WPS500X - Petrol intake manifold pressure, cranking (non-turbo)

The purpose of this test is to assess the intake air and valve system operation whilst cranking the engine. Using the WPS500X pressure transducer to see the resulting pressure waves within the intake manifold.

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.


Ensure both ignition and fuel systems are disabled before starting this test.

How to perform the test

  1. Ensure both ignition and fuel systems are disabled.
  2. Connect WPS500X 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).
  4. Press the range button and select Range 3.
  5. Connect the WPS500X, via a vacuum hose, to the vehicle intake manifold.
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Start the scope.
  8. Crank the engine for around 10 s to produce the waveform.
  9. Stop cranking and with your live waveform on screen wait for the vacuum to decay (trace to 0) then stop the scope.
  10. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.


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.

Waveform notes

Channel A

① indicates intake manifold at atmospheric pressure before cranking with the WPS500 pressure transducer set to Range 3 with no zoom. ② Denotes the commencement of cranking. ③ Indicates the maximum intake manifold vacuum achieved during cranking (-65 mbar approx.). ④ Indicates "buffeting" as the rush of intake air rebounds from the stationary piston and closed throttle (cranking stop).

Green ① denotes atmospheric pressure 0 mbar.

Channel A

① indicates the intake manifold vacuum pulsations (ripple) in relation to valve open/close during cranking, magnified 10 times. The waveform no longer represents the value of the vacuum: see Diagnosis section below.

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

Channel A : When selecting zoom level 1 of your WPS500X, the manifold vacuum value obtained in Figure 2 (-65 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 10 times, allowing you to analyse 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)

  1. Engine off (no cranking)

    Before engine start the intake manifold vacuum should be identical to atmospheric pressure (zero mbar on our scope scale).
  2. Cranking. WPS500X Range 3 zoom off - Approximately 1.0 second to maximum manifold vacuum.

    During engine cranking the manifold vacuum will rapidly increase and decrease to form the pulsations (ripple) in both Figure 2 and Figure 3, indicating sufficient cranking speed with reduced pumping loss.

    With a cranking speed of 250-350 rpm, the intake manifold vacuum should reach a maximum value of approx. -65 mbar (within 1.0 second).

    Note: A reduction in maximum manifold vacuum value 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 or reduced pulsations/ripple in manifold vacuum, possibly accompanied by low intake manifold vacuum value, may indicate cylinder head valve concerns. To identify the source of the pulsations select Zoom level 1 (Step 5 below).
  3. Cranking stop - 0.5 second decay time max to min manifold vacuum.

    During the engine stop 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.
    Note: Vacuum auxiliaries such as the brake servo and vacuum switching valves will contribute to a rapid decay in vacuum if leaks are evident.

    The peak seen at the end of cranking (engine stop) can be attributed to buffeting as the rush of intake air rebounds from the stationary piston and closed throttle.
  4. Cranking - WPS500X Range 3 zoom level 1

    Select Zoom level 1 by pressing the ZOOM button once on the front panel of your WPS500X and crank the engine. 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.

Figure 4 shows 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 2 and 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 depending 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 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


  • PicoScope WPS500X Vacuum tap


  • Universal vacuum adaptor


  • Vacuum hose


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Guided test: Intake Manifold - Intake Pressure (Cranking)