Quote from tristancliffe :Yes, but with exceptions. Modern ECU's sometimes completely stop the injectors on overrun (when the engine is being turned primarily by the gearbox) with the throttle fully closed, but this is in the interests of economy and emissions (though I'm not sure of the benefits to emissions, as it seems like a boat of false promises to me). But on 'older' cars, especially on carburettors, the fuelling was always on (and with carbs proportional to the pressure drop across the carbs caused by the throttle position and engine speed). Also, in your example above, in case #1 the engine will restart delivering fuel around the idle rpm to try and stop it from stalling. The ECU will detect falling rpms (and probably won't know the car is in gear) and just try to maintain a smooth idle. i see. Thanks for the explaining.
Quote from tristancliffe :Close the throttle suddenly, and the engine will run lean (not rich) which causes the popping. I think you (and not just you) must be confusing backfiring (intake), with afterfiring (exhaust). Quote from wikipedia :Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire, occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in a decatted exhaust system. When a driver shifts up and lets off the gas, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.