WPS500X - Petrol intake manifold pressure, cranking (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.

This test evaluates the intake manifold pressure under engine cranking conditions.

Manifold vacuum during cranking can be used as an initial indication of the presence of compression (poor piston ring and combustion chamber sealing will result in insufficient manifold vacuum).

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.

Picoscope and WPS500X

Figure 1 - Scope and WPS500X

How to perform the test

Accessories

  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 3
  2. Channel A ± 140 mbar
  3. Timebase 1 s/div
  1. Connect BNC to BNC lead to Channel A of your scope and the BNC socket of the WPS500X transducer.
  2. With the test port open, 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. Press the range button and select Range 3 (±5 psi) (do not select zoom functions).
  4. Connect the vacuum hose to the test port of the WPS500X.
  5. Insert the vacuum adaptor into the vacuum hose.
  6. Insert the vacuum hose and adaptor into any convenient inlet manifold vacuum source.
  7. Prevent your engine from starting (remove ignition and injection fuse).
  8. Run your scope software by pressing either the space bar on your keyboard or the start button in PicoScope.
  9. Crank the engine, measuring and recording the vacuum level achieved whilst cranking.
  10. Stop cranking and monitor the vacuum decay.
  11. Set the WPS500X to Range 3 Zoom Level 1, and repeat step 9 (continual cranking) and record intake vacuum.

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

Channel A

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

Green denotes atmospheric pressure 0 mbar.

Figure 3 - WPS500X Range 3 Zoom level 1

Channel A

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.

Diagnosis

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?

POWER (IGNITION) - 1. EXHAUST - 2. INTAKE - 3. COMPRESSION

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.

More 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.

AT426-1

Disclaimer
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

    £819.00

  • PicoScope WPS500X Vacuum tap

    £58.00

  • Universal vacuum adaptor

    £20.00

  • Vacuum hose

    £20.00

  • WPS500X Pressure Transducer Kit (with carry case)

    £1,155.00

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Guided test: Intake Manifold – Intake Pressure (Cranking)