identifying non-firing cylinder (rpm drop test)

[übersteuern]

Guys,
I've got an engine that's started running rough, it feels like it's running on 5 cylinders. Embarrassingly, it's been a while since I've done an rpm drop test, so I've forgotten how.

Is it ok to disconnect the plug leads while the engine is running (for the easiest identification of when there's a change)?
Do I just yank the lead off the spark plug?
Since we're playing with high voltage, any precautions to avoid getting zapped?

(it's actually on an M30, but it's Motronic, so I figure it's the same ignition system)

Thanks.

[Sea Aych]

Did you check your spark plugs first?

[ttrousdell]

My M30 ran on 5 cylinders once when one of the spark leads fell off the plug(shitty wire set). As you can imagine sluggish acceleration and rough idle/running. You can pull the leads off. the wires are insulated enough where you shouldn't get shocked. still avoid grounding yourself. RPM drop is easy. Pull the wire leads and see how it affects engine rpms

http://www.freeasestudyguides.com/cylinder-power-balance-test.html

[übersteuern]

Thanks for the replies. I forgot to subscribe so I missed them

Did you check your spark plugs first?

Yeah, I ended up reading the plugs instead (damn, #6 is annoying to get to on an E34!). The mechanic said it was before than a cylinder drop test which could blacken up all the plugs.

I was thinking cylinder drop would be a quicker test, looks like it has this disadvantage though.

Thanks again.