M60 engine idle
I have had all the idle problems we are talking about. Each cylinder has its own coil connected to the spark plug with a rubber boot and a three wire connector with a metal retaining clip for each. You have probably noticed these very easily if you have replaced your plugs and you have already disconected the connectors. Without removing the coil or anything else, one at a time disconnect the connector just enough to break the contact inside is fine. On properly working cylinders you will notice an increase in rough idle while the connector is disconnected as power from the cylinder is no longer functioning. When you come to a cylinder that is having issues you will see no change in the rough idle. Disconnecting a non-working cylinder won't make it work any less. Get it.
Now to the boot. Small cracks at the end are no problem. A split that runs along the side can very easily ground the sparks to engine and bybass the plug making no spark in the cylinder. Oil or water in the boot or area around the boot (can collect liquids here easily) should be removed. You may want to look into doing a "leak down" test if you decide to take out the coils. You can find the device at Harbor Freight or online for around $50. It will tell you the exact mechanical condition of each cylinder without taking anything apart. It helps to have someone who knows how to do it to show you, then you can do again on your own. Works with any vehicle. Important to do before buying a used car.
The only other problem is if you have a Nickasil coated engine block. They are very susceptible to hi sulfur gas made in the 90s and subsequently phased out everywhere in 2007. Some states like Colorado had really "bad" gas and ruined these blocks easily. BMW replaced these with Alumisil block under warranty on many M60 engines, but not everyone bothered. My 540 is a Nickasil block but the previous owner used premium gas and the standards in California were much higher on sulfur and mine engine does not seem to be effected. You no longer need to use premium, it is mostly a myth for the gas companies to make more money.
Hope this helps.