The purpose of this test is to analyze the dynamic pulsations hidden in the vacuum signal to check for irregularities attributed to cylinder head valve operation during cranking.
Manifold vacuum during cranking can be used as an initial indication of the presence of compression (poor piston ring and combustion chamber sealing will result in insufficient manifold vacuum).
Note: Manifold pressure is directly related to intake condition and flow, throttle position, valve timing/lift, engine condition, exhaust flow and any boost pressure applied via forced induction.
All numerical readings quoted in this help topic are typical and not applicable to all engine styles.
All values obtained below with the WPS500X are referenced to gauge pressure.
Intake pressure before the throttle (air inlet side, positive pressure) is described here as atmospheric pressure = 0 mbar.
Intake pressure after the throttle (engine side, negative pressure) is described here as vacuum = below 0 mbar.
Ensure that the WPS500X is fully charged before starting this test.
We advise you to recharge your WPS500X after use to ensure it is ready for future measurements.
① indicates intake manifold at atmospheric pressure before cranking with the WPS500 pressure transducer set to Range 3 with no zoom. ② Denotes the commencement of cranking. ③ Indicates the maximum intake manifold vacuum achieved during cranking (-65 mbar approx.). ④ Indicates "buffeting" as the rush of intake air rebounds from the stationary piston and closed throttle (cranking stop).
Green ① denotes atmospheric pressure 0 mbar.
① indicates the intake manifold vacuum pulsations (ripple) in relation to valve open/close during cranking, magnified 10 times. The waveform no longer represents the value of the vacuum: see Diagnosis section below.
Channel A ①: When selecting zoom level 1 of your WPS500X, the manifold vacuum value obtained in Figure 2 (-65 mbar) is brought up to the zero line (black marker) so removing the static vacuum value. This reveals the dynamic pulsations hidden within the vacuum signal by magnifying the ripple 10 times, allowing you to analyse the waveform for irregularities attributed to cylinder head valve operation.
Refer to vehicle technical data for specific test conditions and results.
Typical values (when engine is at correct operating temperature)
Figure 4 shows how the vacuum pulsation/ripple is formed. As the piston moves down the bore, air is drawn into the cylinder so creating a negative pulse. Now imagine 4 cylinders drawing in air at different times at high speeds. The result is the pulsations /ripple you can see in Figure 2 and Figure 3.
Assessing engine conditions using the WPS500X and PicoScope will reveal more information about the condition of your engine than was ever thought possible given the resolution and speed of both the transducer and scope. For this reason we have to be aware that the variety of engine designs, intake, exhaust systems, and elaborate variable valve timings will all have an effect on the waveform that will differ from vehicle to vehicle.
Be very careful when analysing intake manifold pulsations. Remember what we are looking for is anomalies in waveform patterns, something irregular that stands out in a repetitive fashion.
Knowing how the pulsations are formed is key to a non-intrusive evaluation and diagnosis of an engine's condition.
Once again be aware that we are looking for irregularities in the waveform and so a saw- tooth pattern across all pulsations is most likely to be normal for the style of engine under test as it is highly unlikely for every valve within the engine to be poorly seated or sticking.
When attempting to identify an offending cylinder due to an irregularity in the magnified vacuum pulsations, it is advisable to use an ignition event on an additional channel of your scope.
Below we have used the number 1 cylinder firing events and indicated the position of the crankshaft whilst highlighting the four stroke cycle between each firing event. Note how the trough formed by the piston on the intake stroke of number 1 cylinder occurs 2 pulses to the right of the firing event (offset by 2 strokes/pulses).
The easiest way to remember this is to think of the four stroke cycle and where does the intake stroke occur after the ignition/power event?
POWER (IGNITION) - 1. EXHAUST - 2. INTAKE - 3. COMPRESSION
The Intake stroke occurs 2 pulses to the right of your ignition event.
With a firing event displayed against your vacuum pulsation waveform, using the firing order you are then able to identify the remaining vacuum pulses. In our example above we have a 4 cylinder engine with the firing order 1 > 3 > 4 > 2. Applying the firing order reveals the pulses for the remaining cylinders in the order of 3 > 4 > 2.
The internal combustion engine can be likened to a mechanical air pump, where air is drawn in through the intake and forced out through the exhaust. Engine efficiency relies heavily on this process, which is often referred to as "Engine breathing". During the intake stroke on our petrol engine below, air is drawn into the relevant cylinder, but the flow of air is met with a restriction in the form of our throttle butterfly valve. The butterfly valve will be held near to the closed position leaving a very small area for air to be drawn in and reach the cylinder on the intake stroke. A comparison can be made here with a bicycle pump, where placing your finger over the inlet to the pump while drawing back on the grip will restrict the air flow into the pump and generate a vacuum under your finger.
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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