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The effect of a misfire
Posted to Technical Theory Forum on 3/20/2013 28 Replies

[exhaust pulse waveform]

Back in the old days when I was in charge of the Tech Center at Allen Test Products, we did a lot of things to make the engine fail in order to collect data. I had a budget to hire Automotive students of Western Michigan University to assist, and we had a ball. Besides measuring Torque and HP on the dynamometer, we could measure fuel consumption. One of the test we did was to determine the effect of one cylinder misfire and found a 30 % waste in energy. This was before OBD-2

[6 cylinder - No misfire]

Ironically we can proof the same without all that equipment by looking at the pulse waveform of the exhaust. [66548] Here is an illustration of all the 6 cylinders normally firing after the fix. [66547] This illustration shows the misfire, where we can see the repeat of all cylinders. The condition is 850 RPM in drive. Notice that the misfire caused a stumble but it took all the 5 contributing cylinders to restore to normal, and the cycle starts all over again.

This is extremely valuable when you can print the evidence out (on your letterhead of course) and the owner shows everyone at the water cooler. It is a silent promotion of your shops expertise. The point is: that it is not the loss of one contributing cylinder, but it is affecting all contributors. When driving at 60 MPH it takes a little deeper foot peddle and a more fuel to maintain the desired road speed.

Prolonged driving with a bad plug wire or dripping injector or any absence of a spark can have nasty consequences.. Misfire can damage a converter very quickly, especially the manifold mounted or "close coupled" converter. Unburned fuel, high temperatures and literal flame introduced to the ceramic can actually crack the brick causing a blockage and melt-down

Mac from Michigan

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