Pico is a brand of oscilloscope. It is an awesome tool, very intuitive and user friendly. A relative compression test w/ a scope consist of putting an amp clamp around the positive battery cable and then putting a second lead onto the signal wire for any of the 6 coils. The engine is disabled so that it won’t actually start (on European cars I usually pull the fuel pump relay), then the engine is cranked for several revolutions to capture a waveform.How do you determine which cylinder is cranking at what rate? Pull all the plugs except one, then use some instrument to measure the crank speed during the compression stroke? Try to guess by ear?
Enlighten me: what is Pico?
IMO the OP got low-ish numbers for all cylinders.
As the engine is turned over by the starter, the compression of air in each cylinder causes the starter to draw more amps, which is depicted by peaks on a wave form that correspond to each compression event. On the QV you would count out six peaks on the graph from the oscilloscope and look for them all to be very close to the same maximum height.
The second channel on the scope, the lead attached to the coil signal feed, allows you to determine which cylinder is which—provided you have the engine firing order. In the graph below you can see the engine is a 6 cylinder because the firing event (red waveform) happens every 6 compression events (blue waveform). What you hope to see on a healthy engine is that all the peaks of the blue waveform are close to the same height. (The one depicted is not a healthy engine)
The pressure transducer is used to go “in cylinder” and get a waveform of what is happening inside a particular cylinder. If you find a peak that is lower or higher than the others, you can use the pressure transducer on that cylinder to “see” what is happening in that cylinder—are the intake valves closing all the way, is the engine timing off, are the exhaust valves all closing fully, is the exhaust restricted, etc. I don’t have a pressure transducer, so I haven’t done that testing, but it can be more informative than a leak down test and quicker.
There is a ton of information on the internet, and PIco shares their info and how to do these tests via videos on YouTube. “Trained by Techs” also put out some quality videos of how to use an oscilloscope to do a relative compression test.
I would be interested to see a waveform generated by @kumar_a ; car.
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