It is a little cruder test than I like. Yet if a cylinder is off line at idle the glow plug base will be lower in temperature. Cylinder totally down about a fifty degrees lower temperature than the others at hot idle.

You essentially are just reading the coolant temperature in that area. Where the cylinders that are firing will have a hotter glow plug base.

Increasing the idle will make the engine seem a little smoother in itself. Also from a dynamic perspective the cylinder may fire up at increased revs.

Depends on what is wrong. On a rough running engine at idle I always check to make sure all the cylinders are firing first. Unless something else is obvious.

I could have spend a lot of time and effort otherwise in many cases over the years. You may not find a dead cylinder but you will then know it is not the problem. If you do find one there can be many causes. You at least know where the problem is located though. For the crowd post the temperatures you read when you get to it.

When you said the adjustments on the valves were tight. Do you mean no clearance at all on some? Or just less clearance that what it should have been?

Over the years ahead you will find many applications for the temperature reader.

You could have an injector with no fuel output. If that were the case in all probability it would not be the injector. Too early to even consider it. You just want to establish quickly that all the cylinders are running decently. Or not.