I just got one of these OTC spark plug voltage testers on Ebay:

and it didn't come with any instructions as to interpreting the readings. I tested it on a great running '93 Civic with new plugs and all the plugs read 4kV. Next I tested it on an '81 Olds Omega I just got. This car starts up ok but after a 2 minutes it starts to miss and then dies after which it won't start at all. I put in new plugs and it was fine last week during warmer weather when it ran very smoothly. For the few minutes it did run, I hooked up the tester and it was reading in the 10 to 14kV range. What do these reading mean in general and what specifically would it indicate on this Omega? Weak plug wires?

I have never used that tool so I'm not sure what or how you are reading. In general the voltage put out by the coil is whatever is required to bridge the highest form of resistance. An open plug wire (open means infinite resistance) will take the coil to its maximum output which is usually around 30kv (30,000v). The highest form of resistance is usually the plug gap under load. A plug setting on the motor outside the cylinder, will fire at probably a couple thousand volts, whereas the same plug firing in the cylinder under compression will fire at 10kv approximately.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician