Relative compression test – petrol

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

The purpose of this test is primarily to evaluate the performance of each cylinder based on the electrical load on the starter motor, as a direct result of the cylinder pressure generated during the compression stroke. In addition, the peak and average current required to crank the engine can be measured and qualified with the vehicle’s technical data. 

Note:

Relative compression test results are dependent upon the battery and starter motor condition in conjunction with the associated circuit. It is recommended that you carry out a battery, starting and charging system test with the Battery test in PicoDiagnostics before you perform this relative compression test.

How to perform the test

Accessories
 
PicoScope settings
  • Channel A: -200A .. 1kA DC
  • Sample rate: 1 MS
  • Timebase: 500 ms/div
  1. Choose the current clamp from the probe list under Channel options for Channel A
  2. Connect the current clamp to Channel A on PicoScope and set the clamp to the high range if applicable.
  3. Connect the clamp to one of the two battery connections (live or earth), whichever allows the easiest connection.
    Note: The current clamp must be switched on and face the correct way. There is an arrow pointing to the battery positive (+) on one side and an arrow pointing to the battery negative (-) on the other side. An incorrect connection will lead to an inverted oscilloscope picture.
  4. Make sure that fuel injection is prohibited to all cylinders; the engine must not be able to fire! This will prevent catalyst damage, engine oil contamination and bore washing.
  5. Run the software by clicking the Go button in PicoScope or pressing the space bar on your keyboard.
  6. You may have to adjust the timebase (ms/division) to compensate for faster or slower cranking speeds.
  7. Make sure that the throttle is held wide open and crank the engine for approximately 5 seconds.
  8. When you have stopped cranking you can stop the scope by either pressing the space bar on your keyboard or the Stop button in PicoScope.

Example waveform

All values included in the Example waveforms are typical and not specific to all vehicle types.

Channel A indicates the rise and fall in the starter motor current during engine cranking.

Waveform notes

Refer to vehicle technical data for specific test conditions and results. Typical values with engine cranking:

  1. The peak in-rush current to the starter motor required to rotate the engine from rest. The peak in-rush current recorded during cranking (470 A) can be confirmed using the signal ruler (marker 1) where the value is recorded in the ruler legend (marker 4).
  2. The peak starter motor current during continuous cranking (122 A).
    The starter motor current should be even across all peaks during continual cranking as the peak current is directly proportional to the cylinder pressure during the compression stroke. The signal ruler (marker 2) will confirm equal current peaks and provide an indication of a suspect cylinder. The peak current value during continual cranking is recorded in the ruler legend (marker 4).
  3. The time ruler handle is located at the bottom left-hand corner of the waveform. Align both time rulers with 2 consecutive compression peaks (four-cylinder engine) by clicking and dragging them. For every revolution of a four-cylinder engine, there are two compression events indicated by two starter motor current peaks during continuous cranking. PicoScope will calculate the cranking speed based on the frequency of the signal between the time rulers and record the values in the frequency/RPM legend (marker 5).
  4. The ruler legend records the numerical current and time values relative to the position of the signal and time rulers.
  5. The frequency/RPM legend displays the engine RPM relative to the position of both time rulers.
  6. Measurement table indicating peak in-rush and average starter motor cranking current. Custom measurements can be made using the measurement toolbar located at the bottom right hand of the screen.

Diagnosis

The purpose of the waveform in this test is twofold:

●     To measure the amperage required to crank the engine

●     To evaluate the relative compressions

The amperage required to crank the engine will depend on several factors, including the capacity of the engine, the number of cylinders, the viscosity of the oil, the condition of the starter motor, the condition of the starter motor's wiring circuit and the compressions in the cylinders.

The current for a typical four-cylinder petrol engine is in the region of 80 to 200 amps.

In the waveform above, the initial peak of current (approximately 460 A) is the current required to overcome the continuous friction and inertia to rotate the engine. Once the engine is rotating, the current drops. The small step before the initial peak is also worth mentioning. This is caused by the switching of the starter solenoid.

You can compare the compressions against each other by monitoring the current required to push each cylinder up on its compression stroke. The better the compression, the higher the current demand, and vice versa. It is, therefore, important that you have current draw on all cylinders equal.

The waveform below is from an engine with a loss of compression in one cylinder.

Where a loss of compression is indicated as above, a second PicoScope channel (B for example) can be utilised to connect to cylinder 1 spark plug lead (Channel B now provides “Sync signal”). Repeat the relative compression test as described in “How to Perform the Test”. The test results will now indicate peak starter current during continuous cranking alongside cylinder 1 firing event. Using the firing order, each current peak (and therefore cylinder) can be identified between each firing event captured on channel B.

