Intake manifold pressure during idle (gasoline)

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

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. Press the WPS500X range button and select Range 2.
4. Connect the WPS500X to an intake manifold vacuum source using the kit adaptors.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope.
7. Start the engine and let it run at idle.
8. With your waveform on screen stop the scope.
9. Turn off the engine.
10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• 0 bar is expressed as a relative pressure and indicates atmospheric pressure.
• An average intake manifold pressure around -700 mbar (i.e. a vacuum, relative to atmospheric pressure).
• A series of small depressions, producing a ripple effect, cycling at around 21-25 depressions per second.
• The depressions are equal in amplitude (around 10 to 15 mbar).

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 idling, can be described, as follows:

• The nearly closed throttle valve chokes the air flow from the atmosphere as the engine pumps air mass away from the intake manifold. The cumulative effect is a reduction in intake manifold air density, causing a reduced overall pressure, relative to the atmosphere.
• A small depression is seen for every induction stroke. In a 4-cylinder engine they will be separated by 180° of crank rotation.

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 misfire arising from no, or poor, ignition or injection may reduce the exhaust flow from the affected cylinder and its volumetric efficiency (on induction), causing:
  • No or reduced depressions at periodic intervals.
• A poorly sealed exhaust valve might leak air mass back to the intake manifold via the Exhaust Gas Recirculation (EGR) system on the cylinder’s compression stroke, causing:
  • A raised pressure event at periodic intervals.
• 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.

GT427-EN