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Ignition timing: The forgotten test

Monday, April 1, 2019 - 07:00
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Fugure 4 - A broken crankshaft balancer caused the CKP reluctor to shift

Further investigation, focusing on the crankshaft position sensor, revealed a damaged (Figure 4) crankshaft balancer.

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Figure 5 - A relative compression capture with slightly questionable ignition timing.

Another example could be this next capture of another Ford vehicle. Figure 5 illustrates ignition timing that is questionable. The ignition firing (purple) appears to be near top dead center or even a bit to the right, or retarded. In this case ignition timing is suspect and more testing should be performed.

Method #2:  In-cylinder compression in relation to sync

In-cylinder testing is a much more accurate way to measure ignition timing and would be the next diagnostic step in the case of the vehicle used in Figure 5. This technique will still require an ignition sync, but will also requires the use of a pressure transducer to establish TDC (Top Dead Center) and 720° of crankshaft rotation. Unlike the relative compression test, this test can be done during engine cranking or while the engine is running. In addition, very accurate ignition timing measurements can be made.

To facilitate this test a spark plug is removed and a pressure transducer is installed in its place. The engine is then cranked over or started. The highest point in the pressure capture is top dead center. The ignition sync can then be compared to actual top dead center and, if desired, can be measured with more accuracy.

Figure 6 - Ignition timing while running should be advanced. This capture shows near top dead center.

Figure 6 is an in-cylinder capture from a different vehicle. The vehicle is running at idle and it is obvious that the spark firing event occurs almost exactly at top dead center.

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