as it is highly unlikely for every valve within the engine to be poorly seated or sticking.

When attempting to identify an offending cylinder due to an irregularity in the magnified vacuum pulsations, it is advisable to use an ignition event on an additional channel of your scope.

Below we have used the number 1 cylinder firing events and indicated the position of the crankshaft whilst highlighting the four stroke cycle between each firing event. Note how the trough formed by the piston on the intake stroke of number 1 cylinder occurs 2 pulses to the right of the firing event (offset by 2 strokes/pulses).

Figure 5

The easiest way to remember this is to think of the four stroke cycle and where does the intake stroke occur after the ignition/power event?


The Intake stroke occurs 2 pulses to the right of your ignition event.

With a firing event displayed against your vacuum pulsation waveform, using the firing order you are then able to identify the remaining vacuum pulses. In our example above we have a 4 cylinder engine with the firing order 1 > 3 > 4 > 2. Applying the firing order reveals the pulses for the remaining cylinders in the order of 3 > 4 > 2.

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 indication of the efficiency and running condition of the engine at idle speed and allow you identify vacuum deficiency caused by, air leakage, pumping loss, or intake/exhaust valve irregularity.


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)


  • WPS500X Pressure Transducer


  • Universal vacuum adaptor


  • Vacuum hose


  • PicoScope WPS500X Vacuum tap


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Guided test: Intake Manifold - Intake Pressure (Running)