A coal roller is a pickup truck
with a diesel
engine which has been modified to emit a thick, black cloud of exhaust. This is created by intentionally throttling the engine so that a large volume of fuel enters the engine's compression chambers but is only partially ignited and the resulting exhaust is rich in unburned diesel vapor.
One method of modifying a diesel engine in this manner is to install larger after-market injectors. This results in the greatest "performance" but is also the most expensive approach and requires expertise to re-tune the vehicle. A less invasive, inexpensive and novice approach is to install an aftermarket tuning/performance microprocessor which overrides the timing, intake and/or environmental performance profiles of the vehicle ‘s factory-installed microprocessor. Other modifications include parts to increase intake and exhaust airflow, including exhaust systems which vertically extend from the bed of the pickup truck as a tractor would have.
Originally the province of featured vehicles at tractor pulls, creating the largest smoke cloud was a matter of competition amongst their builder/operators and a source of entertainment common to county fairs.
However more recently, "rolling coal," has become a status symbol of certain American conservative ideologues. A coal roller can therefore also refer to this type of person who owns and operates a vehicle as described to outwardly disdain various environmental attributes they identify with liberalism. The vehicle becomes a status symbol amongst peers who delight in offending "liberals," with their displays.
Such displays are, as expected, hazardous to health and the environment. Rolling coal is illegal in many states and municipalities, violators subjected to various fines. The Obama administration's EPA officially declared the practice to be in violation of the Clean Air Act.