while engine running...

You "COULD" could damage coil insulation internally, it will run rough, you will get a belt!

With a modern high power ignition system its not wise, you could blow the coil or the amplifier- the HT comes out of the coil but has no path to earth and travels back whence it came.

If you grip the HT lead in your hands then your a very brave man indeed or else stupid- the body is an ideal conductor and 40,000volts+[1] into it spells bad news(!)

If the car in question uses a wasted spark system, then by breaking the circuit on one plug you automatically break it for its other half so the car would run on 2 cylinder, if not stall immediately.

If you are checking for a spark on a car in a no start situation then a proper spark tester is an essential tool, you'll invariabley cause more problems and blow something else by just casually pulling off a HT lead and waving it around.

[1] This would be considered the minimum on current engines- alot are running around the 60-80,000volts. Engines with coil on plug towards the upper end of this and above. When the sticker says "danger of death, dont touch with engine running" it means it!

Tim..

The car will misfire, something would get fried and you'd get electricuted!

electricuted!

Depends on the ignition system and how you pull off the plug lead. I heartily recommend using your most expensive and well insulated set of pliers for this purpose. That's if you really must do it at all. It can be used to establish the culprit if you think you have a misfiring cylinder. The one that makes the least difference was the culprit etc...