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Engine mechanical testing: Good, better and best

Friday, June 1, 2018 - 07:00
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As with the relative compression test, the cranking vacuum test should produce a clean, oscillating pattern with good uniformity. Again, it is important to identify which signal event is created by which cylinder in the engine, so I will illustrate the procedure in a captured waveform. There will be a vacuum event every 180 degrees in a four-cylinder engine, every 120 degrees in a six, and every 90 degrees in an eight-cylinder engine. A vacuum stroke occurs 360 degrees after the cylinder fires on the power stroke, so you need to capture an ignition event when performing a cranking vacuum test to properly identify the events on the screen. In the annotated waveform shown in Figure 11, I have used rotation rulers found in the Pico scope software to break up the waveform into eight evenly spaced intake events. The numbering order on top is the firing order, but it is overlaid on the vacuum events, not the ignition event. The vacuum pull happens 360 degrees after the ignition coil firing signal.

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Figure 10 - Cranking vacuum waveform with both a Pico transducer and a First Look transducer connected to the same test port. Note the Pico waveform pulls down from zero volts due to it being an absolute pressure transducer while the First Look transducer’s signal oscillates above and below the zero voltage reference. Both transducers display vacuum events or pulls.
Figure 11 - This cranking vacuum waveform from a 2005 GMC Yukon, 6.0 V8 is considered good. The individual cylinder vacuum events are labeled. There should be the same number of vacuum pulls as the number of cylinders, and they should be relatively even.

Once you capture a cranking vacuum waveform and ID the events, you can begin to make conclusions on what you see relative to the problem on the vehicle. Remember we are looking at mechanical engine sealing with electronic tools, so keep in mind what could go wrong in an engine when analyzing an abnormal pattern. Valve leakage, valve timing, ring sealing and intake or exhaust path restrictions can all have an effect on these patterns. The 2005 Yukon with the broken exhaust valve spring was tested with cranking vacuum. The pattern in Figure 12 shows the effect of the sticking open exhaust valve on cranking vacuum.

Figure 12 - Bad cranking vacuum waveform from broken exhaust valve spring on cylinder #3.

When this pattern is looked at by itself, it is difficult to say the problem is a broken exhaust valve spring, but it is easy to see there is a problem with the engine and the problem is mechanical! When the intake valve opens on cylinder #3’s intake stroke, there is a loss of vacuum in the engine; the top Pico trace shows this because it is an absolute pressure transducer. Both transducers are momentarily connected to the exhaust manifold through the open intake valve in cylinder #3 and the First Look sensor clearly shows a pressure rise as exhaust pressure connects to the intake manifold when the intake valve opens. This is only the beginning of what can be seen when pressure transducers are connected to the engine. We will continue the discussion in the next article with in-cylinder pressure testing. Once you add these tests together on the same scope screen, the operation of the internal combustion engine becomes much clearer. I am sure you will not be disappointed if you acquire the necessary tools and begin using these tests in your diagnostic routine. Practice is important and testing good engines first should be a priority. Remember, an engine can only operate properly if its base mechanical condition is good! The quicker you can determine this, the better. Best of luck!

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