Heavy duty diesel engines are designed for long hours and extreme loads, but even the toughest systems fail without early ...

First thoughts are that the stepper-motor (which controls the idle speed) is faulty. That would explain both the high idle at low engine temps and the stalling and stumbling as you roll to a stop.

The problem could have been the coolant, rather than the cap. A higher-pressure cap can help if it's a minor problem but, in any case, if you run into problems in the future there is an admission of a ...