WPS500X - Petrol engine intake manifold pressure

The purpose of this test is to observe the intake manifold pressure under varying engine running conditions from cranking to idling, wide open throttle, back to idling and engine off.

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.

General connection advice

  • 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.

How to perform the test

  1. Connect the fully charged WPS500X transducer 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. Connect to the intake manifold and select range 2.
  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. Fully depress the throttle pedal allow the engine to reach maximum RPM then release the pedal, allow engine to return to idle before switching off.
  8. Stop the scope.
  9. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Waveform notes

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

  1. ENGINE OFF - Atmospheric pressure
  2. ENGINE CRANK AND START - Note rapid increase in manifold vacuum
  3. IDLE SPEED STABILISING - Varies between idle control systems
  4. STABLE IDLE SPEED - Note the stable manifold vacuum
  5. WOT SNAP TEST - Rapid decrease in manifold vacuum
  6. INCREASED INTAKE "POCKET" DURING OVERRUN - Note the increase in manifold vacuum below the stable and even idle at 4
  7. 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 to minimum 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.

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

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)


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Guided test: Intake Manifold - Intake Pressure (Snap Wide-Open Throttle)