Electronic throttle bodies are most definitely a huge step forward in modern cars, but they are not perfect when used with a manual transmission, here is what causes rev hang.
In more traditional methods, cars would make use of a mechanical throttle body which was directly connected to the driver's inputs by means of a cable connected to the accelerator. But more modern throttle bodies make use of electronic methods, which eliminates the need for a cable, and replaces that with a position sensor. This, in turn, tells the throttle body how open the valve needs to be at any given time.
But what about that irritating rev hang delay that manual cars with an electronic throttle experience? Well, to put it simply, it's done on purpose to quite literally improve on your car's emissions.
So, in a car with a mechanical throttle, you can shut the valve instantaneously by taking your foot off the accelerator pedal. What happens then is that, for a moment, your engine still injects fuel, thus giving you a rich spike. An electronic throttle body, on the other hand, will ignore your inputs and deliberately slow down the close of the valve in order to burn off that excess fuel, making your car release fewer emissions from a rich spike.
Take a look at the video below by YouTube channel, Engineering Explained, on The Big Problem With Manual Transmissions - Rev Hang.
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