Sounds like your fan isn't kicking on.

During idle there is no airflow source to push away hot coolant from your condenser. During cruising there is 30-40 mph winds pushing airflow from your hot condenser.

So this is why fans kick on during idle. Try turning on your ac if equipped. The ac uses every fan possible. Do they turn on?

You may have misunderstood the mechanic. If you have water inside your combustion chamber then that cyliNder wouldn't fire. You would feel the misfire.

Do you have a cold air intake? Maybe he was just warning you of potential problems.

Does the mechanic refer to coolant as water?

Does the vehicle smoke white? Maybe that's why he thinks your burning water or coolant.