Kv waveforms

Looking for an occasional misfire on a ford focus at idle and higher rpm. This picture is from cylinder 4, the Kv barely goes over 1k volts but it still looks like a spark is being produced? This is with the engine rpm held at 2500. All the other cylinders have a Kv of around 5k at this speed. I am new to secondary ignition wave forms and this one sticks out to me as the voltage seems low.

Is the misfire worse at idle or at when accelerating wide open throttle at medium speed ?
A misfire at idle with a general lack of pulling power at low to medium RPM would make me suspect a leaking valve or compression issue.
A definite hesitation type misfire when engine is pulling hard from medium RPM would make me look to an ignition issue. I take you have swap around on the plugs and wires.

In this case I would take do two more tests (1) a snap throttle KV capture from the missing cylinder and a known good cylinder, (2) any type of compression test, ie cranking voltage, cranking amps, leakdown test or old style compression test with a gauge.

Misfire seems more obvious at idle but it is misfiring while revving up as the exhaust starts popping and has high HC and CO. Compression sounds ok but ill try a relative compression test. Only reason I suspected coil is because wave form on cylinder 4 is a lot different from the others, engine code for cylinder 4 misfire.

With Ford coil packs remember you can swap coil wires over between the coil outputs of companion cylinders ie on 4 cylinder 1 & 4 companion cylinders as are 3 & 4.

Does the height of the kv-spike change at all with different rpm or snapping the throttle?

I guess you already checked the spark plug if the plug gap is too small or too much carbon the kV needed for jumping the gap is not much could also be a rich mixture

I am really thinking low compresion due to a leaking exhaust valve on on this one because of the low KV but with a near horizontal spark line

Andy.MacFadyen wrote: I am really thinking low compresion due to a leaking exhaust valve on on this one because of the low KV but with a near horizontal spark line

I was thinking if it was low compression I would have a dead miss here but its more a frequent misfire, sometimes it wont do it for 20-30 seconds. I have not checked if the Kv rises on the snap but I will check this out, looking at it tomorrow evening. Nothing has been removed so not checked spark gap. I am thinking a weak coil, I could try stress the coil and remove the lead and see how far a gap it jumps. Sometimes that line was only around 800 volts.

Here are a couple of waveforms from other cylinders. NIght and day different from number 4. Also notice how the spark line leads up on the 'good' cylinders but leads downwards on C4. Still learning here so what are your thoughts on this?

Intermittent is a good clue look for a plug lead arcing to ground, Paul made good video on how to do this using an old school tungsten bulb test light but you can equally do it with a length of wire conneced to a ground (nb any ground except the battery itself because of explosive gas risk). The simple sweep the exposed metal probe end of the test light along the plug lead to look for a spark taking the easier path to jump to ground. I will try and find the video and post a link.

Happy to say I found the problem after looking at this car again this evening. Concentrating on cylinder 4 I removed the plug to find the gap almost completely closed up with the plug in the right way up, when turned upside down the gap reappeared! The sleeve surrounding the electrode had come loose allowing to to move to open or close the gap. Never seen this before, very happy to have resolved this. Thanks for this comments.

Happy for you cheap fix is a good thing