On systems that use the ECM to compute ignition spark advance, there are only two conditions which are likely to cause spark timing to be incorrect; initial timing or a false input signal to the ECM.
The first step in troubleshooting emissions and driveability concerns should always include a quick check of initial ignition timing. Any error in initial timing will be reflected throughout the entire spark advance curve.
If engine load is miscalculated because of incorrect input signals, spark advance angle will not be appropriate for engine operating conditions. This will result in driveability and emission problems. Refer to course 850 for additional information on spark advance strategy.
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