Autonerdz forums

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Don't you dare Wilf     I think you know that FLS better than anyone.  Certainly better than me 

It's easy to jump to conclusions with that tool as the waves leave a lot to interpretation.  You question everything.  This is good.  With a tool like that you need to substantiate things to draw valid conclusions.  It's easy to assume things that later prove to be less than completely accurate.

Hi Mike,

The difference in 02 content (contained between individual cylinder exhaust strokes) drives differential pressures between the exhaust strokes of good combusting and misfiring cylinders.  The unconsumed O2 (and sometimes HC) from a misfire (or partial misfire) causes a difference in mass between the exhaust strokes (homogeneous gasses and non-homogeneous gasses). Non-homogeneous masses create an obvious pressure differential in the signal created by the FLS.
I think you are equating the “long line” pressure transitions (in the signal created by the FLS) between exhaust pulses as the signature of a misfire and that is correct. Any pressure transition between exhaust pulses is an indication of a differing combustion event between cylinders.  It’s just that the “longer the line”, (greater the pressure transition) the greater the misfire if that helps. Long line misfires are “dead holes” that you and your customers will definitely “feel”. Shorter lines (lesser pressure transitions between exhaust strokes) are partial misfires or combustion inefficiencies that you may “feel” as a quiver or not “feel” at all.

Since the FLS is a Piezo device, I look at the FLS waveforms as the “sound” of each cylinders exhaust stroke. Of course each cylinder’s (entire or total) sound will occur at a predictable point of a 720 degree cycle, differences between individual exhaust stroke “sounds” are easily identified. With the resonance difference that “different design” exhaust systems create through different bends and different muffler baffling however, I’m not sure that that you can dissect and analyze the mini-content of each exhaust strokes “sound” in the same way between “different” manufacturers and engines. What “sounds” like a burnt valve on a Ford may “sound” like a low flowing injector on a Chevy? I dunno, but I think that at least valve "sounds" may be easier done on the intake side of things with the FLS. That’s my take for WIW, but I think that between you and Wilf working these signals so much we'll probably all know sooner or later 


mwebb wrote on Jul 31^st, 2007 at 7:48pm:


A tube within a tube is a venturi (http://en.wikipedia.org/wiki/Venturi_effect) which should smooth the signal somewhat and maybe make the overall pressure differentials easier to pick out but I’m not sure that’s a good idea for what you and Wilf are doing since you will lose some waveform “sound” definition and change the picture.

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Maybe I am confused, you said there was a wave that was between 4 and 8 cylinders in length. I took it to mean that it was not happening every rev or every other rev, relative to the spark firing event or TDC on CKP, say. If it did not sync to th electrical impulses on the car then it would be some kind of noise or whatever.