  1. Starter motor in-rush current during cranking.
  2. Signal ruler indicating current peaks during cranking.
  3. Starter motor current should be even across all peaks during continual cranking as peak current is directly proportional to cylinder pressure during the compression stroke. The signal ruler (marker 2) will confirm equal current peaks and provide an indication of a suspect cylinder.
  4. Ignition trigger signal for cylinder 1 (referred to here as a synchronisation signal (sync).
  5. Missing current peak forms a trough indicating minimal current draw by the starter motor.
  6. Identification of starter motor current peak in relation to compression events.
  7. Scaling button improves vertical resolution of the waveform (increased by a factor of three).

The waveform above highlights a significant and uniform drop in current peaks during cranking (marker 4). Such events can be attributed to a loss of compression as the loading on the starter motor is momentarily reduced. In order to now identify the offending cylinder, a second PicoScope channel (marker 3) can be used to display the ignition events in cylinder 1 to provide a sync signal.

By using the sync signal and knowledge of the vehicle’s firing order, we can identify each peak current event and locate the offending cylinder (marker 5). In the scenario above the current peak for cylinder 4 is missing (marker 4) indicating that you will have to perform a compression test to complete the diagnosis. To improve the vertical resolution of your capture, click on the scaling button (6) and select the level of scaling required to identify the current peaks.

Note: Where a loss of compression is equal across all cylinders the relative compression results based on peak starter motor current during continuous cranking may appear normal! In such a scenario pay close attention to cranking speed which will increase above the manufacturer specified speed. (Typical cranking speeds are 200 to 300 rpm)

Technical information

It is essential for the engine to have sufficient compression for it to run. The compression provided by the rising piston will be determined by the swept area being compressed into the combustion area: this is called the compression ratio. The compression is also determined by the effectiveness of the seal between the cylinder wall and the piston; this seal is maintained by the piston rings. The same applies to the seating of both the inlet and exhaust valves.

Piston rings are made of centrifugally spun cast iron, which produces a radial pressure forming the seal. Cast iron is used also for its excellent self-lubricating properties.

If the relative compression test highlights a problem and the cylinder is identified using channel B connected to an ignition lead (sync signal) you can perform a test with the WPS500X pressure transducer.

Please follow the Guided Tests WPS500X In-cylinder compression – cranking (petrol) and In-cylinder compression – idle (petrol) in order to identify the cause of low compression.

You can also read more about compression testing in our Technical document – Compression testing.

NOTE: when you compression test a petrol engine with a pressure transducer, you should isolate the ignition primary circuit to avoid stray high tension (HT) voltage damaging the electronic circuitry.

A typical compression is between 120 and 200 psi. A low compression can be caused by:

●         An ineffective seal between the cylinder and the piston

●         Poor seating of the inlet and exhaust valves

●         Broken or seized piston rings

●         Incorrect camshaft timing

●         An obstructed induction tract

All the readings should be similar. If one is lower than the others, you can perform a 'wet' test by squirting a small amount of oil into the cylinder and re-testing the compression. The inclusion of the oil ensures a tight seal between the piston and the bore. If the compression is regained, the fault lies within the piston rings. If it makes very little difference, the fault lies within the valves.

There should not be more than 25% difference between the highest and lowest compression readings.

A higher than average compression can be caused by:

●         A build-up of carbon in the combustion chamber (reducing its area).

●         Excessive skimming of the cylinder head.

●         An incorrect thickness of the head gasket.

Note: It is advisable to disable the ignition's low tension circuit to avoid damaging the amplifier or the Electronic Control Module (ECM).

Additional information

You can find more information about Compression testing in our Training section.

PicoScope and PicoDiagnostics

You can measure relative compression with PicoScope, but in most cases, it is easier to use the relative compression test built into PicoDiagnostics. PicoDiagnostics is the companion software to PicoScope, and it is downloaded to your computer alongside PicoScope Automotive. For more information about the test in PicoDiagnostics, please see the User's Guide.     
 

AT004-4

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.

Suitable accessories

  • 200 A / 2000 A (high amps) DC current clamp

    £259.00

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1 comment | Add comment

Matt
September 29 2016

Adding a sync source on another channel (ie:  inductive sync probe, ignition coil trigger signal, ignition coil current ramp, etc) adds a significant amount of helpful information.

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Guided test: Relative compression test – petrol