I read everything I could find from searching and didn't find anything definitive. So, last fall while road testing my bike for inspection, my Indy mentioned that the engine light had come on when he got into the throttle. I said it never came on when I ride it, to which he replied, "Maybe I ride it a little harder than you." He cleared the codes, and I haven't thought much about it til yesterday.
I installed autotune for my PCV a few weeks ago, and have been going through the process of accepting the trims and getting the tune dialed in. In an effort to do so, I have been trying to vary my riding to hit as many different cells in the fuel map as I can. So yesterday I was on my way to the store and decided to romp on it pretty hard. Can't say for sure about RPM as I have no tach, but I think I hit about 85 in fourth gear. Check engine light came on and stayed on for maybe 30 seconds or so then turned off. The bike continued to run fine and had I not seen the light, I would have never noticed any difference at all. I got to the store, and while I was in the parking lot, I checked the codes and found P1358 which is Rear Cylinder Combustion Intermittent,(or so I read on here). I cleared the code, and went into the store. On the trip home, same deal. This time, fifth gear wide open throttle for a few seconds. Light on for a little while, then off. Checked codes when I got home, same deal.
So everything I've read leads toward checking plugs/wires/coil, etc. I am running stock Harley 6R12 plugs. The wires are not HD, but they are supposed to be an OEM equivalent from Drag that my Indy sold me, because that's what he had. Other than that stock 88ci/stock heads/HQ-0034 cams/S&S air cleaner/ MGS true duals/ Power commander five with a map from Fuelmoto and autotune. The bike runs good, even when the light comes on, so why is it happening, and why only under hard throttle? What do I do about it? 2004 Road King. Any thoughts? Thanks.

 

The connections to coil and plugs need some electrical grease. Fixed my same issue with different wires

 

I'm afraid I contribute much but I do remember one of the parts desk guys steering me away from aftermarket plug wires saying he had had people say the aftermarkets would cause engine codes. It was a couple years ago and I stayed with the stockers with no problems

 

I know I greased the plug ends when I put them on, but I don't remember about the coil. Simple enough to try I suppose. But why only under hard throttle?

 

So does anyone have any wisdom about the hard throttle part of this riddle?

 

Vibration...

 

A simple explaination is this:

At W.O.T. under load your combustion pressures are higher, therefore requiring higher ignition voltage to jump the plug gap.

 

^^^^ case closed! Lol

 

Sounds plausible. Assuming this is what's causing my issue, what is the reason for my insufficient ignition voltage? Is this a case where something as simple as checking and greasing my connections could really be all that is necessary? Or do I need to replace the wires? Or perhaps there is even a problem with the coil? Obviously, I will start with simple and cheap and expand as required, just curious as to what I might expect.

 

How many miles on the plugs? I apparently don't ride mine hard enough either because in 10,000 miles my plugs would get dirty enough to misfire and fuel economy dropped off noticeably. Finally went to one heat range hotter plug and they stay much cleaner now and run properly for over 20,000 miles. If you want to go from cheap to expensive while throwing parts at it, plugs would be a good place to start. Or try disconnecting and reconnecting the rear fuel injector plug, then the spark plug if that doesn't fix it.