Intake manifold pressure during cranking (gasoline)

The purpose of this test is to evaluate the intake manifold pressures of a naturally aspirated gasoline engine during cranking conditions using the WPS500X pressure transducer.

How to perform the test

  1. Disable the vehicle’s fuel injection and ignition systems.
  2. Connect the fully charged WPS500X pressure transducer 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 WPS500X range button and select Range 3.
  5. Connect the WPS500X to an intake manifold vacuum source using the kit adaptors.
  6. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  7. Start the scope.
  8. Crank the engine for around 5 s to produce the waveform.
  9. Stop cranking and with your live waveform on screen wait for the vacuum to decay (trace to 0 bar) then stop the scope.
  10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

 

Note

Some Engine Control Modules (ECMs) operate the throttle independently to the accelerator pedal position. This will alter your waveform. If this is the case, you will need to disconnect the throttle body connector and re-test.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

0 bar is expressed as a relative pressure and indicates atmospheric pressure.

The inlet manifold pressure is initially at 0 bar (atmospheric pressure) with the engine stationary.

The pressure drops below 0 bar once the engine has started to crank and an induction stroke has created the necessary conditions.

A series of negative pulses, depressions, occur as the engine continues to crank.

The depression amplitudes and frequencies increase as the engine speed increases.

During steady-state cranking, the peak depression amplitude is around -65 mbar and the intake manifold pressure consistently remains below 0 bar.

The inlet manifold pressure returns to 0 bar at the end of cranking.

A positive pressure peak, reaching around 40 mbar, and then a negative peak, immediately follow.

The inlet manifold pressure returns to 0 bar.

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

An internal combustion engine acts as an air pump. It draws air in through the intake and forces it out through the exhaust. The rate at which the air mass enters the intake is the rate at which the air mass leaves the exhaust (unless it is added to or expelled via other means, such as leaks).

Air mass flow depends on engine speed, engine displacement, and intake manifold air density. Within the intake manifold volume, the air density depends on pressure. Therefore, if we use a throttle valve to regulate the intake manifold pressure, we can control the intake air density.

If engine speeds and throttle valve positions are known, we can use intake manifold pressure measurements to evaluate the air mass flow behaviour within an engine.

Please note: you should only make pressure value decisions based on comparison with manufacturer data.

Waveform features

Intake manifold pressure behaviour, during cranking, can be described, as follows:

  • When the engine is stationary, air does not flow and the throttle body has no effect. Therefore, the inlet manifold remains at atmospheric pressure.
  • A depression occurs on every induction stroke. In a 4-cylinder engine they will be separated by 180° of crank rotation.
  • The piston speed and engine’s pumping effect increase (up to a point) with increasing engine speed. Therefore, as the engine accelerates from stationary at the start of cranking:
    • The depression amplitudes increase.
    • The overall intake manifold pressure decreases.
  • When the engine stops cranking, the inertia of the air mass within the intake system causes it to build up in the intake manifold and briefly increase its pressure.

Waveform diagnosis

The intake manifold pressure reflects the net effect of all cylinder and intake interactions. The relationships are complex; for example, two valve-overlap scenarios occur around the start of every induction stroke:

  • Within a cylinder, the inlet valve opening overlaps with the exhaust valve closing (they are both open for a short period).
  • Across cylinders, the opening of an inlet valve overlaps with the closure of the inlet valve in the cylinder preceding it in the firing order (at least one inlet valve is open at any one time).

Although a uniform pattern should be apparent, the intake manifold pressure waveform characteristics cannot be accurately predicted without exact knowledge of the engine design.

Therefore, diagnosis relies mostly on the identification of periodic anomalies within the waveform. An observed anomaly provides sufficient justification for further investigation.

We can link typical faults to possible waveform effects, for example:

  • A blocked intake (upstream of the throttle) may increase the overall throttling effect, causing:
    • Larger depressions.
    • A lower overall intake manifold pressure.
  • An inlet manifold leak via seals, joins, or auxiliary systems, such as the Evaporative Emissions (EVAP) and Exhaust Gas Recirculation (EGR) systems, may cause:
    • Smaller depressions.
    • A raised overall intake manifold pressure.
  • A worn inlet cam will limit the inlet valve lift and reduce volumetric efficiency (i.e. the engine’s pumping ability) in the affected cylinders (on induction), causing:
    • No or reduced depressions at periodic intervals.
  • A poorly sealed inlet valve will leak air mass back to the intake manifold on the cylinder’s compression stroke, causing:
    • A raised pressure event at periodic intervals.
  • Excessive piston blow-by could reduce volumetric efficiency (on induction) on the affected cylinders, causing:
    • No or reduced depressions at periodic intervals.
  • If piston blow-by was particularly excessive, air mass could be passed back (on compression) to the intake manifold via the Positive Crankcase Ventilation (PCV) system, causing:
    • A period of raised intake manifold pressure at periodic intervals.
  • A head gasket leak to the coolant system may reduce the affected cylinder’s volumetric efficiency (on induction), causing:
    • No or reduced depressions at periodic intervals.
  • A head gasket leak between adjacent cylinders could affect both their volumetric efficiencies. The effects will likely be dependent on their relative position in the firing order, possibly causing:
    • A pair of periodic anomalies or a single periodic, but prolonged, anomaly.
  • A worn exhaust cam will limit the exhaust valve lift and reduce volumetric efficiency (and induction) in the affected cylinders, causing:
    • No or reduced depressions, or a raised pressure event, at periodic intervals.
  • A blocked exhaust may reduce overall volumetric efficiency (on induction), causing:
    • Smaller depressions.
    • A raised overall intake manifold pressure.

GT426-EN

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 Kit (with carry case)

    £1,055.00

  • WPS500X Pressure Transducer

    £749.00

  • PicoScope WPS500X Vacuum tap

    £40.00

  • Universal vacuum adaptor

    £9.00

  • Vacuum hose

    £21.00

Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Intake manifold pressure during cranking (gasoline)