Spark Kv Low in one cylinder

[MrFixIt]

Hello!
I am a noobie to reading scope readings. I am working on a 1985 2.8 Fieror GT. All of my spark Kv reading are around 16Kv, except one cylinder, which runs 5-12 Kv. What could cause this?
Thanks!
Brit

[carmedic]

Low kV

Low compression or a fouled spark plug come to mind. Possibly a plug gapped too narrow, but that doesn't happen often. Any chance you can post a screen shot or two?

Jim

[sdlindsey]

Hello!
I am a noobie to reading scope readings. I am working on a 1985 2.8 Fieror GT. All of my spark Kv reading are around 16Kv, except one cylinder, which runs 5-12 Kv. What could cause this?

Please include some screen shots.

By "spark kV" did you mean the firing kV, the initial spike, or did you mean the kV of the spark line?

Think of secondary ignition like a string of fixed length bent into an "L" shape. There is going to be the same amount of energy per strike, only it may be distributed differently. I'll include some screen shots to back my explanation. If the firing kV demand is high, then the spark kV (the firing line) will be shorter. If the firing kV demand is low, then the spark kV (the spark line) will be longer. The firing kV is the total amount of kV required to bridge all the gaps and resistences in the secondary ignition circuit. The spark kV is the kV required to maintain the spark.

The first capture is from a 1997 Acura Integra that had a wire that was allowing the secondary ignition to go to ground outside of the cylinder. You can see that by it's low firing kV and lengthier spark kV. (If you were seeing this in Raster/Cylinder mode it would look more dramatic. It's kind of squashed in Parade)

The second capture, specifically the second pattern, is from a 1993 Ford Escort that had a cracked coil tower and the spark, under load, was jumping a 1/2" air gap to one of the coil hold down bolts. You can see the extremely high firing kV (energy required to jump all that air) and you can see there is no real spark kV to speak of. Compare it to the one before and after and you'll see what I mean.

I would say that when you look at secondary ignition look at both Parade and Raster. Each mode is strong for a different reason and can expose different issues. 5ms versus 10ms is also useful depending on the RPM you are running it at and how much detail you want. I also advise making use of the cylinder mode so you can really look closely at things. The MODIS and the Vantage Pro both have good buffers so I would advise watching things at idle, off idle, and during the event. You can then pause and review the data -- either manually or even have the device auto scroll. You can change the view (Parade, Raster, etc etc) at any time.

[MrFixIt]

Low compression or a fouled spark plug come to mind. Possibly a plug gapped too narrow, but that doesn't happen often. Any chance you can post a screen shot or two?

Jim

I wish I had a way to

[MrFixIt]

Please include some screen shots.

By "spark kV" did you mean the firing kV, the initial spike, or did you mean the kV of the spark line?

Think of secondary ignition like a string of fixed length bent into an "L" shape. There is going to be the same amount of energy per strike, only it may be distributed differently. I'll include some screen shots to back my explanation. If the firing kV demand is high, then the spark kV (the firing line) will be shorter. If the firing kV demand is low, then the spark kV (the spark line) will be longer. The firing kV is the total amount of kV required to bridge all the gaps and resistences in the secondary ignition circuit. The spark kV is the kV required to maintain the spark.

The first capture is from a 1997 Acura Integra that had a wire that was allowing the secondary ignition to go to ground outside of the cylinder. You can see that by it's low firing kV and lengthier spark kV. (If you were seeing this in Raster/Cylinder mode it would look more dramatic. It's kind of squashed in Parade)

The second capture, specifically the second pattern, is from a 1993 Ford Escort that had a cracked coil tower and the spark, under load, was jumping a 1/2" air gap to one of the coil hold down bolts. You can see the extremely high firing kV (energy required to jump all that air) and you can see there is no real spark kV to speak of. Compare it to the one before and after and you'll see what I mean.

I would say that when you look at secondary ignition look at both Parade and Raster. Each mode is strong for a different reason and can expose different issues. 5ms versus 10ms is also useful depending on the RPM you are running it at and how much detail you want. I also advise making use of the cylinder mode so you can really look closely at things. The MODIS and the Vantage Pro both have good buffers so I would advise watching things at idle, off idle, and during the event. You can then pause and review the data -- either manually or even have the device auto scroll. You can change the view (Parade, Raster, etc etc) at any time.

I only have a Vantage with the Kv Module but it is a start. Looking at the Pico.

I have replaced the cap, coil, rotor and wires. Putting in new plugs (replaced these already once recently).

I wonder if I lashed the valves wrong on that cylinder. How would a fouled plug look versus say low compression on a scope? I had someone put new head gaskets on it, but I can't remember him checking to see if the iron heads were warped

Mid-engine cars are such a pain!

[sdlindsey]

I only have a Vantage with the Kv Module but it is a start. Looking at the Pico.

The kV module is cool because it is a histograph tool. You can see the overall changes in voltages over time. The downside: you can't see the actual secondary ignition pattern itself. It has its ups and downs. The key is understanding the differences in how the data is presented, and where each method shines. Histograph is _really_ good for intermittent issues.

Do you understand what you are actually looking at with the kV module?

I wonder if I lashed the valves wrong on that cylinder. How would a fouled plug look versus say low compression on a scope?

I'll let you decide.

If the condition with the spark plug increased the resistence/distance of the gap, then that would increase the total resistence/gap in the secondary circuit. This would increase the firing kV demand -- total amount of kV required to bridge all gaps and create a spark. If the condition with the spark plug decreased the resistence/distance of the gap, then that would decrease the total resistence/gap in the secondary circuit. This would decrease the firing kV demand. Lower compression means less air in a given space, and higher compression means more air in a given space. If you had more air in a gap, would that require more of less kV to bridge?

One last note: This is where looking at the actual secondary pattern can be helpful. You can look at other aspects of the pattern other than: firing kV, spark kV, and burn time. You can look at the actual shape of things. I'll include a capture to demonstrate.

A quick search on the web produced this: http://www.babcox.com/editorial/us/us90240.htm which is a decent explanation of the basics of a secondary ignition pattern.

The first capture below was taken from a 2004 Ford Ranger with a V6 of some kind. I forget now, and my notes are not complete. This vehicle had a P0304 CYLINDER #4 MISFIRE DETECTED. You can see the download slope of the spark kV line. (This system uses multi-strike at idle and that's why it has two peaks.) The spark kV demand decreased as the spark was firing. This engine had 60% leakage from the piston ring in the #4 cylinder. I actually started this diagnosis with a (very quick) relative cranking compression testing -- as show in my second capture. You can see where one of the humps is much lower, indicating a compression issue. I triggered off an injector so I could confirm the firing order location of the problem.

[bwrench]

You will find that either a modis or a Vantage Pro will have a way better ignition scope than a modis. The reason is, the Snap-On scopes were designed to find problem with easy, to set up easy, and to move around the shop easy. The pico is a true lab scope, but it does not have an ignition scope. It is using the lab scope to view it's ignition waveforms with. Where the difference lies, is in an ignition scope, they use an rpm trigger, so that when you change the speed of the engine, you will still see the same number of cylinders on your scope, no matter what engine speed that you use. When using a lab scope function, the number of cylinders displayed depends on the engine speed. The Pico is great for analyzing things, although it will not store it's information in an endless data stream like the Snap-On scopes do. Snap-On has really done their work on these scopes, and have built a scope that is built for fixing cars, instead of fooling around with cables, buttons, and laptop computers.