Exhaust pressure during cranking

The purpose of this test is to evaluate exhaust pressures 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 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 exhaust probe to the WPS500X and insert the probe into the tailpipe.
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 the scope.
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.
• The pressure waveform has an average around 0 bar (atmospheric pressure).
• A continuous, uniform, series of peaks and troughs.
• All the peaks are at around the same pressure.
• All the troughs are at around the same pressure.
• There are no anomalous peaks, troughs or inflections.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Exhaust / tailpipe 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).

The ability of a given an engine to pump air is known as its volumetric efficiency. As the exhaust output pressure is affected by the engine’s volumetric efficiency, it is an effective measure with which to assess the engine’s function.

Waveform features

The features of an exhaust pressure waveform have the following relationship with the engine cycle:

• A peak occurs for every exhaust stroke.
• The peaks are separated by 180° of crank rotation, given a typical 4-cylinder engine.

Waveform diagnosis

The exhaust pressure reflects the net effect of all cylinder and exhaust path interactions, for example, with the Exhaust Gas Recirculation (EGR), turbo, catalytic converter, particulate filter, and Selective Catalytic Reduction (SCR) systems.

The relationships are complex; for example, there are two valve-overlap scenarios:

• 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 exhaust valve overlaps with the closure of the exhaust valve in the cylinder preceding it in the firing order (at least one exhaust valve is open at any one time).

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

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

There are two main effects an engine-related fault might have on the exhaust waveform:

• An overall reduction in waveform amplitude, caused by an overall loss of volumetric efficiency, such as:
  • A restricted intake.
  • A restricted exhaust.
• Periodic anomalies, resulting from a volumetric efficiency reduction in one or more cylinders, such as:
  • A worn inlet cam, which will limit the inlet valve lift.
  • A poorly sealed inlet valve.
  • Piston blow-by.
  • A head gasket leak (to the coolant).
  • A worn exhaust cam, which will limit the exhaust valve lift.
  • A poorly sealed exhaust valve.

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 and could cause a pair of periodic anomalies or a single periodic, but prolonged, anomaly.

GT882-EN