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 cranking, can be described, as follows:
- When the engine is stationary, air does not flow and the throttle body has no effect. Therefore, the inlet manifold remains at atmospheric pressure.
- A depression occurs on every induction stroke. In a 4-cylinder engine they will be separated by 180° of crank rotation.
- The piston speed and engine’s pumping effect increase (up to a point) with increasing engine speed. Therefore, as the engine accelerates from stationary at the start of cranking:
- The depression amplitudes increase.
- The overall intake manifold pressure decreases.
- When the engine stops cranking, the inertia of the air mass within the intake system causes it to build up in the intake manifold and briefly increase its pressure.
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 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.
