Crankcase pressure during cranking

The purpose of this test is to evaluate crankcase 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 a suitable probe to the WPS500X and insert the probe into the dipstick tube.
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.
• 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 Crankcase pressure waveform.

Further guidance

Combustion gases escape past the piston rings and into the crankcase during an internal combustion engine’s operating cycle. This effect is known as blow-by.

A crankcase ventilation system is used to vent blow-by gases back to the intake manifold, whilst fresh air is drawn in to replace them. This has two effects:

• The subsequent combustion of the vented blow-by gases reduces harmful emissions to the atmosphere.
• No build up within the crankcase of harmful (to the engine) combustion gases and pressure.

Fresh air enters the crankcase via a breather, which has its inlet close to and downstream of the air filter. A Positive Crankcase Ventilation (PCV) valve regulates the flow of blow-by gases from the crankcase to the intake manifold.

The PCV valve is a normally closed valve. That is, when the engine is off and the crankcase and intake manifold pressures have equalised (with atmospheric pressure), the valve is fully closed. This prevents harmful gases from venting to the atmosphere when the engine if off and the vehicle is stationary.

The higher the crankcase pressure above the intake manifold pressure, the greater the opening of the PCV valve. Therefore, the valve position and gas flow vary with engine operation, as follows:

• At idle, the crankcase pressure is much higher than the intake manifold pressure and the valve opens to let the gases through.
• At higher loads, the intake manifold pressure reduces. However, the amount of blow-by increases, which, if there was no other outlet, would means crankcase pressure would increase, therefore the valve opens as necessary to allow increased flow.
• Given a backfire within the inlet manifold, the increased pressure forces the PCV valve closed, which protects the crankcase gases and content from a possible ignition source.

The measurement of crankcase pressure, usually by connecting at the dipstick tube, breather inlet, or other convenient access point, allows us to assess the degree of blow-by and the operation of the ventilation system.

Waveform features

The crankcase pressure waveform features have the following relationship with engine operation during cranking:

• A peak occurs on every compression stroke, where the high in-cylinder pressure causes blow-by.
• The peaks are separated by 180° of crank rotation, given a 4-cylinder engine.
• The pressure peaks have a shape that depends on the build-up and release of pressure during the compression and power (expansion if there is no combustion, as in the above example) strokes.
• The average pressure will be lower than atmospheric pressure, as ventilation requires air flow – from the atmosphere, through the crankcase, and into the intake manifold.

Waveform diagnosis

Diagnoses rely mostly on the identification of periodic anomalies within the waveform. An observed anomaly provides sufficient justification for further investigation.

Periodic waveform anomalies will occur if there is:

•  A loss of volumetric efficiency (i.e. lower than expected cylinder charge) caused by:
  • an inlet or exhaust valve fault, such as a restricted port, worn cam or defective sealing.
  • A bent connecting rod.
  • A head gasket leak.
• Excessive blow-by caused by:
  • Defective piston rings or cylinder bores.
  • A head gasket leak to an oil gallery.

An overall low crankcase pressure will occur if:

• There is an air intake restriction, causing excessive consumption of crankcase gases.
• The PCV valve is stuck open (this will have the same effect as an intake manifold leak).
• A blocked crankcase breather inlet.

An overall high crankcase pressure will occur if:

• The PCV valve is blocked.

A faulty PCV valve or system can affect an engine’s fuel trims and cause either lean or rich running conditions (depending on the fault). In these cases, the engine management system may switch on the MIL light and set a diagnostic trouble code (DTC).

Other related symptoms may include erratic idle/running, oil burning/blue smoke, or the contamination and build-up of carbon, water, or sludge, within the crankcase, breather, PCV valve, and connected systems.

GT883-EN