When diagnosing a dead cylinder, I usually probe the boots of the ignition wires with a test light. I stick the sharp end of the test light probe straight thru the rubber and kill the cylinder temporarily to see if there's any current thru the wire without having to unplug it. It also shows if there's no difference in engine behavior that I have probably found the misfire. When I pull the probe away, the hole closes itself (it's so small to begin with anyways)

Recently, I was told this is a bad way to test. Is it? If so, why? It's not like I'm killing the live cylinders for long periods.

 

One reason is you should never poke a hole no matter how small into a spark plug wire. I have seen the smallest of holes in a plug wire arc.

 

I usually plop some liquid electrical tape over the hole when I'm done. The guy giving me crap was spouting some junk about ruining the ignition coil. I've done it a lot and never had a problem; but I'm not an expert.

 

I have heard of the coil going out on hondas due to this. I haven't ever had it happen to me but have seen and heard about it many times.

 

Why poke a hole in something perfectly good? It just as easy to pull off the plug wire?

 

It shouldn't hurt the coil, but it's not going to do the wires any good.