Engine mechanical testing is generally considered the first step when facing an engine with a misfire. In this article, we will discuss a range of tests that include the PicoDiagnostics, then use PicoScope 7 for relative cranking current to in-cylinder testing using the WPS500X.
Relative compression testing #
With a relative compression test, you can easily determine if all the cylinders in an engine have equal compression. This test is a quick compression check that you can do without removing the spark plugs or glow plugs. It uses the starter motor current to determine the comparative compression values on all cylinders. The advantages of doing this test are that you do not need a pressure transducer/sensor to run the test; all you need is a scope and a current clamp and you do not have to check each individual cylinder; all cylinders can be tested in one go.
The theory behind the relative compression test is that the amount of current required to turn over the engine while a cylinder is on its compression stroke will indicate how much the cylinder is compressing or leaking. This information makes sense when you compare all the cylinders, as you can easily see if one is weaker than the others.
It is important to remember to prepare the vehicle for testing so that the engine will not start during cranking.
PicoDiagnostics (PD) #
In the relative compression test in PicoDiagnostics, the result is scaled to show the highest cylinder as 100%. The bars are displayed in the engine’s firing order and they are named A, B, C, and so on. With only the battery connection, the software cannot determine which cylinder is number one, so the result could shift if you run the test more than once.
Note: the PD Relative compression test is just a quick test and is not to be used to base your diagnosis on (or to hang any parts). Always confirm with a cranking current test with a high amp clamp before moving to a WPS500X in-cylinder test.
In the PD compression test, you can also add your WPS500X
PicoDiagnostics with WPS500X #
If you have a low reading in the results from the relative compression test, the next step is to introduce the WPS500X Pressure Transducer and perform an in-cylinder compression test. The software evaluates the cylinder efficiency in an engine through 720 degrees of crankshaft rotation to gain a peak cylinder pressure. All you have to do is remove one spark or glow plug and insert the pressure transducer.
There is some extra setup required before you can carry out the test. As we are now measuring ‘pressure’, we need to tell the software we are doing so. Click the pressure button. This will give you a number of options and the more information that can be filled in, the more accurate the results will be.
In the absolute compression test, PicoDiagnostics will display the true compression in each cylinder. The bars are named 1, 2, 3, and so on.
When using the PD Relative compression test with WPS500X, there are 2 important things to remember.
First, the cylinder number is not the cylinder ID. It is the position from where the WPS500 is located in relation to the firing order. ( #1 is where the WPS500 is located 2 is the next cylinder in the firing order, not Cylinder #2)
Next, placing the WPS500X in the weak or dead cylinder will cause the whole test to be skewed and display inaccurate results.
Cranking current test (relative compression) with a high amp clamp #
The Guided Test can be viewed here for Gas vehicles:
And for Diesels:
In the previous waveform, knowing the misfire is on cylinder #4 makes identifying it on the waveform easy, but what if you have no code or the engine doesn’t start?
Using another channel to monitor spark on cylinder #1 (or an easy cylinder to access, remembering where to start your firing order from) can be easily performed using the COP probe or a secondary ignition pickup:
https://www.picoauto.com/accessories/ignition-accessories
In-cylinder testing with the WPS500X #
All values included in the example waveforms are typical and not specific to all vehicle types.
Refer to vehicle technical data for specific test conditions and results.
The WPS500X pressure transducer simply provides a means to convert positive or negative pressures into voltage. When combined with PicoScope 7, you have the distinct advantage of displaying pressure values against time. This gives the technicians the ability to view dynamic changes in cylinder pressures during the four-stroke cycle. If you assess the cylinder efficiency with the WPS500X, you can reveal additional information about the condition of your engine and the individual valve timing. Because of the resolution and speed of the pressure transducer and PicoScope 7, you can see more than ever thought possible. For this reason, we have to be aware that the variety of engine designs, intake, exhaust systems, and elaborate variable valve timing will all have an effect on the waveform/results, which will differ from vehicle to vehicle.
Based on the cylinder efficiency test and analysis procedure in the Guided Test In-cylinder compression – cranking (petrol), we can confirm the following:
- Correct peak cylinder pressure for the engine under test. (Allow for standard compression hose volume of 5 ml.)
- Uniform and symmetrical compression towers confirm the cylinder efficiency, not only as the pressure builds but also as it naturally decays during the expansion stroke. For the compression towers to be symmetrical, the valve timing, valve train and mechanical integrity of the piston/cylinder must be efficient.
- The presence of the expansion pocket confirms that our cylinder can hold a vacuum and must, therefore, be airtight (the integrity of the valve seat and piston ring is OK).
- Sufficient cranking speed
- Adequate intake and exhaust flow to achieve the correct peak pressure (no restrictions).
- Repeatability of peak compression for every completion of the four-stroke cycle.
- Cylinder pressure remains at zero bar during exhaust and intake events, which confirms there are no airflow restrictions during either stroke.
Based on the cylinder efficiency test and the analysis procedure in the Guided Test In-cylinder compression – idle (petrol), you can confirm the following:
- Correct peak cylinder pressure at idle speed when compared to adjacent cylinders. (Allow for the Pico standard compression hose volume of 5ml.)
- Uniform and symmetrical compression towers confirm the cylinder efficiency, not only as the pressure builds but also as it naturally decays during the expansion stroke. For the compression towers to be symmetrical, the valve timing and duration, the valve train, and the mechanical integrity of the piston/cylinder must all be efficient.
- The presence of the expansion pocket confirms that the cylinder can hold a vacuum and that it must, therefore, be airtight (the integrity of the valve seat and piston ring is OK).
