I really believe some cars completely shut off the injectors, so I wouldn’t say “never”. Some vehicles may leave a little fuel to burn under decel to keep catalytic converter temps up, but to me that would be the only reason. My standalone has the ability to completely shut off the injectors on decel. Fuel is needed to keep an engine idling, but not needed while an engine decelerates.
Just found this on the web… other searches yield similar results.
DECEL EMISSIONS. When decelerating, the engine will typically either lean out the fuel mixture or shut the fuel off completely (some fuel-injected engines). The computer typically uses inputs from the Vehicle Speed Sensor, TPS, Airflow sensor, and engine rpm to determine when this occurs. When the throttle closes and manifold vacuum shoots up, the computer cuts back on the fuel. Normally, HC, CO and NOX emissions drop during deceleration because the engine is no longer under load and is receiving little or no fuel.
If CO emissions remain high during deceleration, the engine is receiving too much fuel.
Causes may include:
• Leaky fuel injectors; and
• Faulty VSS, TPS, or airflow senso
from http://www.swedishbricks.net/700900FAQ/EnginePerformance-Symptoms1.html