R53 started stumbling a bit recently (mild erratic idle), then missing a bit, then missing a lot on a drive yesterday. Pulled the coil wires and there was a substantial amount of reddish/tannish powder around the coil contacts as well as some pitting. Sprayed both the coil contacts and the plug wire connectors with electronic contact cleaner and, BOOM, problem fixed. I'm running a 15% S/C pulley, 380cc injectors, aftermarket air intake, stock coil and a tune and use nothing but 93 octane fuel. Just curious if anyone knows why the powder buildup would be happening and if you've also run into this issue? Possible fixes?

 

Congratulations, you’ve discovered rust. It’s a common problem, especially on the back two terminals.

 

Simplistic answer to be sure (and more than a bit snarky) but it doesn't explain the problem or the fix. Since this has never happened on any other car I've ever owned (and I've owned plenty), why would this occur on a MINI?

 

Clean it up, slap some dielectric grease on it, and run it. Or better, replace the coil and the plug wires if it's badly pitted because it'll probably just do it again. I don't think I've ever seen a great explanation as to why it's so endemic. Though I believe the stock coil is plated steel, hence the rust, and I don't think mine had any extra protection on it from the factory.

 

It's caused by vibration induced "fretting." https://en.wikipedia.org/wiki/Fretting
As deepgrey said, dielectric grease should help.
Also consider squeezing the terminals on the wires with a pair of pliers for a tighter fit on the coil contact. Or, try better quality wires. I've been running the same set of Magnecor wires on my R53 for about 15 years and the corrosion is barely noticeable.

 

Huh, I hadn't considered fretting. If it's that, then it's actually not rust (or at least not all of the debris is rust). I wonder why the terminals are never all equally bad though. Engine harmonics?

 

Yeah, harmonics. The back wires are longer so they will have a different resonance.
Zip tie-ing them together will change this and may make the problem better (or worse!).