There is a definate difference between a miss...dead cylinder... and a vibration. If it has a dead cylinder, it will shake at idle and you will hear it. Instead of a nice smooth rattling marble sound, the sound the engine makes will have an audible break in the sound you hear...kinda hard to discribe it to someone who has never really worked on a compression fired engine...forgive me on the next lines...kinda like r r r k r r r k r r r k instead of r r r r r r r r r. Also, the more RPM, the faster it will shake. You will SEE, hear and feel a miss. Alot of times it is accompanied by smoke of some sort. If the cylinder was "dead" due to a valve, piston or head or cylinder issue, there would be white smoke caused by low compression. If the miss was caused by a failed injector, there could be black, white or no smoke. For a vibration, they usually happen in multiples of engine cylinders. Example, a 6 cylinder will vibrate at around 550 to 650, 1100 to 1300, 1700 - 1900, etc. An 8 cylinder will do it at 800 + or - a hundred or so RPM, 1600 + or -, 2400 + or -, etc, and SOUND smooth. You will not see it vibrate, only feel it in the floorboard, shifter, steeringwheel, brake, clutch and throttle pedals, door handles, windshield wipers, eye teeth, etc, and, the more RPM, the more it will vibrate.
In answer to the cracking lines to find the miss, yes, that's how it's done. If it does it cold, wait 'till first thing in the morning. Fire it and feel the exhaust manifold. If you have a miss, the manifold will be cold at that cylinder long after the others are warm. If it helps, put spit or water on your finger as you check the manifold. The firing ones will dry right away, the missing one will stay wet for a bit. Whew! My fingers hurt now! Good luck.
Oh yeah, glow plugs won't cause a miss or vibration unless the tip has broken off. In that case, see the "smoke" thing above...damaged cylinder components.
