4.2.1 Description
The misfire detection monitor runs continuously and is designed to detect levels of misfire that can cause thermal damage to the catalyst and/or result in excessive tailpipe emissions. Determination of a misfire is made by analysis of changes in crankshaft speed, since a misfire will cause a drop in acceleration after an anticipated firing event. This data is analyzed in four ways to ensure the detection of all possible combinations of misfire.
The results of the misfire judgement process for each firing event are used to determine whether two failure levels have been met, 'catalyst damage' misfire and 'excess emissions' misfire. Each misfire judgement process has its own failure threshold and calculation period.
The following fault conditions can be identified by the monitor
- Cylinder 1 (1A) misfire
- Cylinder 3 (2A) misfire
- Cylinder 5 (3A) misfire
- Cylinder 7 (4A) misfire
- Catalyst damage misfire
- Low fuel level misfire
- Cylinder 2 (1B) misfire
- Cylinder 4 (2B) misfire
- Cylinder 6 (3B) misfire
- Cylinder 8 (4B) misfire
- Excess emissions misfire
- Random/Multiple cylinder misfire
The misfire monitor operates continuously within the boundaries of the regulated monitor operation window, as shown below.
After engine start, the monitor will be enabled as soon as the engine speed rises above the minimum operation speed (150 RPM below fully warm stabilized idle speed). Two revolutions of crank angle data, i.e. one sample of data from each cylinder firing, must then be 'buffered' before any decisions can be made by the monitor. Before engine speed has reached the top of the start flare the monitor will be ready to make misfire judgements, which are then made on every cylinder firing, irrespective of whether the monitor is enabled or not.