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Don't miss on the misfire graph

Saturday, August 1, 2009 - 00:00
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The IDS has the ability to display a live misfire graph by utilizing the information from the Crankshaft Position (CKP) sensor and Camshaft Position (CMP) sensor collected by the Powertrain Control Module (PCM). The CMP sensor identifies which cylinder should be on its compression stroke. The CKP sensor charts the effective increase or decrease in RPM each of the cylinder firing events has on the crankshaft.

Notice the RPM vertical scale on the left side of Figure 1. The center number is 0 RPM, the top of the scale is +50 RPM and the bottom is -75 RPM. Along the top of Figure 1, the cylinder numbers are listed in firing order with each cylinder having its own vertical RPM record column below it.

If the engine is not misfiring and there is not much of a change in RPM cylinder to cylinder, the thin blue horizontal graph line stays very close to the green horizontal 0 RPM line. The gray horizontal band is a record of where previous RPM changes tracked for those cylinder columns; it is known as a histograph.

Below the graph screen and to the left is the live RPM reading box, and to the right of that the blue bar graph also tracks current RPM. This blue horizontal bar graph has another function in that it will turn yellow when the misfire function of the scanner becomes inaccurate. Because the misfire graph is charting changes in RPM from one cylinder firing event to the next, the graph can get lost if you snap the throttle too quickly. The graph cannot tell if the change in RPM was due to the snap or a misfire/fire sequence.

It is a common technique to quickly snap the throttle open in order to create high firing voltage demands as a test of the ignition system. This scan tool test cannot handle this scenario. If the engine is barely running on any cylinders at all the graph will be unstable and constantly turn yellow and blue, rendering the results questionable.

Let's move on to what a misfire event looks like (Figure 2). I unplugged the No. 1 cylinder injector. Notice that in the center of the Cyl. No. 1 column the downward sloping line ends in a triangle pointing down to about the -50 RPM mark. The line slopes down because the RPM dropped between the successful Cyl. No. 2 firing event at the end of the firing order and the misfire on Cyl. No. 1 at the top of the firing order.

The RPM greatly increases between the Cyl. No. 1 event and the next cylinder in order, which is Cyl. No. 3. The drastic change in RPM is evidenced by the line corner in the middle of the Cyl. No. 3 column. Between Cyl. 3 and 4 there is not much of a change, so the line corner barely forms a point. Between Cyl. 4 and 2 there was slightly more of a change, but because the RPM dropped with the misfiring No. 1 cylinder, the "corner" in the middle of the Cyl. No. 2 column is almost imperceptible.

Some Fords use CMP sensors that are mounted to a removable housing and drive shaft. This shaft can be installed indexed to the wrong cylinder by accident. If the CMP housing is installed wrong, the wrong cylinder will be identified as misfiring. To check this, try pulling a known cylinder spark plug wire or injector connector. Then see if the misfire graph IDs the correct cylinder.

The crankshaft profile must be properly learned for the misfire graph feature to operate. This profile has to be relearned when battery voltage to the ECM is lost or Keep Alive Memory (KAM) is reset.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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