Definition
An intercooler, also called an aftercooler or charge-air cooler, cools the compressed intake air leaving a turbocharger before it enters the engine, increasing the air density available for combustion.
Compressing air heats it, and hot air is less dense. Cooling the charge after the turbocharger restores density, which means more oxygen per cylinder filling, which means more power can be made cleanly from the same displacement.
What neglect does
Aftercoolers foul on both sides — raw water scaling the cooling passages, and oil mist and debris coating the air side. The symptoms are gradual: reduced power, higher exhaust temperature and black smoke under load. Because it declines slowly, owners commonly attribute it to the hull or to fuel.
Aftercooler service is a scheduled item on turbocharged engines and one of the most commonly skipped ones — worth asking about specifically when buying a used boat with a turbocharged engine.
Where you meet it
On turbocharged engines, in Cummins and Caterpillar service schedules, and in the diagnosis of gradual power loss.