Relative compression (gasoline)
The purpose of this test is to evaluate engine cylinder and starter motor condition from the electrical current drawn by the starter motor during engine cranking.
How to perform the test
- Disable the vehicle's fuel injection.
- Connect the high amps clamp to PicoScope Channel A.
- Select the highest range and zero the clamp.
- Connect the clamp to either the positive or negative battery lead.
- Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
- Start the scope.
- Crank the engine for around 5 s to capture your waveform.
- Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.
Further guidance
The test uses the following property of a Direct Current (DC) starter motor to provide a convenient, non-invasive comparison of engine cylinder compression:
With a fixed supply voltage, the current drawn by a DC starter motor is proportional to the load on the motor; when the load increases the current increases.
During engine cranking, both the starter motor's load and current increase as each piston travels through its compression stroke. Therefore, a continuous measurement of starter motor current indicates the relative degree of compression across the cylinders as they cycle through in their firing order.
Ideally, the cylinder compressions and their associated current draw would be equal. In reality, there will be some slight variation. Any repetitive low peak in the waveform indicates a cylinder with low compression and provides justification for further investigation.
Compression related issues can cause poor running, misfire and emissions related symptoms. Typical faults that cause compression issues are:
- Intake or exhaust valve timing or sealing faults.
- Piston, bore and/or compression ring faults.
- Blocked air filter or intake.
- Blocked exhaust.
- Turbo faults.
- Cylinder bore washing.
- Engine lubrication problems.
In-cylinder pressure testing can be used to confirm compression issues with engine cranking or running conditions.
Disclaimer
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.