- Correct exhaust valve timing and duration based on the transition in cylinder pressure at approximately 180 and 360 degrees of crankshaft rotation. Refer to the vehicle repair manual for exhaust valve timing and duration
- The integrity of the exhaust system is based on cylinder pressure during the exhaust stroke (approximately 180 to 360 degrees of crankshaft rotation). Catalyst restrictions will be revealed with excessive positive pressure above 0 bar.
- Event timing at approximately 360 degrees of the crankshaft rotation by observing the valve overlap. Both the exhaust and the intake valves will momentarily remain open (refer to the vehicle repair manual for valve timings and duration).
- Correct intake valve timing and duration based on the transition in cylinder pressure at approximately 360 and 540 degrees of the crankshaft rotation (the intake pocket). Refer to the vehicle repair manual for the exhaust valve timing and duration.
- Equal depth (vacuum level) between the expansion and intake pocket, revealing an efficient balance between the expansion and intake strokes. Manifold pressure (vacuum) is proportional to the swept volume of the cylinder by the piston, and therefore, the vacuum produced by the piston in a sealed cylinder during the expansion stroke (expansion pocket) should be nearly equal to the vacuum produced during the intake stroke, acting against a closed throttle (intake pocket). This will vary with high-performance engines.
- Engine speed, based on 360 degrees of crankshaft rotation.
- Correct cylinder pressures during 720 degrees of crankshaft rotation (no restrictions) are achieved by adequate intake and exhaust flow.
- Repeatability of peak compression for every completion of the four-stroke cycle.
Rotation Rulers and Partitions #
In PicoScope 7, you can divide the distance between compression events into four equal partitions, revealing the position of the crankshaft (degrees of rotation) with the rotation rulers and the partitions. If we know the position of the crankshaft, we can identify each of the four-stroke cycles between compression events.
The rotation rulers are used to denote 0 and 720 degrees of rotation about the captured cylinder pressure waveform. Given that peak cylinder pressure occurs at TDC of the compression stroke, positioning the rotation rulers at two consecutive compression peaks/towers will denote 0–720 degrees of crankshaft rotation relative to TDC / peak compression. Based on our knowledge of the four-stroke cycle, we know the events that should take place between the rotation rulers at 0 and 720 degrees (TDC Compression to TDC Compression). To assist with diagnosis, dividing the distance between 0 and 720 degrees into four equal divisions will reveal the position of the crankshaft at key stages throughout the four-stroke cycle (TDC and BDC). Once we know the position of the crankshaft, we can identify each of the four-stroke cycles between compression events and attributing anomalies found to specific four-stroke events/activities.
The image above shows the main parts of our absolute compression testing (In cylinder waveform). The diagram is an approximation of the timing of the events happening.
The purple arrows at the top of the image indicate the movement of the piston from Top Dead Center to Bottom Dead Center.
The blue trace from Channel A is the waveform captured by the WPS500X. Notated are the power stroke expansion pocket, the intake vacuum pocket, and the compression tower.
Towards the bottom of the diagram, you can see the four strokes of the engine labelled Power, Exhaust, Intake and Compression. EVO, EVC, IVO and IVC are the intake/exhaust valve opening and closing events. The color-matched arch between EVO–EVC and IVO–IVC shows approximately how long each valve is open during the 720-degree revolution of the crankshaft. The specific timing for these events is dependent on the base engine timing and camshaft lobe profiles for each specific engine model.
Troubleshooting #
Based on the knowledge gained above, we can now look at a number of mechanical issues revealed in waveforms captured with the pressure transducer.
The image above reveals an additional pressure peak during the exhaust event that should not be present. As the piston rises from the Bottom Dead Center (BDC) of the expansion stroke (approximately 180 degrees after the Top Dead Center (TDC) compression), the exhaust valve will open and release the cylinder pressure out to the atmosphere (atmospheric pressure) via the exhaust system. The waveform indicates a pressure increase to 2.056 bar during the exhaust stroke because the exhaust is not allowed (or blocked) to flow! In effect, we have another compression stroke during the four-stroke cycle – Compression-Expansion-Compression-Intake.
Air can be drawn into the cylinder and compressed, hence the peak compression value (13.41 bar), but air cannot escape during the exhaust stroke.
We can quickly compare this cylinder to a different cylinder (but not the companion). This is cylinder #5 on the same engine.
You can see that both the peak compression is higher, and it clearly has the normal exhaust stroke.
In this case, the new exhaust manifold had slag that almost blocked the entire port!
Here is the matching Guided Test
Another excellent test for engine mechanical is the cranking compression test. This is very effective in finding upper engine sealing issues (valves, etc.)
Here is the matching Guided Test for cranking compression
As you can see, the engine is disabled from starting and cranked using 500ms/div or greater.
The ruler shows the peaks are very close.
Let’s zoom in on these peaks closer.
Let’s look at a case study of a cylinder that is missing and setting a misfire code only.
Let’s start on a good cylinder. Vehicle is a Chrysler Town and Country with a 3.6l V6
Now let’s go to the cylinder that is setting misfire codes (not a companion of the one we tested first).
Wow! Almost a 30psi (1.1bar) difference between cylinders.
A leak-down test on this cylinder had previously been run and was inconclusive (probably because of the valve face as it rotates).
In this case, you can see that the rotation of the valve changes how it seals and almost causes a step-up and step-down function. Of course, not all valves rotate on many modern engines.
The head was removed, and the valves and seats reground to properly repair this engine.
All of the psdata files used in this article are below. You may find you have to go in and change your settings if you use PSI versus BAR, for example (in this article, I used both settings).
The WPS500X product line can be found here: https://www.picoauto.com/products/pressure-sensors/wps500x-overview
Hope this brief look into compression testing helps speed up and increase the accuracy of your diagnostics!
