When it comes to engines, the term "deleting" refers to the removal of one or more of the emissions control devices that the engine came with. When an emissions device is "deleted", the device and any other relevant components in question are physically removed and the engine is then re-tuned to run without said device.
"Deleting" is different from "defeating" in the sense that "deleting" refers to physically removing emissions devices, but "defeating" involves disabling emissions devices while leaving all of them in place on the engine. For example, an EGR (exhaust
gas recirculation) cooler can be defeated by welding it shut in some spot or blocking it off from
one or both ends by inserting a metal plate between the cooler and the exhaust or intake manifold. DPF's (
Diesel Particulate Filters) can be defeated by being drilled out or by installing bypass pipes.
On
Diesel engines, people who do deletes
will typically remove EGR (Exhaust
Gas Recirculation) systems and DPF's (
Diesel Particulate Filters), as these two systems are infamous for drastically increasing a
Diesel engine's fuel consumption.
Here in
Michigan we no longer have any kind of emissions testing, which means that
today you can find quite a few deleted Diesel trucks, and even some cars and
SUV's, running around in the state.