Intake manifold pressure during snap test (gasoline)

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

How to perform the test

1. Ensure the engine is at its correct operating temperature.
2. Connect the fully charged 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 WPS500X range button and select Range 2.
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 engine and allow it to reach a stable idle speed.
8. Start the scope to see live data.
9. Fully and rapidly depress the accelerator pedal to snap the throttle open, hold the position for 1 to 2 s, then quickly release the pedal so the throttle snaps shut.
10. Allow the engine to return to a stable idle speed.
11. With your waveform on screen stop the scope.
12. Stop the engine.
13. 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.
• The pressure is around -700 to -720 mbar when the engine is stabilised at its idle speed.
• There is a hash on the waveform.
• The pressure rapidly rises to 0 bar when the throttle is snapped open.
• The waveform hash increases with increasing engine speed.
• The pressure remains at 0 bar whilst the throttle is held open.
• The pressure rapidly decreases when the throttle is snapped shut.
• The pressure drops to around -850 to -900 mbar with the throttle closed on overrun.
• The pressure returns to around -700 to -720 mbar as the engine returns to a stable idle.

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).

We can use a snap test to see how the intake manifold pressure responds to a rapid change of throttle position from fully closed to fully open and back again. It tells us how well air can move through the engine.

Waveform features

Intake manifold pressure behaviour, during a snap test, can be described, as follows:

• A lower intake manifold pressure than atmospheric pressure when the engine is at idle. In these conditions, the throttle valve chokes the air mass flowing into the manifold from the intake ducting, reducing the manifold pressure.
•  A small depression in the overall pressure is caused by every induction stroke, which occur every 180° of crank rotation on a 4-cylinder engine. The depressions are seen as the waveform hash.
•  An immediate increase in the intake manifold pressure when the throttle is snapped open. The throttle opening causes air mass to rush into the intake manifold from the atmosphere and increase the manifold pressure.
• Conversely, there should be an immediate decrease in the intake manifold pressure when the throttle is snapped shut. Initially, the engine should be at relatively high speed and will quickly pump away air mass from the manifold, reducing its pressure.
• The pressure will drop below the intake manifold pressure at idle during overrun conditions. During overrun, the throttle valve is fully closed and no air can flow past to replace the air mass being pumped away by the engine.

Waveform diagnosis

A snap test reveals broader trends in the overall intake manifold pressure behaviour. Please note that you should only make pressure value decisions based on comparison with manufacturer data.

The waveform may be affected by possible faults in the following ways:

Reduced intake manifold pressures, as caused by:

• Restricted intake upstream of the throttle valve, which increases the overall throttling effect.
• Incorrect valve timing.

Increased intake manifold pressures, as caused by:

• Leaks, or equivalent, within:
  • Intake manifold sealing.
  • Brake servo vacuum system.
  • Evaporative (EVAP) emissions system (e.g. valve stuck open).
  • Exhaust Gas Recirculation (EGR) system (e.g. valve stuck open).
  • Positive Crankcase Ventilation (PCV) valve (stuck open).
  • The crankcase, from excessive blow-by past the pistons and then the PCV valve.
• A loss of volumetric efficiency (from a lower than expected cylinder charge or compression) in one or more cylinders, from:
  • Incorrect valve timing.
  • An inlet or exhaust valve having a worn cam, broken valve spring, poor valve sealing, or a port restriction.
  • A bent connecting rod.
  • A misfire from no, or poor, ignition or injection.
  • A head gasket leak.
  • Defective piston rings or cylinder bore.
  • A restriction in the exhaust system.

GT381-EN