The purpose of this test is to evaluate exhaust pressures during idle conditions using the WPS500X pressure transducer.
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.
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.
The features of an exhaust pressure waveform have the following relationship with the engine cycle:
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:
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:
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.
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.
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