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Engine mechanical testing using an in-cylinder pressure transducer

Saturday, September 1, 2018 - 06:00
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After seeing a higher compression event for cylinder #1 and knowing this is the problem cylinder a running compression test is performed. The result is quite revealing. A good cylinder (Figure 12) will be compared to the problem cylinder.

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Figure 12 — Running compression test from cylinder #3 with a throttle snap is shown. This is a known good cylinder
Figure 13 — This zoomed capture of a cranking compression test from cylinder #3 is measuring the point of intake valve closing which occurs on the compression stroke.

The good cylinder shows a compression peak during a snap throttle application reaching over 330 PSI and the intake valve closing at 45 degrees after bottom dead center (Figure 13). Comparing the same test from cylinder #1 shows peak compression during a snap at only 287 PSI (Figure 14) and the intake valve closing at 20 degrees after bottom dead center (Figure 15).

Figure 14 — Running compression test from the problem cylinder showing lower peak compression during a throttle snap.
Figure 15 — Zoomed cranking compression waveform showing early intake valve closing point.

The early intake valve closing and reduced peak cylinder pressure during a snap throttle illustrate a worn intake camshaft lobe causing an intake path restriction. Cylinder pressure is a function of airflow and the effective compression ratio which is determined by the point at which the intake valve closes so that pressure can be built. When an intake valve closes earlier than normal, the effective compression ratio increases and will cause higher compression values. It must be clear at this point that engine mechanical problems cannot escape detection when using the tests performed in this article. Once these tests are mastered your accuracy in determining engine mechanical problems will be spot on without requiring engine disassembly.

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